From perspicuum, sublimis, is that science which describes and explains the various phenomena which occur in the region of our atmosphere. It has deeply engaged the attention of men in every state of society, from the roving savage to the refined votary of wealth and pleasure. The moment we cross our doors, we commit ourselves to the influence of the weather. But the hardier classes of the community, the shepherd, the ploughman, and the mariner, whose labour creates or procures the staple articles of life, are always exposed, by their occupation, to the mercy of the elements. They were hence led, by the strongest motives, to examine closely the varying appearance of the sky, and to distinguish certain minute alterations which commonly precede the more important changes. No doubt they would often mistake the indications of those aspects, and would infer conclusions from mere casual circumstances. Those tokens which portend the more violent convulsions of the atmosphere, the pelting storm or the careering tempest, are generally of a decided character; but the symptoms which go before the ordinary fluctuations of the weather can only be dimly conjectured by long experience and sagacious observation. This shadowy knowledge, this dubious and very limited anticipation of the changes which arrive in the medium we breathe, is merely the fruit of personal assiduity and application. Like the conclusions which men of discernment form to themselves on the subject of physiognomy, it is scarcely communicable; it receives therefore no accessions in the progress of ages, but perishes with the individual.

The vague rules which experience had formed on the subject of atmospheric changes were adopted by the philosophers of antiquity, and incorporated into their cosmological systems. But in attempting to explain the phenomena, they sought merely to satisfy the imagination; and the supposition of certain elements, each ended with peculiar properties, appeared, on a superficial view, to connect harmoniously the general facts.

In the infancy of science, however, it was very difficult to distinguish between such appearances as are only incidental, and those constant presages which invariably anticipate their effects. The different fluctuations that happen in the lower atmosphere seemed dependent on the influence of the heavenly bodies, which are continually altering their places. Not only the sun and moon appeared to rule the seasons, but the planets, and even the fixed stars, were conceived to retard or accelerate, by their feeble action, the revolution of the grand cycle of changes. The application of the term aspect to the positions of those remote bodies, implied a principle of intelligence, and the stellar influence shed by them might therefore be deemed capable of producing all the variety of effects. Hence the origin of Physical Astrology, which has, during so many ages, maintained ascendency in the world, and still colours the verses of our poets, and pervades the creed of the vulgar.

The heat evolved by the luminous particles transmitted from the sun, as they lose themselves among the lower strata of our atmosphere, or under the surface of the land and the sea, varying in its intensity, according to the obliquity of their incidence, and the extent of medium which they traverse, produces the most important effects in the great economy of the globe. But the correct knowledge we have at length acquired concerning the mutual action of bodies, contracts the limits of the supposed celestial influence almost to a point. The light of the moon does not amount to the 100,000th part of that of the sun, and the heat which this very feeble illumination excites or communicates has never been detected by the most delicate instruments, or the best-devised experiments. But all the rays shot from the planets, and from the whole constellations of fixed stars, are quite lost in the contrast with those lunar beams. Their combined impressions, during the lapse of countless ages, on the atmospheric temperature, would hence elude the utmost powers of calculation.

It is only by their attractive force that the heavenly bodies, except the warming energy of the sun, can ever affect the constitution of this globe. As, by their action upon the earth, they must likewise agitate the atmosphere with a corresponding reciprocation. This effect, however, depends much more on the proximity of the disturbing body, than on its magnitude or density. Thus the sun, with twenty-three million times the mass of the moon, yet being about four hundred times more distant, has only about the third part of her influence in causing the tides. But, according to the computations of Laplace, the joint action of the sun and moon is only capable of producing a tropical wind, flowing westward at the rate of about four miles a day; a quantity evidently too small to be ever subjected to observation, and certainly insufficient to occasion any immediate and practical results.

It may be calculated that Jupiter and Venus, in those parts of their orbits which approach the nearest to the earth, affect the aerial tides by a force 76,000 and 82,000 times less than the sun. Those planets could therefore excite a most minute shifting in the atmosphere, limited to a foot in the space of fourteen or fifteen days, or about a single mile in twenty years. As the mass of Mars is unknown, its disturbing force cannot be estimated, though it is less than that of Jupiter. But the combined influence of all the rest of the solar system is, from their great distance, incomparably smaller.

Nothing can be so utterly groundless, therefore, as the inefficacy disposition to refer the ordinary changes of the weather to the influence of the moon. But, compared with this, the far fancied efficacy of the stellar aspects, which was for ages once firmly believed by the learned world, vanishes into the shadow of a vision. Nor has the most elaborate examination of numerous registers of the weather disclosed any precise and constant connection between the phases of the moon or the positions of the stars, and the succession of atmospheric phenomena. The inferences which seem to indicate a different conclusion may be fairly disregarded, as drawn only from a loose and imperfect inspection of facts.

It cannot be disputed, however, that all the changes which happen in the mass of our atmosphere, involved, capricious, and irregular as they may appear, are yet the necessary results of principles as fixed, and perhaps as simple, as those which direct the revolutions of the solar system. Could we unravel the intricate maze, we might trace the action of each distinct cause, and hence deduce the ultimate effects arising from their combined operation. With the possession of such data, we might safely predict the state of the weather at any future period, as we now calculate an eclipse of the sun or moon, or foretell a conjunction of the planets. After the protracted chain of combinations has been completed, the same series of events must again be repeated through the boundless lapse of ages. As all the anomalies of the planets are periodical, so likewise the successive varieties follow some cycle of vast extent. In the remotest annals of atmospheric phenomena, we might descry a gleam of futurity, and read the changes which still lie hid in the womb of time.

But, besides this great cycle, there probably are much shorter interior and subordinate periods, in which the weather nearly, though not absolutely, returns after the same order. Whatever principles concur to modify its succession, the revolutions of the sun and moon are certainly the primary and predominant causes. The character of the weather is accordingly strongly marked by the vicissitudes of day and night, and by the annual repetition of summer and winter. The menstrual action of the moon escapes correct observation; but, accumulated during a sufficient time, it may possibly produce a decided influence. Every nineteen years the motions of the moon come to coincide with those of the sun, and her nodes perform their revolutions in nearly the same time, or about eighteen years. This period, or the half of it, has therefore not improbably some slight connection with the usual series of the changes of the weather.

"If the sages of antiquity be justly blamed for adopting implicitly the notions and prejudices of the vulgar, those of modern times may be charged with too eager a disposition to reject whatever savours of popular opinion. A collection of the numerous rules and remarks formed in the course of ages among different orders of men, deeply interested by their occupations in watching the changes of the sky, would undoubtedly contain some important truths, which the diligence and sagacity of the philosopher might discriminate, and employ for the basis of beneficial speculations. The most sanguine can hardly uphold the prospect that mankind will ever arrive at such a pitch of knowledge as to be capable of prognosticating the future modifications of the atmosphere, with the same precision with which they can foretell the successive revolutions of the heavenly bodies; yet the motions, however irregular in appearance, which prevail in the element that we breathe, are, equally with those performed in the regions of space, the result of certain fixed laws. The variable aspect of the sky proceeds partly from the direct action of the sun-beams, but principally from the winds which they excite and maintain. The unequal gravitation of the different portions of our atmosphere to the sun, and to the moon especially, must occasion some small effect in producing or altering the aerial currents; and even the disturbing forces of the planets have a remote share, how minute soever, in the formation of meteors. Nor can we hesitate to conclude, with the late ingenious and elegant M. Bailly, whose fate demands the tribute of a tear, that the notions, so widely spread among men, of the aspects and influences of the celestial bodies, are only the corrupted remains of astronomical science, already advanced to high perfection in some distant age of the world. If motions were to rise and cease instantaneously with the operation of their causes, the same succession of seasons would exactly attend on each revolution of the sun; but the currents of air acquire velocity by degrees, and thenceforth continue to flow till their force is spent. The varied face of our terraqueous globe will, therefore, modify the direction, the power, and the duration of the winds, raised by the action of the solar rays. Hence an extreme complication of causes, which will produce an immense series of fluctuating events. That profound geometer M. de Lagrange has established by demonstration, that all the changes arising from the disturbing forces in the planetary system are subjected to certain vast cycles, on the return of which, the same motions are perpetually renewed. Similar periods, but of an extent that affrights the imagination, probably regulate the modifications of the atmosphere; for, whenever a coincidence of circumstances prevails, the series of appearances must inevitably recur. The aggregate labours, indeed, of men continually transform the face of our globe, and consequently alter the operations of natural causes; but, if the agency of animals be stimulated and determined solely by the influence of external objects, it follows that the reactions of living beings are comprehended in the same necessary system, and that all the events within the immeasurable circuit of the universe are the successive evolution of an extended series, which, at the returns of some vast period, repeats its eternal round during the endless flux of time. Besides the grand cycle, there must evidently be many intermediate smaller periods, at the lapse of which our atmosphere will present nearly, though not exactly, the same fleeting aspects. Whether these bear any decided relation to the lunar revolutions, cannot with certainty be affirmed. A copious collection of registers formed in the course of ages, will probably, at some future time, lead to the discovery of certain remarkable periods, which will enable men to conjecture, with tolerable precision, the succeeding changes of the weather. It would be most advisable, perhaps, to begin the inquiry with the tropical countries, in which the seasons are more uniform, and to advance by degrees into the temperate climates. In the meantime, our prognostications may be greatly assisted by observing and studying the concatenation of phenomena. Certain coincidences of aspects mark the near recurrence of some small portions of the general series, and afford scope for the doctrine of chances." (Monthly Review for September 1795, art. i. p. 14.)

Such views of the cyclical returns of the varied seasons may expand the imagination, but will be considered rather as curious speculations, than as definite conclusions which can ever be reduced to real and actual application. To improve meteorology, we must submit to tread a humbler path; we must study closely, and by the aid of delicate instruments, the constitution and modifications of the atmosphere, and pursue, under various circumstances, and in different parts of the globe, a lengthened train of careful and precise observations. The ordinary registers of the weather are of a very modern date, and, besides being for the most part unskilfully kept, they seldom record more than the state of the barometer and thermometer, which afford not any complete indication of the disposition of the atmosphere. No wonder than meteorology, though cultivated from the earliest times, has advanced the most slowly to the perfection of science.

To prepare the way for establishing meteorology on a solid basis, we shall first inquire into the extent and con- stitution of the medium which we breathe; and shall next review the different philosophical instruments which assist external observation, and indicate at all times the exact condition and qualities of that mutable fluid.

The ancients imagined that our atmosphere, the seat of care, mortality, and corruption, reached as far as the moon, beyond which was a boundless expanse of resplendent ether, the abode of celestial beings, exempt by their nature from all anxiety, and absorbed in the enjoyment of eternal bliss. But the discovery of the weight and pressure of the air destroyed at once this magnificent vision. Comparing the length of the mercurial column with the density of the aerial medium, it followed, that if the atmosphere had been an uniform fluid, it could not exceed the elevation of five miles. But the air being very dilateable, the higher portions of the mass which covers our globe, sustaining a diminished pressure, must swell upwards, and occupy a proportionally wider space. This property hence removes the boundary of the atmosphere to a much greater elevation. By means of an excellent pneumatic machine, air can be rarefied about a thousand times; but this degree of rarefaction would not occur below the height of forty-two miles in the atmosphere. Such is nearly the limit deduced from another consideration, first proposed by the famous Kepler. This most original and inventive philosopher conceived, that the depression of the sun below the horizon, when twilight closes around us, might furnish the data for discovering the altitude of the portion of sky which reflects his latest parting rays. Let C (see fig. 2, Plate CCCLV.) be the position of a spectator on the surface of the earth, and A represent the point where the sun sets; the tangent AB will mark the track of his lowest ray, which illuminates the upper part of the atmosphere at B, whence the emission of a secondary ray BC will barely reach the ground at A. But assuming the mean estimate of astronomers, that twilight expires when the solar depression or the arc CA amounts to eighteen degrees. In the right-angled triangle COB, the base OC, or the radius of the globe, being 3956, and the acute angle COB nine degrees, or the half of AOC, the hypotenuse OB is easily found, and hence about 49 miles is the excess BD of CB above CD, or the elevation of the boundary of the atmosphere, illumined by the latest beams of the sun. A correction might be required for the deviation of the rays from their rectilineal path, in consequence of the unequal refraction of the different strata of the atmosphere; but in a question of this kind, resting on rather loose or doubtful observations, it seems superfluous to affect any delicacy of calculation.

The inference drawn from the limits of twilight is not so conclusive as might at first appear. The feeble slanting rays, shed from the higher regions of the atmosphere, may not have been received directly from the sun, but derived, after repeated reflexions, from the more distant parts of the sky. The very ingenious Lambert attempted to distinguish, besides the primary twilight, a secondary, and even a ternary crepusculum. It is easy to see, that the solar ray SA (fig. 3, Plate CCCLV.), after grazing along the surface, will illuminate the upper atmosphere at B, from which some light will be darted in the direction of the tangent BCD, to tinge another elevated portion of the sky at D, which may cast a few expiring rays to the spectator at E, or shoot onwards to the opposite sky at F, and thence reach, in a state of almost extinction, the ground at G. Whilst the first crepusculum, therefore, sets in the west, the second will travel like a bow over the heavens, followed at a regular distance by the dusky veil of the third, or the vanishing train of the fourth. But it may be computed, from the vast length of the tracks which the light would have to traverse, that those shades would in succession be ten thousand times darker. The clearest sky, however, on the close of evening, never appears marked by such contrasted boundaries. The vault of heaven seems to darken by insensible transitions, from the western to the eastern horizon. It is most probable, therefore, that the diminution of light, after the close of evening, is owing to the prodigious rarefaction of much higher portions of the atmosphere, which still catch some faint rays of the sun, without being able, from excessive attenuation, to reflect them efficiently to the earth. But since, unless the sky be overcast, there is total darkness in no climate, even at midnight, we may therefore infer, that the body of air extends to such an altitude, as to receive the most dilute glimmer, after the sun has attained his utmost obliquity, and sunk ninety degrees below the horizon. It would thence follow, that the elevation of the atmosphere must be equal at least to 1638 miles, or the excess of the hypotenuse of an isosceles right-angled triangle having 3956 miles, or the radius of the earth, for its base.

This very great extension of a rare expansive atmosphere appears conformable to the general phenomena. But the thin investure of our globe, at least near the equator, may stretch out much farther; and yet its elevation can never exceed a certain absolute limit. The highest portions of the atmosphere, which is carried round in 23 hours and 56 minutes, by the rotation of the earth about its axis, would be projected into space, if their centrifugal force at that distance were not less than their gravitation towards the centre. But the centrifugal force is directly as the distance, while the power of gravity is as its square. Consequently, when the centrifugal force at the distance of 6'6 radii of the earth is augmented as many times, the corresponding gravitation is diminished by its square or 437 times, their relative proportion being thus changed to 289. Now, the centrifugal force being only the 289th part of gravity at the surface of the equator, it will therefore just balance this power at the distance of 6'6 radii from the centre, or at the elevation of 22,200 miles. On this hypothesis, fig. 7, Plate CCCLV. will represent our globe encompassed by its atmosphere, of which the equatorial diameter extends from A and B.

Such is the extreme boundary of atmospheric expansion. Though it surpasses all our ordinary conceptions of the space occupied by that dilateable fluid, it yet scarcely exceeds the twentieth part of the distance of the moon, which was held by the ancients to communicate with our atmosphere. If it really spreads out to the limit now assigned, it must, in its remote verge, attain a degree of tenuity which would utterly baffle imagination to conceive. Perhaps the fluid itself may change in those lofty regions, and pass into a sort of ethereal essence, more analogous to diffuse light than to a mass of air.

The constitution of the atmosphere forms the next object of inquiry. The analysis of that rare medium is one of the finest discoveries of modern chemistry. It appears to consist of two distinct expansible fluids combined in different proportions, a single portion of oxygen gas being united to three parts by weight, or four parts by bulk, of azote. There is also a very slight admixture of carbonic acid gas, amounting to perhaps the thousandth part of the whole. It may be doubted, however, whether this analysis be complete. The combination of those gases obtained artificially, generates a fluid in which we can hardly recognise the ordinary qualities of atmospheric air. Some fugacious elements must therefore escape during the process of decomposition. Indeed, the air may be considered as an universal solvent. It is the medium of all smells, and must therefore dissolve the different odorous effluvia, and transmit them to the olfactory nerves. The presence of moisture may perhaps assist the solution, but the mass of air is still the great receptacle of those diffusive emanations. We can readily distinguish several earths and stones by their difference of smell. Nay, the metals themselves, especially when rubbed, emit peculiar odours. What can be more variously contrasted, for instance, than the smells of iron, of tin, and of copper? The air must hence actually dissolve some traces of those metals, highly attenuated indeed, and almost evanescent. If a lump of asafetida loses but a grain by exposure for several weeks, a bright surface of copper may, in similar circumstances, suffer the waste of only the thousandth part of a grain. The metal, if not encrusted by oxidation, would consequently experience a certain diminution, however small, in the course of ages.

The atmosphere is thus charged with emanations of all the various substances which it sweeps. To detect such dilute corpuses wholly transcends the powers of chemical analysis. It seems probable, that the air holds some matters in more copious solution than others; and the phenomena of the aerolites lead us to suspect that it attracts iron and nickel with greater force than the rest of the metals. The quantity of those adventitious particles contained in a given mass of air may be exceedingly small, and yet the aggregate weight diffused through the whole atmosphere would form a considerable amount. It appears from numerous endometrical observations, which agree tolerably well when performed in a similar way, that the constitution of our atmosphere is the same in all places on the surface of the earth, and at every elevation which has been yet explored. Such experiments have been made at very distant points, repeated on the summits of the loftiest mountains, and applied to portions of air brought down by balloons from the altitude of five miles. The result is what we should expect from the perpetual agitation and commixing of the lower strata of the atmosphere.

But a variety of circumstances render it extremely probable, that an expanse, far above the region of the clouds, is filled by some peculiar fluid, very different from the grosser element spread below. The shooting stars which are seen every clear night, the bolides or fire-balls, and the luminous arches which not unfrequently occur, and which must traverse the sky at the height of several hundred miles, all seem to indicate the existence of a very ignitable medium. Nor is it difficult to conceive how such a collection of highly inflammable fluids could be formed. Not to mention the multiplied processes of art which emit those products, the great laboratory of nature is incessantly at work in generating and pouring forth hydrogen gas, and its various compounds. The volcanic mountains cover a considerable portion of the surface of the globe; and their innumerable spiracles, with scarcely any interruption, continue to discharge immense streams of inflammable aerial fluids, a great part of which escape conflagration. But, as hydrogen gas has little attraction to common air, it rises upwards by its buoyancy, without suffering much loss in the passage through that fluid. The largeness of their volume, and the celerity of their projection, conspire to favour the ascent of those inflammable gases to the loftiest regions of the atmosphere.

A comparison of the several quantities of astronomical refraction, at different altitudes, points to a similar conclusion. The refraction which the rays of light suffer in slanting across the higher regions of the air, is greater than what calculation assigns to the corresponding density of the medium. But the discrepancy would entirely disappear, if we suppose those strata to consist of hydrogen gas, which is known to possess, in a remarkable degree, the power of refracting.

It seems very probable, that the higher range of atmosphere has a sort of phosphorescent quality, or shines with a certain feeble light, for some time, after it has been heated or excited by the incident rays of the sun. Such may be the principal cause, and not any reflex illumination, of that lucid glow which, even at midnight, is diffused over the clear canopy of heaven.

The phenomenon of what is called the zodiacal light might, perhaps, be traced to the same source. That remarkable appearance, which is more conspicuous in the finer climates, and near the vernal equinox, on the approach of evening, has often been ascribed to the extension of a supposed luminous atmosphere about the sun. But Laplace has shown, that such an atmosphere, far from stretching to our earth, would not even reach the orbit of Mercury. The zodiacal light must therefore have only a terrestrial origin. Supposing the uppermost regions of our atmosphere to consist of diffuse inflammable matter, we might infer from analogy, that, like the calcined sulphate of barytes and other incinerated substances, they are endowed with a phosphorescent quality, and capable of scattering a tenuous gleam on being excited by the beams of the sun. But this luminary darting perpendicular rays, will evidently affect most powerfully the range of atmosphere which occurs in his diurnal track. The expansion of the gaseous investure of our globe above the equator will hence, from its vast elevation, be descried in places even beyond the tropics, glowing with a gentle flame. The luminous cone which converges from the sun to the distance of perhaps 30 or 40 degrees in the circle of the equator, gradually contracts and grows fainter in proportion as that body sinks farther below the horizon.

It thus appears, that the opinion entertained by the ancient philosophers concerning the existence of a vast shining aetherial expanse beyond our atmosphere, is, with some modifications, consonant to the principles of sound philosophy. This is not the first occasion in which we have to admire, through the veil of poetical imagery, the sagacity and penetration of those early sages. It would be weakness to expect nice conclusions in the infancy of science; but it is arrogant presumption to regard all the efforts of unaided genius with disdain. Seldom has a discovery been made without some distant ray of anticipation.

Having ventured to state that the highest region of the atmosphere is probably occupied by some very diffuse phosphorescent gas, we shall hazard a conjecture which will appear bolder, and even paradoxical,—that perhaps air, in its most concentrated state, occupies the bottom of the ocean, and forms a vast bed, over which the incumbent waters roll. Air has actually been condensed above a hundred times, and during this process it betrayed no deviation from the fundamental law, that its elasticity is directly proportional to its density. There seems no reason, therefore, to doubt, that if an adequate compressive force could be exerted, air might be reduced to the thousandth part of its ordinary volume. But this elastic fluid would then be denser than water, and, consequently, instead of rising, would fall through the liquid. Suppose, for instance, a bladder filled with air, and having a small bullet attached to it, were thrown into the sea; in continuing to sink, it would reach a depth where the enormous weight of the column of water would compress it to the same density with the surrounding mass; and if the bullet were now disengaged, the bladder would remain suspended in that stratum, or if carried a little lower, it would precipitate itself to the bottom.

To form some estimate of this singular event, a simple hypothesis calculation will be required. Air of the ordinary temperature is 840 times lighter than distilled water, and is therefore 865 times lighter than sea-water, assuming the density of this to be 1·03. But the mean pressure of the atmosphere being equal to that of a column of thirty-four feet of distilled water, is hence equal to the weight of a column of 323 feet of sea-water. Wherefore 323 X 864, or 28,296 feet, is the depth of the ocean where the necessary compression would obtain. But a small correction must be applied, on account of the augmented density of the sea-water itself under the load of such a column. The logarithm of this density is found very nearly by multiplying the height of the column by six; and striking off seven decimal places; whence the modulus of the compression of water may be reckoned 723,824 feet. Of this large number, the former depth is about the twenty-fifth part; consequently, to procure an equilibrium between the condensed air and the corresponding stratum of sea-water, it is requisite that the air should be contracted one twenty-fifth part more, or reduced to 901 times less than its ordinary volume. But $32 \times 900 = 29,475$ feet, from which deduct the fifth part for the mean condensation of the column, and there remains 28,885 feet, the correct depth.

This computation is to be considered as only a near approximation, yet sufficiently accurate for the object in view. Nor shall we fatigue our readers by the investigation of a strict formula, including exponentials. It is enough to mark the conclusion, that any portion of air carried five miles and a half below the surface of the sea will never ascend again. Now, this limit is only half the depth which the theory of tides assigns to the waters of the ocean. There is more difficulty in conceiving by what process air can be conveyed to its abyss. Increase of pressure, however, enables water to hold a larger share of air; and the effect is hence the same as an augmented attraction. The minute globules of air may therefore be gradually drawn downwards from stratum to stratum, till they are at last detached from the body of water by their own superior density. The precipitation and accumulation of concentrated air under the ocean would thus be the results of some unceasing operation. Such a process may perhaps constitute a part of the great economy of nature. It seems probable, that the existence of a subaqueous bed of air is necessary to feed the numerous fires which continually rage in the bowels of the earth, and occasionally burst forth on the surface in volcanic spiracles.

The variable disposition of the atmosphere is the main cause of all the meteorological phenomena. It is of importance, therefore, to examine the theory and application of the different instruments which can be employed to explore the state of that medium. Some of these have been long in common use; but others, of a more delicate and refined construction, are only beginning to be known, and promise, when generally adopted, to expand our views, by opening a store of new and ulterior prospects. The ordinary observations are confined to the weight and temperature of the air. There are other data still wanted to determine at any time the actual condition of that medium. The dryness or humidity of the atmosphere, its brightness or degree of illumination, the different depth of the cerulean hue of the sky, and the variable disposition to chill the surface of the earth by impressions of cold transmitted from the higher regions; these objects of inquiry should be confounded with others of a more practical tendency, depending immediately on the mutable state of the weather. Such are the attempts to measure the daily evaporation from the ground, to register the quantity of rain which falls, and to mark the direction and indicate the force or velocity of wind. A complete apparatus of meteorological instruments will, therefore, include primarily the Barometer, Thermometer, Hygrometer, Photometer, Athroscope, Gyrometer; and comprehend likewise the Atmosphere, Rain-Gage, Drosoimeter, and Anemometer. We shall review this series in the order of enumeration.

Barometer.—The capital discovery of the weight and pressure of the atmosphere, achieved by Torricelli in 1648, as the first step in the progress of meteorology to the rank of a science. Prior to that memorable period it rested on principles altogether loose and conjectural. But the construction of the barometer, as an accurate instrument of observation, soon disclosed what was passing in the more elevated and distant regions of the atmosphere. The column of mercury, it was perceived, seldom remained long stationary, but rose and fell through a sensible space, according to the different state of the heavens. In fine calm weather it generally stood high, but commonly subsided a short while before rain or wind. The barometer came, therefore, to be regarded as a weather-glass, announcing the proximate changes of the sky, and owed its general reception to this belief. Its indications, however, were found by farther experience to be liable to much uncertainty. Though the extreme depression of the mercurial column appears invariably to foretell the ravage of a hurricane, yet its partial variations often pass away without being followed either by rain or wind. The barometer evidently can mark only the pressure of the atmosphere, and intimate, by consequence, the state of the weather, in as far as it depends on that cause.

Philosophers have eagerly sought to explain the fluctuations of the mercurial column. They have tried every principle that might appear to exert any influence in modifying the local weight of the atmosphere; but their very numerous attempts, it must be confessed, have hitherto proved singularly unsuccessful. It was requisite to show that such causes would not only give results of the kind expected, but were, besides, fully adequate to the production of the phenomena. In most instances, however, either none of those effects could have followed, or they would occur in a very inferior degree and disproportionate extent.

All the proposed explications of the changes of the barometer may, perhaps, be referred to three distinct sources of its heat. 1. The action of heat on the atmosphere; 2. the influence of moisture on that fluid; and, 3. the impression made by its rapid motion in wind. Now, the heating or cooling of the air above any given place could in no way affect its pressure. The only change occasioned would be in the density of the fluid; and, by the influx of the portion below, or the efflux of a similar portion above, the pressure of the atmospheric column would soon become adjusted to an equilibrium with the surrounding mass. The diurnal variations of the barometer within the tropics, seldom exceeding the twentieth part of an inch, as they follow a different course from the progress of heat, are obviously not derived immediately from this cause. According to the accurate observations of Humboldt in South America, at all seasons the barometer falls from nine o'clock in the morning till four in the afternoon; it then rises till eleven at night; and from this time it again descends till half-past four in the morning, and next mounts till nine o'clock.

The transition of the air, from a state of dryness to humidity, seemed to furnish the most plausible explication of the changes of the mercurial column. But the indication of the barometer was, in this case, distinctly at variance with the ordinary feelings of men. Those who suffer under a delicate or enfeebled constitution are accustomed in damp weather to complain of the air as heavier and less elastic. This language is, no doubt, metaphorical only, and descriptive of the disordered state of the nervous system; but it shows the utter fallacy of trusting in philosophical matters to the loose results of vulgar observation and experience. Moisture, so far from loading the air by its weight, communicates, like heat, increased expansion and elasticity. But even supposing a column of air to become suddenly charged with humidity, before its subsequent dilatation had, by diffusing it, produced an equilibrium, still the additional pressure would have been extremely small, not exceeding, at a moderate computation, the fifteenth part of a mercurial inch. Any transition of that medium from dryness to humidity would be quite inadequate, therefore, to the production of the effects actually observed.

Some philosophers imagine that moisture, in separating from the air, ceases to press that fluid by its gravitation, and would hence explain the fall of the barometer on the formation of clouds and the precipitation of rain. But when the aqueous particles are disunited in their solvent, whether dispersed in minute globules or collected in large drops, they must evidently descend till they acquire the celerity sufficient to maintain a resistance in the medium equal to their weight. The pressure of the whole atmospheric mass upon the surface of the earth must therefore continue exactly the same under all the changes of the constitution of the medium.

The combined action of winds seems at first to promise the most satisfactory explication of the variations of the barometer. It is evident that opposite currents rushing to the same quarter will occasion an accumulation of the air; and, on the other hand, different streams flowing from any point must reduce the vertical column. But such conclusions are quite vague, without being subjected to the ordeal of calculation. Now it is easy to compute that a concourse of winds, blowing at the rate of fifty miles an hour, and hence approaching to the violence of a hurricane, would be required to raise the barometer only the tenth part of an inch. The utmost power of a tempest could not, therefore, affect the mercurial column the twentieth part of what is frequently remarked in such circumstances. But trifling as this influence appears, it would be still at variance with actual observation. So far from rising in strong winds, the barometer almost invariably sinks; and instead of continuing depressed in the place where those currents originate, and where a calm must prevail, it generally stands high.

To explain the descent of the barometer during wind, a very ingenious idea has been proposed, which, being apparently confirmed by experiment, has obtained general reception. It is conceived, that a current of air, in sweeping over the surface of the earth, must cease to exert any vertical pressure. But this assumption can hardly be reconciled with any strict principle in science, for the particles of air will not for a moment cease to gravitate, nor will any horizontal motion of them produce the slightest derangement in a perpendicular direction. A remarkable experiment, however, was made by the ingenious Mr Hauksbee about the beginning of the eighteenth century, "showing," as he says, "the cause of the descent of the mercury in the barometer in a storm." Having connected the square box cisterns of two barometers by a horizontal brass pipe of three feet in length, he inserted in the side of one of the boxes another pipe opening outwards, and connected the opposite side with a pipe attached to the neck of a large globular receiver, into which three or four charges of atmosphere had been compressed. On turning the stopcock, the imprisoned air rushed with vehemence over the surface of the mercury in the cistern, and effected its escape, while both columns fell simultaneously about two inches, and gradually rose again as the force of the blast diminished. From this experiment Mr Hauksbee formally derives four distinct corollaries: 1. "That we have hence a clear and natural account of the descent and vibrations of the mercury in violent storms and hurricanes;" 2. "That not only the different forces, but also the different directions, of the winds, are capable of producing a difference of the subsidence of the mercury;" 3. "That strong winds may affect the animal economy upon this very account, of their altering the pressure of the atmosphere;" and 4. "That the weight of the atmosphere being diminished in one place, it is also as much diminished at the same time in another place which holds a communication with the former."

This experiment has a specious appearance, and might even seem to warrant the conclusion drawn from it. But a closer examination dispels the illusion. Since the air had been condensed four times, it must issue from the receiver with the immense velocity of 2700 feet in a second, or the double of that with which air of the ordinary density would rush into a vacuum. This is a rapidity, however, twenty times greater than the most tremendous hurricane. The very small change of the four hundredth part of an atmosphere would hence have been sufficient to produce the strongest wind ever observed, and therefore its influence in passing over the mercurial surface must have been quite insignificant.

But the experiment itself was absolutely fallacious. The peculiar result proceeded from a mere casual circumstance, the exit-pipe from the mercurial cistern being wider than the pipe which introduced the current of air. This incidental arrangement is not mentioned in the description of the apparatus, and we have, therefore, caused the original figure to be engraved on a reduced scale, not only to prove the fact which we have just stated, but also to warn experimenters of the necessity of noting scrupulously every necessary circumstance blended in their operations. (See Fig. 2, Plate CCCLIV.) It is easy to perceive that the tube G, which discharges the air from the box F, is much wider than the tube E, which conveyed it from the receiver A. This air, previously condensed, and still restrained in its passage through E, on entering the cavity of the box, immediately expands beyond the limit of equilibrium, and finding an easy escape through G, allows that state of dilatation over the mercury during the time of the horizontal flow. But the air contained in the other cistern K must, from its communication by the slender pipe L, suffer a like expansion, and, consequently, the columns L and H will, in the same time, subside equally.

Such is unquestionably the true explication of this curious fact. Were any confirmation needed, it could easily be derived from a very simple experiment. Let A (fig. 3, Plate CCCLIV.) be any cylinder of tin, suppose three inches long and two inches in diameter, having an open pipe inserted at B, a quarter of an inch wide, and perhaps two inches long, and another opposite pipe inserted at C, about three eighths or half an inch wide, and one inch long; at right angles to these a recurved glass tube or syphon G H E, of the tenth of an inch bore, is cemented below, descending twelve inches, and rising again six inches to the open swell at E, which contains coloured water terminating at F. Holding the cylinder in a horizontal position, and applying the mouth at B, let a sudden blast be injected into the cavity; the water will rise instantly to G, thus showing the diminished pressure, and, consequently, the rarefaction of the air above it. But if a cap D, with a narrow projection of perhaps only the eighth part of an inch, be adapted to the exit-pipe C, on repeating the experiment, an opposite effect will take place, and the column of water, so far from mounting to G, will now sink to H. It is evidently the difficulty of the escape through D that occasions the accumulation of the air within the cylinder, and the consequent depression of the water in the syphon. These different results are perfectly analogous to the local fall or rise of the surface of a river occasioned by the widening or contracting of its channel.

After the complete failure of so many theories for explaining the variations of the barometer, which it would be tiresome to enumerate, we may be charged with presumption in attempting the solution of a problem that has racked and exhausted so much ingenuity. Yet in all these various efforts a principle seems to have been overlooked, of extensive influence, accordant with the general phenomena, and sufficient, we think, on a close investigation, to produce the measure of effect which is required. It is obvious that a horizontal current of air must, from the globular form of the earth, continually deflect from its rectilineal course. But such a deflection, being precisely of the same nature as a centrifugal force, must hence diminish the weight or pressure of the fluid. The only question is to determine the amount of that disturbing influence. Though it should appear quite inconsiderable in the interval of a short space, it may yet accumulate to a very notable quantity through the wide extent over which the same wind is known to travel. Suppose a current to begin to flow from A (fig. 9, Plate CCCLV.) in the direction of a tangent, it will successively bend from a rectilineal track at the points B, C, D, E, F, G, &c. on the surface of the earth. The particles of the fluid are, therefore, drawn incessantly from their course by the action of gravity. Their vertical pressure is consequently diminished by the force spent in producing this deflection. Wherefore, during the prevalence of the wind, the atmospheric column will press with inferior weight at B than at A, at C than at B, at D than at C; thus gradually decreasing through the whole chain. Suppose the intervals AB, BC, CD, DE, &c. to be each of them a mile, and that the current reaches the points B, C, D, E, &c. in successive minutes, a celerity which frequently happens; the deflection at B, owing to the curvature of the earth, would be eight inches, or two thirds of a foot; but the space through which a body would descend in a minute, by the action of gravity, is $60 \times 60 \times 16 = 57,600$ feet, or 86,400 times greater than the deviation from the tangent. Wherefore the atmospheric pressure would, on that hypothesis, be diminished by the 86,400th part for each interval of a mile from A to G. In the space of 288 miles this diminution would consequently be the 300th part of the incumbent weight; and over an extent of 2880 miles it would amount to the 30th part.

If we assume the very probable estimate, that storms involve the whole region of the clouds, or attain an elevation of near three miles, the diminution of pressure, occasioned by a long series of deflections in the stream, would affect one half of the atmosphere. Wherefore, a wind which has blown over a track of 2880 miles, at the rate of 60 miles an hour, might cause the mercurial column to subside half an inch. If the velocity of the wind were doubled, which is probably the limit of the most tremendous hurricane, the fall of the barometer would be four times greater, and amount to two inches.

That the same powerful wind can sweep over an immense track of surface is well ascertained. The effects of a hurricane originating in the West Indies, at the distance of 5000 miles, have often been felt on our shores. But a wind arising at A must evidently be followed, at succeeding intervals, by a current from K, L, M, &c. the range of influence being thus extended over a larger track. During the flow of the air, the depression of the barometer at G will be maintained, or rather augmented.

These conclusions are perfectly accordant with the facts observed. It appears, from comparing the most accurate registers, that the pressure of the atmosphere is subject to very nearly the same variations through a vast extent. A barometer may be considered as in unison with another placed at the distance of perhaps 200 or 300 miles, and the mercurial columns in both of them rise and fall by an almost simultaneous movement. But, in stormy weather, the separated barometers, however they may approximate in their indications, cannot absolutely correspond. The gradation of atmospheric pressure from G to F, from F to E, &c. may be very slow; and yet the minute differences may accumulate to a very sensible amount in the long range from G to A, or to M. Thus, though a barometer may stand only the twentieth part of an inch lower than another separated 200 miles, it would be half an inch below a barometer at the distance of 400 miles.

The theory now stated seems to furnish the most satisfactory explanation of a great variety of phenomena. Thus the minute diurnal alternations to which the barometer is subject within the tropics, are caused by the influence of the land and sea breezes. During the heat of the day those gentle airs blow from the ocean, and, therefore, near the shore, and in the interior of the islands, the mercurial column subsides. But after the vigour of the sun's beams has declined, they flow in an opposite direction, and consequently the mercury rises again. The fluctuations are only partial, however, because such breezes hold a very short sway, and do not perhaps extend beyond 100 or 200 miles.

The common remark, that a north-east wind, so far from depressing the mercurial column, generally causes it to rise, might appear at variance with the principles which we have advanced. But that wind which really comes from the north has probably a short course, and may be dependent on a more extensive current which maintains a flow in the higher regions towards the poles. The existence of opposite streams, though incompatible with the supposition of a wide-spreading hurricane, may yet be admitted in local and partial derangements of the atmosphere.

Hitherto we have contemplated the winds as describing only arcs of great circles about the earth; but they may be constrained to bend in smaller circles, and perhaps trace the parallels of latitude. Their flexure from a rectilineal course being in this case augmented, they would in the same extent exert an influence proportionally greater on the barometer. Such, near the arctic regions, may be the effect of a westerly wind of no very distant origin.

The same principles will explain the phenomena of eddies, whirlwinds, and tornadoes. Suppose (fig. 4, Plate of CCCLV.) a horizontal stream of air, rushing from A to whirlwind B, meets a contrary current blowing from C to D, and winds that some obstacle E occurs in their line of separation. The flow will evidently diverge on both sides of E (see figs. 5 and 6, Plate CCCLV.) till it swells by degrees into a vertiginous revolution. Such is the origin of the famous maelstrom, or whirlpool, on the coast of Norway, occasioned by the meeting of opposite tides. The aerial vortices are evidently produced by a similar cause. If the opposite streams have equal force, the circulation will be maintained in the same spot; but if the one current flows with greater rapidity than the other, it will transport the vertiginous motion by the excess of its celerity. A motion of progression and revolution is thus upheld at the same time. Such appearances are frequently witnessed in summer, especially in the hotter climates. A whirlwind arises on a sudden, and runs over the surface of the ground, drawing into its vortex all the light substances which occur in the track. Immense havoc is often thus produced in the fields of rice and plantations of sugar-cane. It is easy to show, that if r denote in feet the radius of the extreme circle described by the whirlwind, and t the time of circumvolution in seconds, the elasticity, or pressure of the column at the verge will suffer a diminution corresponding to the fraction $\frac{5r}{4t}$. The amount of this diminution over the whole base would be reduced to three fourths; and consequently h, expressing the height of the revolving column of air, $\frac{15rh}{16t}$ would represent the mean effect of centrifugal action. Suppose the whirlwind to have an elevation of 200 feet and a radius of fifty, and to circulate in three seconds, the diminished pressure would be equal to the weight of a column of $\frac{15 \cdot 50 \cdot 200}{16 \cdot 9}$, or 1040 feet. This example, assuming a celerity of sixty-five miles an hour, might be reckoned an extreme case, but it would occasion the mercury to sink in the barometer more than an inch, or 1:12.

The formula for the depression of the barometer caused by a rapid whirl or tornado in the lower atmosphere, may be changed into an expression that shall embrace the velocity instead of the time of revolution. This velocity in miles each hour being denoted by \( v \), the diminished pressure of the vertiginous column will be

\[ \frac{360}{50} = \frac{360 \times 9}{50} \]

It thus appears that the effect is directly as the square of the celerity, and inversely as the radius of circumvolution. And since the rapidity of whirl can never exceed certain limits, the action of a tornado must diminish in proportion to the extent of surface which it occupies. Suppose the height of the cylinder of air to be 300 feet, the radius of the sweep 500 feet, and the celerity of its extreme circulation eighty miles an hour: Then

\[ \frac{36400 \times 300}{50 \times 500} = 230 \text{ feet, which corresponds to a descent of } 1:247, \]

or about a quarter of an inch in the mercurial column.

II. The Thermometer.—The invention of this instrument has not only dispelled the illusions which formerly prevailed on the subject of heat, but has mightily contributed to extend our acquaintance with the actual condition of the atmosphere. A lump of iron brought into the house, from its exposure during a frosty night, feels intensely cold, yet becomes gradually warmer; and if it be put into the fire, it will soon grow extremely hot, till it acquires the faculty of burning. On removing it from the fire, and laying it again out of doors, it will, through all the steps of a contrary progress, relapse imperceptibly into its former state. This obvious fact, and many others of a similar kind, might have been sufficient to show that hot and cold are nearly relative conditions, which every substance is capable of assuming. The schools upheld very different notions, which, after a long series of descents, have at last found shelter among the vulgar. It was maintained that, of the four elements, air and fire are hot, and water and earth essentially cold; and that the compound bodies, from their constitution, partake of those qualities in different degrees. While the senses were the sole arbiters of heat and cold, substances became classed according to the sensations which they excited. If a body, such as lead, rapidly abstracted heat from the touch, it was reckoned cold. But the same quality was likewise bestowed on other substances of a very different kind, for instance, on vinegar and hemlock, because they affect the stomach with a sort of chilling sensation. Soft wood and feathers might be deemed warm, since they draw off heat feebly from the touch; but pepper was also ranged in the same class, because it stimulates the organs of taste. Such a confusion of ideas disfigured physical science, and perverted the practice of medicine.

The construction of the air-thermometer, by the famous Sanctorio of Padua, in 1590, was the commencement of a salutary revolution. As it was at first intended for exploring disease, it has ultimately rendered signal service to the medical practitioners. About twenty years afterwards, those instruments were manufactured by Drebbel, who carried them from Holland into England. They were very rude, however, and adapted to no constant scale, but regarded merely as weather-glasses. The discovery of the barometer opened new views, and showed that the former instruments marked the conjoint effects of heat and pressure. To distinguish the separate influence of heat, it became hence requisite to employ some different fluid from air. Alcohol was preferred, as being very expansible; and, from the year 1655, thermometers consisting of rather wider tubes, terminating in balls or large bulbs filled with that liquid, were manufactured by Italian artists, who imitated an arbitrary standard adopted by the Accademia del Cimento. Such thermometers were clumsy, and susceptible of only a low range. Römer, who made the fine discovery of the progressive motion of light, proposed mercury, as a fluid sufficiently expansible, and capable of bearing an intense degree of heat. The origin of the scale was then fixed at the melting point of ice or snow, and the scale itself divided into degrees corresponding to the ten thousandth part of the capacity of the bulb. Yet this mode of construction seemed tedious, and liable to some inaccuracy. The capital improvement was made in 1724, by Fahrenheit, who took another standard point from the boiling of water under the mean pressure of the atmosphere. For many years that ingenious artist manufactured thermometers in Amsterdam on correct principles, very neat and small, adapted especially to medical purposes. The multitudes of young physicians who at that period studied in Holland quickly dispersed them to every part of the globe. The observations thus obtained gave juster ideas of the comparative temperatures of different climates, and in many cases reduced the exaggerations of travellers to moderate bounds. It thence appeared that the heat of the torrid zone was not so excessive, nor the cold of the arctic regions so intense, as had been commonly represented. The tropical plants could, therefore, enjoy in our hot-houses all the warmth of their native climate. The thermometer was first applied to direct the operations of horticulture, and afterwards extended to regulate the process of brewing and other arts more immediately depending on practical chemistry.

Quicksilver was deemed preferable to alcohol in the filling of the thermometers, not only because of the wide range which it embraces, but on account of the remarkable property which it was afterwards found to possess, of expanding equably with equal accessions of heat. But it failed in the lower part of the scale. The reported sinking of the mercurial thermometer in Siberia appeared to indicate an intensity of cold beyond all conception. But the discovery made by Professor Braun, in 1759, of the actual congelation of mercury itself, reduced the extent of refrigeration to moderate limits. This fluid metal suffers a large contraction in passing into the state of solidity; and, therefore, though it freezes about thirty-nine degrees below the zero of Fahrenheit, it yet shrinks through a space of more than a hundred degrees before it becomes fixed. As alcohol has never been congealed, though brought, in some experiments we could mention, to 150° below zero, thermometers filled with it are now employed to explore, if not to measure, intense cold.

Metallic thermometers are likewise well adapted for examining the state of the atmosphere. They are commonly constructed on the principle of the compensation balance of a chronometer, a spiral or circular spring being composed of two soldered opposite plates of distinct metals, for instance of brass and steel, or of zinc and platinum, which expand very differently under the action of heat, and therefore continually change their incurvation. Some instruments of this kind, made by Breguet at Paris, are remarkably elegant; and those more lately manufactured by that ingenious artist, with a very slender form, surpass all other thermometers in the exquisite sensibility of their indications. Fig. 9, Plate CCCLIV., represents the first form, reduced to about the fourth part of its natural size. The main piece of mechanism is a circular spring, fixed at one end, and composed of steel and brass soldered together. The other end carries a clip, that acts on a short train of wheels, which turn an index on the dial-plate, the extent of the scale, including the range of atmospheric tempera- The dial with its index are shown by A; and B exhibits generally the interior mechanism. Fig. 4, Plate CCCLIV, delineates the latest improvement. To prevent the cracking or dislocation which a large motion of the spring sometimes occasions at the junction of the two metals, a thin plate of a third metal is interposed. Three plates, consisting of platinum, gold, and silver, are united by the rolling press into a single ribbon, of the thickness of only the 1200th part of an inch. The instrument is formed with about twenty-seven spires or circumvolutions of this very slender spring, bearing an index which travels in its circle over fifty centesimal degrees, or ninety degrees by Fahrenheit's scale. The metallic film is, from its extreme tenuity, almost instantly penetrated by the impressions of heat or cold, and the sensibility of this thermometer accordingly surpasses all conception.

It is of much importance, in keeping meteorological journals, to have thermometers that shall indicate the extreme changes which occur during the absence of the observer, such as the greatest heat of the day, and the lowest cold of the night. For this object the metallic thermometers are easily adapted, since their index may push forward or draw back any moveable mark, and thus indicate the limits of its variation. Large mercurial thermometers, also, if mounted like the wheel barometers, to turn an index, will answer a similar purpose. But smaller instruments, though of a more complex construction, have been generally preferred. A sufficient account of the self-registering thermometer proposed by Lord Charles Cavendish in 1757, and of the more complete instrument described by Mr Six in 1782, may be found in any work which treats of the subject. The latter has now come into pretty general use, though we are sorry to remark, that it seems to have fallen into the hands of very inferior artists, the scale consisting merely of boxwood, rudely and inaccurately subdivided.

Both these instruments have been employed to ascertain the coldness of the ocean at great depths. It becomes requisite, however, to make some allowance for the contraction which the glass bulbs must suffer under the enormous compression of the superincumbent columns of water. This can easily be computed, from the effect of perhaps ten atmospheres in a condensing engine; and such corrections were actually applied to the observations made in Captain Phipps' voyage in 1773 towards the north pole. It is a matter of equal surprise and regret, that all such niceties were overlooked in the late expeditions of Captain Ross and Captain Parry to the arctic regions.

The simplest, and by far the best, self-registering thermometers, are those invented and constructed by the late Dr John Rutherford, of Middle Balilish, and first described in the Transactions of the Royal Society of Edinburgh for the year 1794. The one, which marks the minimum, is filled with alcohol, and the other, which indicates the maximum, is filled with quicksilver; and they are both attached to the same frame, or, what is still better, affixed to separate frames, placed nearly horizontal, or rather elevated about five degrees, to prevent the separation of the thread of liquid. The tubes have bores from the twenty-fifth to the fifteenth part of an inch wide, and include a minute tapered or conical piece of ivory, or of white or blue enamel, about half an inch long. This mark having in either thermometer its base turned towards the bulb, is drawn to the lowest point by the alcohol, which again freely passes it; but it is always pushed forward to the highest limit by the mercury, which afterwards leaves it. These marks, however, are now made cylindrical, a little thickened at the ends, and about three eighths of an inch in length. (See figs. 7 and 8, Plate CCCLIV.) Fig. 7 exhibits the alcohol or minimum thermometer; and fig. 8, the one which indicates the maximum, and is filled with quicksilver.

This instrument was first employed by Dr Rutherford, in some interesting experiments, to ascertain the temperature of germination, and determine the depth to which frost penetrates into the ground in different soils and situations, at the suggestion of his neighbour Dr Coventry, the very able and intelligent professor of agriculture in the University of Edinburgh, both of them proprietors in the small county of Kinross. The few instruments of this kind, for some time circulated, were made by the hands of the inventor; but artists have since learned to imitate and improve the original construction, and nothing is wanted now but to promote their diffusion. We regret that this very useful register-thermometer seems to be not yet so well known in London as it deserves.

The minutest changes which take place in the constitution of bodies are, for the most part, attended by corresponding alterations of temperature. To explore those abstruser operations of nature, which often betray the influence of more extensive principles, it was now requisite to improve the delicacy of the thermometer. To this object the writer of the present article had early turned his attention. At first he enlarged the capacity of the bulb, and thus procured degrees of such a size on the stem as to be capable of a very distinct subdivision. These instruments, however, received their impressions very slowly; and therefore tubes of extremely fine bores being selected, he had small bulbs blown, and filled with quicksilver, not in the ordinary way, but by the aid of a compressing force. With such exquisite thermometers, it was easy to procure much nicer observations, and to detect even the finer modifications of corpuscular action. But, to include the usual range of temperature, it was necessary to draw the stem to an immoderate length. The attempt to remedy this inconvenience led to the construction of the first kind of differential thermometer. The main object was evidently attained, if the mercury should always be made to start from some given point. The tube was therefore left open, and a cap adapted to the top, containing a surplus portion of the fluid. When the thermometer was kept inverted, this mercury closed round the orifice, and joined the thread in the stem, as in fig. 17, Plate CCCLIV. But when the instrument was reversed, the excluded fluid instantly separated, and sunk into the cavity under the top of the tube, as in fig. 16. In this situation, the descent of the mercurial thread in the stem marked the depression of temperature.

A differential thermometer of this construction was used for three or four years, in a variety of experiments, without ever failing. But an open thermometer must be liable to some uncertainty, and unavoidably subject to a continual deterioration. The loose mercury will undergo a slow oxidation, while the bore of the tube is apt to be soiled and tarnished by the insensible introduction of moisture. Such an instrument, however, is well adapted for many researches.

In the beginning of the year 1795, another form of the Its simple differential thermometer was devised, which, from its simplicity, its delicacy, and extensive application, has contributed essentially to the progress of physical and chemical science. This instrument is now so generally known, that a few remarks on its general construction will be here judged sufficient. The differential thermometer is a modification of the air-thermometer, but susceptible of the impressions of heat only, and exempt altogether from the influence of the variations of atmospheric pressure. The tube to which the scale is applied has a bore of equal calibre, from the fiftieth to the fifteenth part of an inch wide, the other branch being commonly wider; the terminating balls are not less than four tenths of an inch in diameter, and seldom exceeding an inch and a half. A glass tube terminated by a ball containing air, is joined hermetically, or by the flame of a lamp, to another longer tube, terminated by a similar ball containing air also, but including a small portion of some coloured liquid. The tubes are then bent, generally into a recurved or double stem, like the letter U, and the liquid is adjusted to the proper height by making bubbles of air pass from the one ball to the other, from the little enlargement of the bore left at the junction of the tubes. If both balls have the same temperature, the liquid must evidently remain stationary; but if the ball of the shorter tube be warmed, the air, expanding and exerting more elasticity, will depress the liquid in the stem; or if this ball be cooled, the air, by its contraction, allows the liquid to ascend, from the superior elasticity of the air contained in the opposite ball. The fall or rise of the liquid will, therefore, mark the excess of heat or cold in the adjacent ball, and the space through which it moves will measure the precise difference of temperature.

Alcohol and other volatile fluids were avoided in filling the tube, lest their vapour should affect unequally the elasticity of the air contained in the balls, and thus disturb the accuracy of the indication. Linseed oil tinged with alkanet root was first used; but it was found to be sluggish in its movements, leaving along the inside of the tube a sort of trail, which sometimes collected into globules. But the great objection to this and other fixed oils was, that they did not remain at the same point of the scale, but slowly shifted their place, owing apparently, to a partial absorption of air in one of the balls, while their orange tint was found to fade away by exposure to the light. Deliquinate potash, coloured with archil, was next employed, and with tolerable success. By degrees, however, it deposited the colouring matter, and became almost limpid. Hydrogen gas, instead of common air, was then adopted for filling the balls. This mode of construction prevented the deposition of the colouring matter; but it was experienced to be troublesome, and attended with other impediments. After numerous discouraging trials, it was at last discovered that strong sulphuric acid tinged with carmine fulfils every condition, remaining permanently stationary in contact with confined air, and never losing in the slightest degree its colour from the action of the strongest light. Since the year 1801, this liquid has been constantly used in the construction of the differential thermometer. When the exhibition of striking effects rather than scrupulous accuracy is sought for, tinged alcohol, indeed, has in a very few cases been preferred, on account of the great facility and amplitude of its motions. But, in every instance, a column of liquid, terminating in the cavity of one of the balls, or in a small cylindrical reservoir under it, was preferred; and not, as proposed by some experimenters, a single drop of the liquid, which forces its way through the bore by successive starts, and is therefore liable to much uncertainty and derangement.

The differential thermometer is capable of some diversity of form. It may consist either of a single branch, pendant or horizontal, or it may be bent into two perpendicular branches, whether contiguous or placed at a short distance. Since the motion of the column in the tube is occasioned by the difference of the elasticity of the air contained in the two balls, it is not essential that those balls should be of equal dimensions; for an equipoise must obtain whenever the augmented elasticity of the air of the hotter ball is balanced by its expansion on the one hand, and the corresponding contraction in the opposite ball, joined to the pressure of the ascending column of the sulphuric acid. In general the balls are blown to a certain degree unequal, either to suit the particular instrument, or to please the eye; but in making observations, it must be kept invariably in the position for which it was designed, whether vertical or horizontal, since the pressure of the balancing column would be effected by an obliquity.

The differential thermometer, in its pendant form, and extended to a suitable length, from one foot, perhaps, to three or four feet, may be employed with great advantage in comparing the different temperatures of adjacent strata of air near the surface of the earth during the progress and decline of the day; and to detect the variations in a cloudy or a clear sky, and those occasioned by winds, as modified by the quality of the ground, whether naked or clothed with vegetation. But the main use of this instrument in meteorological researches arises from the various modifications of which it is susceptible. The minute changes of temperature which it marks discover the existence and intensity of other disturbing causes. It is thus found that fresh ploughed ground is more affected by the solar rays

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1 It is not difficult to compute, in general, the size of the scale of the differential thermometer. Let the diameters of the two balls be expressed in inches by \(a\) and \(b\), the diameter of the bore of the tube being denoted by \(d\), and the measure of a centesimal degree by \(x\). The capacities of those balls, \(w\) representing the ratio of the circumference to the diameter of a circle, would hence be \(\frac{wa^3}{6}\) and \(\frac{wb^3}{6}\); and the portion of liquid raised through the space \(x\) would be \(\frac{wa^3x}{4}\). Consequently the opposite elasticities exerted are \(\frac{3a^2x}{2a^3}\) and \(\frac{3b^2x}{2b^3}\); or, expressed by the altitudes of a column of sulphuric acid, of which 220 inches may be reckoned equivalent to the weight of the atmosphere, they become \(\frac{660a^3x}{2a^3} + \frac{660b^3x}{2b^3}\), or \(\frac{330a^3x}{a^3} + \frac{330b^3x}{b^3}\); wherefore, since air expands one 250th part for each centesimal degree, \(\frac{330a^3x + 330b^3x}{ab^3} + x = \frac{220}{250}\), and, by reduction, \(x = \frac{22a^3b^3}{8250(a^3 + b^3) + 25a^3b^3}\). This corresponds to ten of the millesimal degrees adopted for the differential thermometer, but the length of an hundred of those degrees, which may be preferred as a larger basis for the scale, will be \(\frac{44a^3b^3}{1650(a^3 + b^3) + 6a^3b^3}\). Suppose \(a = \frac{4}{5}\), \(b = \frac{4}{3}\), and \(d = \frac{1}{35}\) of an inch; then

\[ \frac{44 \times 27}{64 \times 27} = \frac{64}{27} \]

\[ \frac{1650 \left( \frac{1}{1225} \right) \left( \frac{27}{64} \right) \left( \frac{64}{27} \right)}{5 \times \frac{27}{64} \times \frac{64}{27}} = \frac{1650 \times 4825}{1225 \times 1738 + 5} = \frac{376 + 5}{5} = 5 \text{ inches}. \]

If the balls be of equal diameters, this expression for the length of an hundred degrees in inches will become simply \(\frac{44a^3}{3300a^3 + 5a^3}\). Thus, suppose \(a = \frac{4}{5}\) and \(d = \frac{1}{30}\) of an inch; then

\[ \frac{44 \times 512}{3300 + 5 \times 512} = 3.62 \text{ inches}. \]

If the density of the sulphuric acid were reduced from 1.85 to 1.62, the expression for the length of ten degrees would pass into this very simple form, \(\frac{a^3}{756a^2 + a^3}\). than a green sward, on which also the breeze has little influence.

III. The Hygrometer.—The mutable condition of the atmosphere, as it inclines to dryness or humidity, is the main source of all the variety of meteorological phenomena. The changes which it undergoes with respect to moisture have a marked influence on a very numerous class of substances, and even on the animal frame. But unfortunately those indications are always vague, and often fallacious. To ascertain the portion of humidity which a given quantity of air holds, or is capable of sustaining, is a problem of the first importance; but our advances to the constructing of an instrument fit to measure with accuracy that disposition have been extremely slow.

Most substances of a loose and spongy texture, or possessed of an absorbent quality, are affected, though in very different degrees, by the presence of humidity. Accordingly, the variations, both in weight and bulk, which absorbent bodies undergo, have been employed to indicate the disposition of the air with respect to moisture. For this reason such substances are likewise termed hygroscopic, since they are always affected by the state of the ambient medium, though they may not precisely measure its degrees of humidity or dryness. But neither heat nor moisture is passively diffused, or yet shared among different bodies in equal proportions. From some experiments made by Sir John Leslie, it appears that, under a like change of circumstances, a hundred grains of ivory will attract from the atmosphere seven grains of humidity; the same weight of boxwood, fourteen grains; of cider-down, sixteen; of wool, eighteen; and of beech, twenty-eight. Other substances, in their respective measures of absorption, exhibit still wider differences.

The dry or humid state of the air is therefore discovered from the variable weight of certain bodies exposed to its influence. Rock salt has been applied to that purpose; but potash, the muriate of lime, sulphuric acid, and most of the deliquescent substances, whether in a solid or a liquid form, act the most powerfully. Other materials of a firm or adhesive consistence manifest the same properties, though in a lower degree. Plates of slate-clay or of unglazed earthenware, the shavings of box or horn, paper or parchment, wool or down, all act as hygrometers. But these substances, especially the harder kinds of them, unless they be extremely thin, receive their impressions very slowly, and hence they cannot mark with any precision the fleeting and momentary state of the ambient medium. Nor is the weight which they gain by exposure proportioned to the real dampness of the air; for the measures of their successive absorption are found to increase in a most rapid progression as they approach to the point of absolute humidity.

But to weigh the substances with the accuracy befitting such experiments is a very delicate and troublesome operation. Those thin bodies are liable, besides, to become time covered with dust, which, while it must evidently augment their weight, cannot be detached from them without injuring their slender texture. The increase of bulk which they acquire from the portion of moisture attracted into their substance, furnishes therefore a more certain and convenient indication of the state of the atmosphere. The old vegetable and animal fibres are connected by a fine soft netting, in which the power of absorption appears briefly to reside. Hence the presence of moisture always enlarges the breadth of such substances, without affecting in any sensible degree their length. This effect is visible in the swelling of a door by external dampness, and in the shrinking of a panel from the opposite cause. But the substances, such as paper or parchment, which have a diffused or interlaced texture, are extended by the absorption of humidity almost equally in every direction. On the contrary, twisted cord and gut, being swelled by moisture, suffer a corresponding longitudinal contraction, accompanied likewise, if not confined, by some uncoiling of their fibres.

All these properties have severally been employed in the construction of hygrometers. The expansion of the thin cross sections of box or other hard wood, the elongation of the human hair or of a slice of whale-bone, and the untwisting of the wild oat, of cat-gut, of a cord or linen thread, and of a species of grass brought from India, have at different times been used with various success. But the instruments so formed are either extremely dull in their motions, or, if they acquire greater sensibility from the attenuation of their substance, they are likewise rendered the more subject to accidental injury and derangement; and all of them in time appear to lose insensibly their tone and proper action.

An attempt was made by Sir John Leslie to revive Ivory by the method of measuring the expansion of absorbent cohesive substances by their enlargement of capacity when disposed into a thin shell; and, by successive steps, he carried the hygroscope thus formed to as high a state of improvement as perhaps such an imperfect instrument will admit. A piece of fine-grained ivory, about an inch and a quarter in length, was turned into an elongated spheroid, as thin as possible, weighing only eight or ten grains, but capable of containing, at its greatest expansion, about 300 grains of mercury; and the upper end, which was adapted to the body by means of a delicate screw, had a slender tube inserted, six or eight inches long, and with a bore of nearly the fifteenth part of an inch in diameter. (See fig. 19, Plate CCCLV.) The instrument being now fitted together, its elliptical shell was dipped into distilled water, or lapped round with a wet bit of linen, and, after a considerable interval of time, filled with mercury to some convenient point near the bottom of the tube, where is fixed the beginning of the scale. The divisions themselves were ascertained by distinguishing the tube into spaces which correspond each of them to the thousandth part of the entire cavity, and equal to the measure of about three tenths of a grain of mercury. The ordinary range of the scale included about seventy of those divisions. To the upper end of the tube was adapted a small ivory cap, which allowed the penetration of air, but prevented the escape of the mercury, and thereby rendered the instrument tolerably portable.

This hygroscope was largely, though rather slowly, affected by any change in the humidity of the ambient medium. As the air became drier, it attracted a portion of moisture from the shell or bulb of ivory, which, suffering in consequence a contraction, squeezed its contained mercury so much higher in the tube. But if, on the contrary, the air inclined more to dampness, the thin bulb imbibed moisture and swelled proportionally, allowing the quicksilver to subside towards its enlarged cavity. These variations, however, were very far from corresponding with the real measures of atmospheric dryness or humidity. Near the point of extreme dampness, the alterations of the hygroscope were much augmented; but they diminished rapidly as the mercury approached the upper part of the scale. The contraction of the ivory answering to an equal rise in the dryness of the air, was found to be six times greater at the beginning of the scale than at the seventieth hygroscopic division, and seemed in general to be inversely as the number of hygrometric degrees, reckoning from twenty below. Sir John Leslie therefore placed another scale along the opposite side of the tube, the space between the zero and the seventeenth division of the hygroscope being distinguished into 100 degrees, and corresponding to the unequal portions from the number twenty to 120 on a logarithmic line. This very singular property will be more easily conceived from the inspection of the figure. The scale might probably be extended farther by continuing the logarithmic divisions. Thus, 320 degrees by the hygrometer would answer to 108 of the hygroscope, or to a contraction of 108 parts in a thousand in the capacity of the bulb. But at the dryness of 300, the contraction of the ivory seemed never to exceed 105.

Boxwood was likewise formed into a hygroscope of the same shape and dimensions; but this absorbent material swells twice as much with moisture as ivory does, and therefore requires its inserted tube to be proportionally longer or wider. Its contractions are still more unequal than those of ivory; for, near the point of extreme humidity, those alterations in the capacity of the bulb appeared to be more than twenty times greater than, under like changes in the condition of the atmosphere, take place towards the upper part of the scale. The space included between the commencement and the hundred and fortieth millesimal division of the scale might hence be marked with a hundred hygrometric degrees, corresponding to the decreasing portions of a logarithmic line from five to a hundred and five.

In noticing the rapidly declining contractions which ivory and boxwood undergo, Sir John Leslie did not mean, however, to state the quantities with rigorous precision. Much time had been consumed in attempting to trace out the law of those gradations; and such experiments are rendered the more tedious, from the protracted action of the hygroscope, which often continues travelling slowly for the space of a quarter, or even half an hour. This tardiness is indeed the great defect of all instruments of that nature, and utterly disqualifies them for every sort of delicate observation.

The very large expansions which the hygroscope shows on its approach to extreme humidity, explains in a satisfactory manner the injury which furniture and pieces of cabinet work sustain from the prevalence of dampness. On the other hand, the slight alteration which the instrument undergoes in a medium of highly dry atmosphere, seems to have led most philosophers to believe that there is an absolute term of dryness, on the distance of which from the point of extreme moisture they have generally founded the graduation of the different hygoscopes proposed by them. This opinion, however, is far from being correct, and might give occasion to most erroneous conclusions. No bounds can be set to the actual dryness of the air, or the quantity of moisture which it is capable of holding, which, by the joint application of heat and rarefaction, may be pushed to an almost indefinite extent.

The ivory hygroscope, after being for several hours immersed in air remarkably dry, was apt of a sudden to split longitudinally. But if the bulb endured such a range of contraction, it appeared in some instances to take at least another set, or to accommodate its constitution, by imperceptible changes, to the state of the surrounding medium.

But though the bulbous hygroscope is, in extreme cases, liable to much uncertainty and some risk, it may yet be used with visible advantage in certain peculiar situations. The very sluggishness of the instrument, when the value of its divisions has been once ascertained, fits it so much the better for indicating the mean results. After being long exposed in situations hardly accessible, it may be conveniently transported for inspection, before it can suffer any sensible change. The hygroscope could be therefore employed with success to discover the degree of humidity which prevails at considerable elevations in the atmosphere. It might be likewise used for ascertaining readily the precise condition of various goods and commodities. Thus, if the bulb were introduced, for the space perhaps of half an hour, into a bag of wool, a sack of corn, or a bale of paper, it would, on being withdrawn from their contact, mark the dryness or humidity of those very absorbent substances.

Other hygroscopic substances have at different times been proposed, which, though possessed of greater sensibility, are yet liable to the same general objections. Thus, quills, reeds, gold beaters' skin or pellicle, the skins of frogs, or the bladders of rats, were made to act like the bulbs of thermometers, and to cause, by their contraction or dilatation, as they inclined to dryness or humidity, the included quicksilver to rise or fall in rather a wide tube. These instruments, however, being subject to injury or derangement from the smallest accident, can scarcely be applied to any practical use.

The hygoscopes which depend on the elongation of the fibres are perhaps on the whole preferable. The slice of whalebone proposed by M. Deluc, and the human hair afterwards employed by M. de Saussure, are both of them sufficiently sensible to external impressions; but the difficulty is to determine the precise relation subsisting between those impressions and the state of the atmosphere. Humidity is not distributed in equal shares through the air and among the several absorbent substances exposed to its penetration; nor are the degrees of expansion which it communicates either uniform or proportional to its quantity. The graduation of such instruments, being thus in a great measure arbitrary, can furnish no correct data of the hygrometric state of the atmosphere. The assumption of two fixed extreme points as the basis of the scale is evidently erroneous. Air contained within a glass receiver may be rendered as damp as possible, by the porous suspension of water on the sides; but it can never be absolutely deprived of its moisture, which adheres the more powerfully in proportion as it becomes diminished. Caustic alkalis, concentrated acids, and some of the deliquescent salts, aided by the action of heat, all render the air drier, but without being able to complete the desiccation. By the combined application of other agents, and even by mechanical pressure, the driest air can always be made to deposit some farther portion of moisture.

M. de Saussure directed all the resources of his ingenuity to correct the anomalies of the instrument which he proposed, and at last succeeded, by multiplied precautions, in rendering it as perfect, perhaps, as its nature and composition will admit. The hair-hygroscope (for it is not entitled to the name of hygrometer) certainly shows mobility; but the degrees which it marks can afford no steady or tolerably correct estimate of the dryness of the atmosphere.

To arrive at an accurate measure of the dryness of the Expansive air, it is necessary to pursue a different route. Steam, in whatever way it be formed, whether by the application of heat, or the diminution of atmospheric pressure, has near-

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1 From the few observations which we have made with an instrument of this kind brought from Geneva, we are sorry to say that it falls much below our expectations. Its motions are slow and irregular; and it seems to be little affected by very considerable alterations of the state of the encircling medium. ly the double of the elasticity of common air, or it would, under the same compressing force, occupy about twice as much space. In uniting with that fluid, and forming vapour, it must hence communicate an expansion exactly proportional to the quantity dissolved, or to the share of moisture required for the complete saturation of the air. This principle suggested to Sir John Leslie the means of constructing an accurate hygrometer, to which his researches had been early directed. Inverting a barrel-tumbler, he ground the mouth perfectly flat, and having drilled a hole through the bottom, he cemented into it a syphon-gage, or slender recurved tube, with a narrow bore, and an intermediate swell or cavity, passing through a perforated cap of lead, and holding a portion of nut oil, tinged with alkanet root. (See fig. 6, Plate CCCLIV., where it is represented half the natural size.) To form the scale, he divided 4-2 inches into a hundred equal parts, so that each degree corresponded to the ten thousandth part of the air's elasticity, the whole atmospheric pressure being equal to that of a column of the oil 420 inches in height.

Having now spread a few drops of water over a surface of plate glass, and slipped the tumbler upon it, the included air quickly dissolved as much moisture as was sufficient for its saturation, and marked the expansion thence acquired, by forcing the oil to rise proportionally. The quantity of effect varied much, but was often very considerable, amounting, in fine weather, to 110 or 120 degrees. This little apparatus appeared to answer the purpose intended; but it was not portable, and it always required some address. It soon gave way, therefore, to other instruments, which promised to be more easily and readily managed.

As an hygrometer of this kind exhibits the actual expansion or increase of elasticity which the air acquires from complete humefaction, it seemed calculated for indicating the variable power of a drier or a moister atmosphere in refracting the rays of light. The barometer and thermometer had long been employed to correct the quantity of refraction; but the application of an accurate hygrometer seemed no less necessary for delicate observations. An instrument of the composition now described was accordingly intrusted, in the course of the year 1794, to the late Dr Maskelyne, and deposited in the Royal Observatory of Greenwich. Other objects, however, interfered, and an investigation, which promised important results, was never prosecuted on a regular and digested plan.

This instrument might be rendered still more accurate, by combining it with the principle of the differential thermometer. Let two similar tumblers (see fig. 5, Plate CCCLIV., where it is contracted to about the fourth part of its size), A and B, inverted and loaded, have their mouths ground to fit a glass plate, the air contained in each of them acting by its elasticity on the column of a recurved tube C, which connects their cavities. Slide both of these tumblers in a dry state upon their bases, dip a hair-pencil in oil, and pass it round the outside of the mouth of A, to make it quite tight; then having removed B, and spread a few drops of water over its place, slip it on the plate again. The air included in B, now absorbing moisture, will continue to expand and to press upon the liquid column, till it has become absolutely saturated. Should any change take place in the temperature of the room during this process, it can have no effect in deranging the indication of the instrument, since it must influence precisely in the same degree the elasticity of the air contained in both balls, and thus produce an exact counterbalance.

This compound instrument is adapted to various delicate physical inquiries. In the union of different substances, a certain alteration of volume, however minute, almost invariably takes place. To ascertain such changes under various circumstances greatly extends our views of the empire of chemical affinity. For instance, a small bit of fresh charcoal introduced under one of the tumblers will mark its absorption of the air, by the consequent rising of the coloured liquid. But the tumbler being wetted over the inside, and a portion of dry, though not recent, charcoal, with a pared surface, placed within it, after the air has ceased to dilate from humefaction, if the charcoal be brought in contact with water, the liquor will again rise, and continue for some time to indicate a renewed expansion. As the water is imbibed by the charcoal, minute globules of air will appear to escape. But if the water, by its insinuation, had merely dislodged the air from the pores, there would have been no change of volume, and consequently no alteration in the height of the liquid on the scale. Those aerial globules must have, therefore, expanded as they emerged, or they had existed in a state of condensation united to the charcoal.

On the other hand, if a roll of unsized paper, linen rags, slips of wood, or saw-dust, be successively introduced under a tumbler in which a little water is easily shed, the coloured liquor will sink rapidly in proportion as the moisture is absorbed. In this case, there is an obvious diminution of volume, and an union produced between a liquid and a solid, quite analogous to chemical solution; whereas capillary action, such as the ascent of water through sand, is attended by no change whatever of the space occupied by the compound.

Another application of this instrument was to place under the tumbler separate capsules containing different substances, liquid or solid, which exert a mutual attraction. The included air in this case served as the vehicle of transfer, and a sort of distillation was supported, of a peculiar kind. The indications of this invisible process were variable, and often striking. But to enlarge on this subject would be foreign to our purpose. We have only taken the opportunity of noting a few of the results of an inquiry which was not pushed to any extent, but which deserves to be resumed, as likely to open new paths, and to unfold the more abstruse relations subsisting among different bodies.

Philosophers have long entertained very crude notions respecting the union of moisture with air, and the different circumstances which regulate or influence the process of evaporation. Dr Halley supposed that fire, uniting with the particles of water, communicates a vesicular constitution, which enables them to rise and spread through the atmosphere. A similar idea was entertained by Leibnitz, and was extended still farther by Muschenbroeck and Desaguliers. Kratzenstein adopted the vesicular system in 1743, and endeavoured to calculate the attenuation of the fluid which should produce the requisite buoyancy. Hamberger, in 1750, advanced, however, a capital step, by rejecting that hypothesis entirely, and attributing evaporation at once to a real solution of moisture in the air. This simple theory was in the following year explained at large by Le Roi of Montpellier, and fortified by some striking arguments drawn chiefly from analogy. In pursuance of his theory, he proposed to pour ice-cold water into a deep glass, and to ascertain the dryness of the air, by noting the depressed temperature at which the dewing or deposition of vapour began to form on the outside of the glass, or was again about to disappear. Could this method have been easily and nicely reduced to practice, it might certainly have furnished an accurate estimate of the hygrometer and state of the atmosphere.

Wallerius, in his researches, was drawn aside from the right path, by a fact first noticed by Muschenbroeck, who asserted that evaporation is always more copious from a deep than from a shallow vessel; and this curious and apparently anomalous fact has been confirmed by other sub- sequent experiments. But he found, from a series of careful observations, that the quantity of water exhaled in the same situation is exactly proportional to the extent of surface exposed. He likewise made some experiments which seemed to furnish an explanation of the peculiarity remarked by Muschenbroeck, though their force was not generally perceived at the time. But Richman rather darkened the subject by his strange conclusion, that the measure of evaporation depends on the difference merely between the temperature of the air and of the water, whether in excess or defect.

It was not difficult to perceive that evaporation is promoted by the application of heat, and the agitation of the aerial medium. No attempts, however, were made to determine the relation of that augmented effect to the actual velocity of the wind. But experiments on the influence which an increase of temperature exerts on the quantity of evaporation have been prosecuted with various success by Lambert, Saussure, and Kirwan. The results thus obtained unfortunately differ very widely; and though the researches of the celebrated naturalist of Geneva were those conducted with the most care and address, yet they seem, from the vagueness of their elements, not entitled to much confidence.

If the estimate of the causes which regulate the quantity of evaporation was unsatisfactory, still more perplexing appeared the ordinary account of the source of that depression of temperature which always accompanies the process. It was commonly referred to the operation of different concurring circumstances, among which the agitation of the air was conceived to perform the principal part. The dryness of that medium, on which we shall find the effect alone to depend, was in a great measure overlooked, or was confounded with other occasional agents. An evident confusion of ideas prevailed. The celerity of evaporation was mistaken for its intensity, and the coldness induced on the exhaling surface was viewed as the accumulated effect of a rapid dissipation of moisture. It was not perceived that in a free atmosphere vaporization proceeds with unabated energy, while the corresponding depression of temperature must advance by a relaxing progression, since otherwise the accession of an accelerated movement might push it to any extent. A little reflection, indeed, should have convinced philosophers, that the reduced temperature caused by vaporization must, in given circumstances, have a certain limit beyond which it cannot pass. But simple as this conclusion may now appear, it had escaped the most sagacious inquirers. Even Saussure, a patient and accurate experimenter, persuaded himself that, when water evaporates slowly, the cold produced is scarcely perceptible. To render this sensible, therefore, he thought it necessary to accelerate evaporation. Inserting the ball of a thermometer in a wet sponge, and attaching a cord to its stem, he whirled it briskly round his head, and thus produced a cold of eighteen degrees by Fahrenheit's scale, which he considered as much greater than could be obtained by other processes. To prosecute the inquiry, he had a sort of whirling table constructed, of about five feet diameter, by means of which a thermometer, with its bulb incased in wet sponge, could be made to revolve with the velocity of forty feet in a second. This machine he was at the trouble and expense of having carried up to the summit of the Col du Géant, where, during a residence of several days, he performed a series of interesting and valuable experiments. It did not occur to this philosopher, that by such a contrivance he was only creating to himself a vast deal of unnecessary fatigue, and that his wet thermometer, if left simply at rest for the space of two or three minutes, would have indicated exactly the same results. By all his exertions, he merely shortened the very moderate time required for attaining its extreme limit of depression.

Such were the imperfect notions which still prevailed on the subject of evaporation so late as the year 1796, the date of the publication of the last volume of the Voyages dans les Alpes. It is curious to remark, that Dr Black, in his Lectures, never mentions the dryness of the air as any way promoting evaporation, but ascribes the acceleration of the process entirely to the warmth and agitation of that medium. His friend Dr James Hutton, whose acuteness and penetration were conspicuous, had probably studied the phenomenon more closely. "I never had a hygrometer," he says; "but I used to amuse myself in walking in the fields, by observing the temperature of the air with the thermometer, and trying its dryness by the evaporation of water. The method I pursued was this: I had a thermometer included within a glass tube, hermetically sealed; this I held in a proper situation until it acquired the temperature of the atmosphere, and then I dipped it into a little water also cooled to the same temperature. I then exposed my thermometer, with its glass case wetted, to a current of air; and I examined how much the evaporation of the atmosphere, by holding the ball of the thermometer, or the end of the tube in which the ball was enclosed, towards the current of air; and I examined how much the evaporation from that glass tube cooled the ball of the thermometer which was included." He then proceeds to relate some hasty and very inaccurate observations made in this way. The passage now quoted occurs in a quarto volume published in 1792, buried in a repulsive mass of diffuse reasoning and paradoxical speculation, unsupported by any definite experiment. But Dr Hutton had evidently considered it as a conjectural hint, on which he laid little stress; for he speaks immediately afterwards of our possessing accurate hygrometers, which could not be admitted if the assumption he seemed to make had been strictly true.

Whilst such loose and imperfect notions prevailed respecting evaporation, it was expedient to review the process with attention, and analyse the several changes which accompany it. The depression of temperature which it always occasions had been hastily supposed to be proportional to the rate with which the moisture is dissipated, and to be therefore augmented by every circumstance that can accelerate this effect. But if water contained in a porous vessel expose on all sides its surface to a current of air, it will cool down to a certain point, and there its temperature will remain stationary. The rapidity of the current must no doubt hasten the term of equilibrium, but the degree of cold thus induced will be found still the same. A little reflection may discover how this takes place. It is well known that the conversion of water into steam is, in every case, attended by the absorption of the heat requisite to support a gaseous constitution. Though the humid surface has now ceased to grow colder, the dispersion of invisible vapour, and the corresponding abstraction of heat, still continue without intermission. The same medium, therefore, which transports the vapour, must also furnish the portion of heat required for its incessant formation. In fact, after the water has been once cooled down, each portion of the ambient air which comes to touch the evaporating surface must, from its contact with a substance so greatly denser than itself, be likewise cooled down to the same standard, and must hence communicate to the liquid its surplus heat, or the difference between the prior and the subsequent state of the solvent, which is proportioned to the diminution of temperature it has suffered. Every shell of air that in succession encircles the humid mass, while it absorbs, along with the moisture which it dissolves, the measure of heat to convert this into steam, does at the same instant thus deposit an equal measure of its own heat on the chill exhaling surface. The abstraction of heat by vaporization on the one hand, and its deposition on the other, at the surface of contact, are therefore, opposite contemporaneous acts, which soon produce a mutual balance, and thereafter the resulting temperature continues without the smallest alteration. A rapid circulation of the evaporating medium may quicken the operation of those causes; but so long as it possesses the same drying quality, it cannot in any degree derange the resulting temperature. The heat deposited by the air on the humid surface becomes thus an accurate measure of the heat spent in vaporizing the portion of moisture required for the saturation of that solvent at its lowered temperature. The dryness of the air is, therefore, under all circumstances, precisely indicated by the depression of temperature produced on a humid surface which has been exposed freely to its action.

It may insure perspicuity, however, to recapitulate the great principle on which the formation of the hygrometer depends. When water passes into steam or vapour, it enlarges its capacity, and absorbs a very large share of heat. Any body, therefore, having a wet surface, becomes generally colder, if exposed to the access of air. But this decrease of temperature soon attains a certain limit, where it continues stationary, though the dissipation of moisture still proceeds with undiminished activity. The same medium which transports the vapour must hence furnish also the portion of heat required for its incessant formation. In fact, after the humid surface has been cooled, each portion of the ambient air which comes to touch it must likewise be cooled down to the level of the dense substratum. The addition of heat at the surface of contact is thus a contemporaneous act with its subtraction by the process of vaporization; and it quickly advances to the same degree of intensity, after which a mutual balance of opposite effects is maintained, and the coolness hence induced continues unaltered. An augmented circulation of the evaporating medium may hasten the process; but while it has the same drying quality, it cannot in the least affect the depression of temperature. As soon as such an equilibrium is attained, the deposition of heat on the humid surface must become just equal to its abstraction. But this deposition is evidently proportional to the diminution of temperature, which is hence a measure of the share of heat abstracted, and therefore of the dryness of the air, or its distance from saturation.

This analysis of the process of evaporation appears so conclusive as to banish all doubt and objection. But it was desirable to confirm the deductions of theory by an appeal to direct observation. Accordingly, on setting the hygrometer upon a table in the middle of a room, and blowing from some distance against the wet ball with a pair of bellows which had acquired the temperature of the apartment, the instrument still indicated the same measure of dryness. The experiment was repeated more accurately on a larger scale, by exposing the hygrometer out of doors to the action of a strong and arid wind, a small screen being interposed and again removed, at short intervals of time, during which alternations no change whatever could be perceived in the quantity of the depression of temperature.

Having, therefore, ascertained the great law of evaporation, and proved that the coldness occasioned by it is not in any degree affected by agitation or other extraneous influence, nothing seemed wanting to construct an hygrometer on just principles, but to contrive a thermometer that should mark the smallest alterations of temperature. At first Mr Leslie employed a very delicate thermometer with a short range, open at the top, where a small cap of glass or ivory was fixed, containing a small portion of surplus quicksilver. (See fig. 16 and 17, Plate CCCLIV.) When this thermometer was heated by the hand till the thread of quicksilver filled the whole of the stem, and formed a little globule at the top, it was inverted, as in fig. 17, and all the quicksilver united into one mass; but when it was restored, as in fig. 18, to its first position, the quicksilver fell back from the cap, and lodged about the end of the stem, leaving the bore completely filled. A cup (see fig. 1, Plate CCCLIV.) made of thin porous earthenware, nearly of the shape of a lady's thimble, but somewhat larger, and filled with water, was exposed to the air, while the thermometer lay beside it in a horizontal position. After a few minutes, the thermometer was lifted up and plunged vertically into the cup; and the thread of quicksilver, which had extended through the whole length of the bore, being by this change of position cut off at the top of the tube, immediately contracted, and marked, by the space of its descent, the diminution of temperature in the liquid.

The very severe winter of 1794-5 afforded Sir John Leslie Leslie's an opportunity of making experiments on the evaporation of hygro-ice; in the course of which he was led to the construction ter. of the differential thermometer, now so generally known. At first he employed it merely as an hygrometer; the one ball being always naked, and the other covered with cambric, wetted as often as occasion required. These balls were about an inch and a half in diameter, and blown to the ends of the same tube, one of them having a projecting point or aperture, which was sealed, after the branches had been bent and a portion of coloured oil introduced. The graduation of the scale was determined by placing the instrument erect between two boxes, each containing a body of water, which encompassed one of the balls, and by noticing the rise of the oil on changing the relative temperature of the bath. An hygrometer, thus formed, was sent to the late Sir Joseph Banks, in the summer of 1795; and Mr Gilpin, clerk to the Royal Society, deposited a copy of it in their cabinet. But this model is so rude and clumsy, as hardly to recall the original. In the hands of the inventor, however, the instrument was soon improved, and reduced to a convenient and portable size. He had the satisfaction of showing an hygrometer of this construction, in January 1796, to Dr James Hutton, only a few months before the death of that very ingenious philosopher, who was delighted with seeing the application of a principle which he readily comprehended, and which his sagacity had obscurely anticipated.

But the sluggishness of the oil, and its tendency to deposit its colouring matter, on exposure to the influence of light, still opposed obstacles to the perfection of this hygrometer. These were entirely removed, however, about the year 1801, by the substitution of concentrated sulphuric acid tinged by carmine. The most powerful and continued action of the solar beams was found to produce no change whatever on that purpurine liquid, when precluded from the access of the external air. Any improvements which have since been effected on this instrument consist chiefly in its mechanical arrangement, in the selection of the tubes, the better proportion of the balls, and the elegance and conciseness of the general shape.

The hygrometer has two distinct forms; the one portable and the other stationary. The former (see fig. 13, Plate CCCLIV.), having its balls in the same perpendicular line, is protected by a wooden or metallic case, and fitted for carrying in the pocket; two or three drops of pure water, from the tip of a quill or a hair pencil, being applied to the surface of the covered ball, and the instrument held exactly in a vertical position whenever it is used. The latter form (see fig. 12) is susceptible of rather greater accuracy, having its balls bent opposite ways at the same height. In some instances, it is preferable to retain merely the simplest form of the differential thermometer, the vertical stems being more distant, and the balls not reflected. In both these constructions, the two balls, since they occupy the same level, cannot be affected in the smallest degree by the unequal temperatures of the different strata of air in a close heated room. After the covered ball has been wetted, the instrument will continue to perform unimpaired, for the space, perhaps, of two or three hours. The addition of a few drops of water will then restore its action. But the hygrometer may be made to supply itself with moisture. It is only wanted to pass some fibres of floss-silk close over the humid ball, and immerse them at the distance of a few inches in a tall glass decanter, full of water, with a stopper which leaves open a small projecting lip.

The hygrometer has its opposite balls made to exhibit nearly the same colour and opacity, in order to exclude the admixture of photometrical influence, or prevent any derangement which the unequal action of light might otherwise occasion. The naked ball is blown of red, green, or blue glass, and the papered one is externally covered with thin silk, of rather a fainter shade, as it takes a deeper tint when moistened.

Its theory completed. The theory which we have given of the hygrometer, corroborated by its accurate performance, might appear complete in every part. But the progress of science commonly detects the existence of some collateral causes which come to mingle their influence with the action of the great dominating principles. In our explication of the hygrometer, we stated that the same air which abstracts moisture, and consequently the portion of heat necessary to convert it into the gaseous form, must likewise communicate to the wet ball another portion of heat equal to its depression of temperature, which is hence maintained at a constant point. This analysis, however, involves the supposition, that the air conveys heat from bodies merely by its actual transfer. But having discovered that air transmits a certain share, at least, of the heat by a sort of pulsation, or internal tremor, depending on the quality of the surface from which the impression originates, it was requisite to examine anew the process of evaporation. The effect cannot be produced solely by the quickened recession of the contiguous portions of the ambient medium. The conterminous air must communicate heat to the humid surface also by pulsation; and hence the balance of temperature would be liable to incidental variations, if moisture, with its embodied heat, were not likewise abstracted by some corresponding process. And such is the harmonious adaptation of these elements: the discharge of vapour appears to be subject precisely to the same conditions as the emission of heat, and in both cases the proximity of a vitreous or a metallic surface produces effects which are entirely similar. Let two pieces of thin mirror-glass, or what is called Dutch plate, be selected, about four inches and a half square; and having applied a smooth coat of tinfoil, four inches square, to one of these, cover them both with a layer of the thinnest goldbeater's skin, which will adhere closely on being wetted; and after it has again become dry, cut it on each into an exact square of four inches and a quarter: now place the two glass plates horizontally in the opposite scales of a fine balance, and adjust them to an exact counterpoise: then, with a hair pencil, spread two grains of water over the surface of each pellicle: in a few seconds, the plate which is coated with tinfoil will preponderate, and after the former has lost all its moisture, this will be found to retain still three tenths of a grain. The proximity of the subjacent metal to the humid surface, therefore, impedes the process of evaporation, in the ratio of seventeen to twenty: the very same as, in like circumstances, had been ascertained to be the retardation of the efflux of heat. From this and other experiments, we learn, that some constant portion from a humid surface is always abstracted by the pulsation of the aerial medium. The steam exhaled, in uniting with the air, communicates to this elastic fluid a sudden dilatation, which will continue to propagate itself in successive waves.

In further illustration of this matter, cover with a thin pellicle of goldbeater's skin both the balls of the pyroscope, or that form of the differential thermometer which has one ball naked and the other enamelled with gold or silver, and wet them equally. The coloured liquid will remain for several minutes stationary at the beginning of the scale, and will then mount slowly, perhaps ten or fifteen degrees. Evaporation had, therefore, produced the same cold or depression of temperature upon the surface of the metal as upon that of the glass; from the glass, however, it was more copious than from the metal, having left the former dry, while the latter still exhaled some portion of moisture. But this action soon ceased, and the liquid fell back to its former level. On applying another pellicle, the liquid continued longer stationary, and rose only about five degrees. With repeated pellicles a difference was perceptible in the time of drying the two balls, till the thickness amounted to the 600th part of an inch.

The method employed for the graduation of the hygrometer is not only very convenient, but susceptible of great accuracy. The instrument, with a temporary scale affixed to it, is introduced into a magazine of dry air, and compared with a standard then put in action. To procure the dryness of the included medium, a flat saucer of thirteen inches in diameter, and holding a body, about half an inch deep, of concentrated sulphuric acid, and set on a ground plate of glass or metal, is covered by a very large inverted receiver, containing more than 1500 cubic inches of air, and having at the top an opening of three inches wide, on which rests a smaller plate, with two or three hooks projecting down from it. The scale of the standard instrument was determined by suspending beside it, under the receiver, two delicate thermometers, one of which had its bulb coated with several folds of wet tissue paper. The descent of the coloured liquid of the hygrometer, corresponding to the difference of ten centesimal degrees of the parallel thermometers, was hence computed, and this length afterwards divided into an hundred equal parts, to form the standard degrees. To graduate any other instrument, it was only requisite to attach a scale of inches, and mark the simultaneous measures when a steady equilibrium had at last obtained. The space of half an hour is generally sufficient to bring this about. A simple proportion, therefore, discovers the length answering to an hundred millesimal degrees, from which the subdivisions of each particular scale are derived.

The condition of the atmosphere with respect to dryness is extremely variable. In our climate, the hygrometer will, during winter, mark from five to twenty-five degrees; but, in the summer months, it will generally range between fifteen and fifty-five degrees, and may even rise, on some particular days, as high as eighty or ninety degrees. On the continent of Europe, it maintains a much greater elevation; and in Upper India it has frequently stood at 160 degrees.

When the indication of the hygrometer does not exceed fifteen degrees, we are directed by our feelings to call the air damp; from thirty to forty degrees, we begin to reckon it dry; from fifty to sixty degrees, we should account it very dry; and from seventy degrees upwards, we might consider it as intensely dry. A room is not comfortable, or perhaps wholesome, if it has less than thirty degrees of dryness; but the atmosphere of a warm occupied apartment will commonly produce an effect of upwards of fifty degrees.

But this hygrometer will perform its office even if it should be exposed to frost. The moisture spreads over the surface, and, imbibed into the coat of the papered ball, will first cool a few degrees below the freezing point, and then congeal quickly into a solid compound mass. The moment in which congelation begins, a portion of heat liberated in that act brings the ball back to the temperature of freezing; and the coloured liquor, in proportion to the coldness of the external air, starts up in the opposite stem, where it remains at the same height till the process of consolidation is completed. After the icy crust has been formed, evaporation again goes regularly forward; and if new portions of water be applied, the ice will, from the union of those repeated films, acquire a thickness sufficient to last for several days. The temperature of the frozen coat becomes lowered in proportion to the dryness of the atmosphere. The measure of heat deposited on the chill surface by the contact of the ambient air is then counterbalanced by the two distinct though conjoined measures of heat, abstracted in the successive acts of converting the exterior film of ice into water and this water into steam; which transformations that minute portion must undergo before it can unite with its gaseous solvent. But the heat required for the melting of ice being about the seventh part of what is consumed in the vaporization of water, it follows that the hygrometer, when the surface of its sentient ball has become frozen, will, in like circumstances, sink more than before, by one degree in seven. This inference is entirely confirmed by observation. Suppose, in frosty weather, the hygrometer, placed on the outside of the window, to stand at twenty-eight degrees, it may continue for some considerable time at that point, until the congelation of its humidity commences; but after this change has been effected, and the equilibrium again restored, the instrument will now mark thirty-two degrees.

The theory of this hygrometer will enable us to determine, not only the relative, but even the absolute dryness of the air, or the quantity of moisture which it can absorb, by comparing the capacity of that solvent with the measure of heat required to convert a given portion of water into steam. To discover the capacity of air is, however, a problem of great difficulty, and it has not perhaps even yet been ascertained with much precision. It was formerly estimated, we are convinced, by far too high. Thus, Dr Crawford made it to be 185 times, or nearly double that of water. But, from several concurring observations, we should reckon the capacity of air to be only three tenths parts of that of water. But 600 centesimal degrees, or 3000 on the millesimal scale, being consumed in the vaporization of heat, this measure of heat would prove sufficient to raise an equal mass of air 20,000 millesimal degrees, or those 6000 degrees augmented in the ratio of ten to three. Now, at the state of equilibrium, the quantity of heat that each portion of the aerial medium deposits touching the chill exhaling surface, or what answers to the depression of temperature which it suffers from this contact, must, as we have seen, be exactly equal to the opposite measure of heat abstracted by it in dissolving its corresponding share of moisture. Wherefore, at the temperature of the wet ball, atmospheric air would take up moisture amounting to the 20,000th part of its weight for each degree marked by the hygrometer. Thus, supposing the hygrometer to mark fifty degrees, the air would then require humidity equal to the 320th part of its eight for saturation at its reduced temperature. When the papered ball of the hygrometer is frozen, the degrees of this instrument must have their value increased by one tenth, so that each of them will now correspond to an absorption of moisture equal to the 17,000th part of the eight of the air.

But the value of those degrees becomes augmented in much higher proportion if the hygrometer be immersed in hydrogen gas. This very dilute medium appears to have about eight times the capacity of common air, and the quantity of heat which under similar circumstances it will deposit on the evaporating surface must likewise, from the same principle of mutual balance, be eight times greater, and, consequently, each hygrometric degree will indicate an absorption of moisture equal in weight to the 750th part of the solvent. The energy of hydrogen gas is therefore scarcely less remarkable in dissolving moisture than in containing heat. Confined with a powerful absorbent substance, whilst common air marks eighty degrees of dryness, hydrogen gas will indicate seventy. This gas must, in similar circumstances, therefore, hold in solution seven times as much moisture as the atmospheric medium.

To discover the precise law by which equal additions of heat augment the dryness of air, or its power to retain moisture, is a problem of great delicacy and importance. Two different modes were employed in that investigation, but which led to the same results. The one was, in a large close room, to bring an hygrometer, conjoined with a thermometer, successively near to a stove intensely heated, and to note the simultaneous indications of both instruments; or to employ two nice thermometers, placed beside each other, and having their bulbs covered respectively with dry and with wet cambric. By taking the mean of numerous observations, and interpolating the intermediate quantities, the law of aqueous solution in air was laboriously traced. But the other method of investigation appeared better adapted for the higher temperatures. A thin hollow ball of tin four inches in diameter, and having a very small neck, was neatly covered with linen, and being filled with water nearly boiling, and a thermometer inserted, it was hung likewise in a spacious close room, and the rate of its cooling carefully marked. The experiment was next repeated by suspending it to the end of a fine beam, and wetting with a hair pencil the surface of the linen, till brought in exact equipoise to some given weight in the opposite scale; ten grains being now taken out, the humid ball was allowed to rest against the point of a tapered glass tube, and the interval of time, with the corresponding diminution of temperature, observed, when it rose again to the position of equilibrium. The same operation was successively renewed; but, as the rapidity of the evaporation declined, five, and afterwards two, grains only were, at each trial, withdrawn from the scale. From such a series of facts, it was easy to estimate the quantities of moisture which the same air will dissolve at different temperatures, and also the corresponding measures of heat expended in the process of solution.

By connecting the range of observations, it would appear that air has its dryness doubled at each rise of temperature, answering to 15 centesimal degrees. Thus, at the freezing point, air is capable of holding a portion of moisture represented by 100 degrees of the hygrometer; at the temperature of 15 centigrade, it could contain 200 such parts; at that of 30, it might dissolve 400; and, at 45 on the same scale, 800. Or, if we reckon by Fahrenheit's divisions, air absolutely humid holds, at the limit of congelation, the hundred and sixtieth part of its weight of moisture; at the temperature of 59 degrees, the eightieth part; at that of 86 degrees, the fortieth part; at that of 113 degrees, the twentieth part; and at that of 140 degrees, the tenth part. While the temperature, therefore, advances uniformly in arithmetical progression, the dissolving power which this communicates to the air mounts with the accelerating rapidity of a geometrical series.

It hence follows, that, whatever be the actual condition of a mass of air, there must always exist some temperature at which it would become perfectly damp, as M. Leroi had first advanced. Nor is it difficult, from what has been already stated, to determine this dewing point in any given case. Thus, suppose the hygrometer to mark 52, while its wet ball has a temperature of 20 centesimal degrees, or 68 by Fahrenheit; the dissolving power of air at this tem- perature being 252, its distance from absolute humidity will therefore be 200, which is the measure of solution answering to 15 centesimal degrees, or 59 by Fahrenheit. The same air would consequently, at the depressed temperature of 59 degrees, shrink into a state of absolute saturation; and if cooled lower, it would even deposite a portion of its combined moisture, losing the eightieth part of its weight at the verge of freezing.

Annexed is a small table of the solvent power of air, from the temperature of 15 centesimal degrees below zero to 44 above it, or from — 5° of Fahrenheit's scale to 111°-2.

Quantities of Moisture dissolved in Atmospheric Air at different Temperatures by the Centesimal Scale.

| Temp. | Moist. | Temp. | Moist. | Temp. | Moist. | |-------|--------|-------|--------|-------|--------| | -15 | 50-0 | 15 | 200-0 | 30 | 400-0 | | -14 | 52-4 | 16 | 209-5 | 31 | 418-9 | | -13 | 54-9 | 17 | 219-4 | 32 | 438-7 | | -12 | 57-4 | 18 | 229-7 | 33 | 459-5 | | -11 | 60-1 | 19 | 240-6 | 34 | 481-2 | | -10 | 63-0 | 20 | 252-0 | 35 | 504-0 | | -9 | 66-0 | 21 | 263-9 | 36 | 527-8 | | -8 | 69-1 | 22 | 276-4 | 37 | 552-8 | | -7 | 72-4 | 23 | 289-5 | 38 | 578-9 | | -6 | 75-8 | 24 | 303-1 | 39 | 606-3 | | -5 | 79-4 | 25 | 317-5 | 40 | 635-0 | | -4 | 83-1 | 26 | 332-5 | 41 | 665-0 | | -3 | 87-1 | 27 | 348-2 | 42 | 696-4 | | -2 | 91-2 | 28 | 364-7 | 43 | 729-4 | | -1 | 95-5 | 29 | 381-9 | 44 | 763-9 |

These temperatures, and the corresponding quantities of moisture dissolved, may be represented by the abscissae and ordinates of the logarithmic curve, as in fig. 8, Plate CCCLV., where some of the principal terms are marked.

The influence of warmth in augmenting the dryness of the air, or its disposition to absorb moisture, affords also the most satisfactory explication of the singular fact already noticed. If two equal surfaces of water be exposed in the same situation, the one in a shallow and the other in a deep vessel of metal or porcelain, the latter is always found, after a certain interval of time, to have suffered, contrary to what we might expect, more waste by evaporation than the former. Amidst all the changes that happen in the condition of the ambient medium, the shallow pan must necessarily receive, more completely than the deeper vessel, the chilling impressions of evaporation, since it exposes a smaller extent of dry surface to be partly heated up again by the contact of the air. The larger mass being, therefore, kept invariably warmer than the other, must in consequence support a more copious exhalation.

From the principles which have been explained, it likewise results that the hygrometer does not indicate the actual dryness of the air, but only the dryness which it retains after being reduced to the temperature of the humid ball. The real condition of the medium, however, could easily be determined, from the gradations already ascertained in the power of solution. Suppose, for example, that the hygrometer should mark 42 degrees, while the thermometer stands at 16 centigrade; the moist surface has therefore the temperature of 11-8 centigrade, at which the dissolving energy is less by 37 degrees than at 16 centigrade; and hence the total dryness of the air, at its former temperature, amounted to 79 degrees. The following table will greatly facilitate such reductions. It is computed for as wide a range of dryness and temperature as will probably occur in any climate.

Correction of the Hygrometer, and Position of the Point of Saturation at Different Centesimal Temperatures.

| Hyg. | Dryness | Point Sat. | Hyg. | Dryness | Point Sat. | Hyg. | Dryness | Point Sat. | Hyg. | Dryness | Point Sat. | Hyg. | Dryness | Point Sat. | |------|---------|------------|------|---------|------------|------|---------|------------|------|---------|------------|------|---------|------------| | 10 | 12 | 21-0 | 12 | 19-5 | 12 | 18-3 | 17-8 | 13 | 16-3 | 10 | | 10 | 13 | 15-0 | | 20 | 24 | 29-2 | 24 | 27-6 | 25 | 26-1 | 24-7 | 25 | 23-2 | 20 | | 20 | 26 | 21-5 | | 30 | 36 | 42-7 | 36 | 39-8 | 37 | 37-2 | 35-0 | 38 | 32-9 | 30 | | 30 | 38 | 30-1 | | 10 | 13 | 15-0 | 13 | 13-8 | 13 | 12-5 | 11-4 | 13 | 10-0 | 10 | | 10 | 14 | 9-3 | | 20 | 26 | 21-5 | 26 | 19-8 | 26 | 18-2 | 16-8 | 27 | 15-4 | 20 | | 20 | 27 | 14-0 | | 30 | 38 | 30-1 | 39 | 29-3 | 39 | 26-1 | 24-4 | 40 | 22-2 | 30 | | 30 | 40 | 20-3 | | 10 | 14 | 9-3 | 14 | 7-9 | 14 | 6-8 | 5-6 | 14 | 4-5 | 10 | | 10 | 15 | 3-4 | | 20 | 27 | 14-0 | 27 | 12-6 | 28 | 11-3 | 9-9 | 28 | 8-6 | 20 | | 20 | 29 | 7-4 | | 30 | 43 | 12-2 | 43 | 10-6 | 44 | 9-1 | 7-7 | 45 | 6-3 | 30 | | 30 | 43 | 12-2 | | 40 | 57 | 18-2 | 58 | 16-3 | 59 | 14-5 | 12-7 | 60 | 11-0 | 40 | | 40 | 57 | 18-2 | Correction of Hygrometer, &c.—Continued.

| Hyg. | Dryness | Point Sat. | Dryness | Point Sat. | Dryness | Point Sat. | Dryness | Point Sat. | Dryness | Point Sat. | Dryness | Point Sat. | |------|---------|------------|---------|------------|---------|------------|---------|------------|---------|------------|---------|------------| | 10 | 16 | 2·1 | 16 | 3·1 | 16 | 4·3 | 16 | 5·4 | 17 | 6·5 | 10 | | | 20 | 31 | — 1·1 | 32 | 0·0 | 32 | 1·3 | 33 | 2·4 | 33 | 3·6 | 20 | | | 30 | 46 | — 4·9 | 47 | — 3·6 | 48 | — 2·2 | 49 | — 0·9 | 50 | 0·4 | 30 | | | 40 | 61 | — 9·4 | 62 | — 7·8 | 63 | — 6·3 | 64 | — 4·7 | 66 | — 3·3 | 40 | | | 50 | 76 | — 15·0 | 77 | — 13·0 | 79 | — 11·1 | 80 | — 9·4 | 81 | — 7·6 | 50 | |

| 10° | 17 | 7·5 | 18 | 8·6 | 18 | 9·6 | 18 | 10·7 | 19 | 11·8 | 10 | | | 20 | 34 | 4·8 | 35 | 5·9 | 35 | 7·1 | 36 | 8·2 | 37 | 9·4 | 20 | | | 30 | 50 | 1·9 | 52 | 3·0 | 53 | 4·3 | 54 | 5·5 | 55 | 6·7 | 30 | | | 40 | 67 | — 1·8 | 68 | — 0·4 | 69 | 1·0 | 71 | 2·4 | 72 | 3·7 | 40 | | | 50 | 83 | — 5·9 | 84 | — 4·3 | 86 | — 2·7 | 88 | — 1·1 | 89 | 0·4 | 50 | | | 60 | 98 | — 11·0 | 100 | — 9·0 | 102 | — 7·1 | 104 | — 5·4 | 106 | — 3·7 | 60 | |

| 15° | 19 | 12·9 | 20 | 13·9 | 20 | 15·0 | 20 | 16·0 | 21 | 17·0 | 10 | | | 20 | 38 | 10·5 | 39 | 11·6 | 39 | 12·7 | 40 | 13·8 | 41 | 14·9 | 20 | | | 30 | 56 | 7·9 | 57 | 9·1 | 58 | 10·3 | 60 | 11·5 | 61 | 12·7 | 30 | | | 40 | 74 | 5·0 | 75 | 6·4 | 77 | 7·6 | 79 | 8·9 | 81 | 10·2 | 40 | | | 50 | 91 | 1·8 | 93 | 3·3 | 95 | 4·7 | 97 | 6·0 | 100 | 7·4 | 50 | | | 60 | 108 | — 1·8 | 111 | — 0·2 | 113 | 1·3 | 116 | 2·8 | 118 | 4·4 | 60 | | | 70 | 125 | — 6·3 | 128 | — 4·5 | 131 | — 2·6 | 134 | — 0·8 | 136 | 0·9 | 70 | | | 80 | 142 | — 11·8 | 145 | — 9·5 | 148 | — 7·2 | 151 | — 5·2 | 154 | — 3·2 | 80 | |

| 20° | 21 | 18·1 | 22 | 19·1 | 22 | 20·1 | 23 | 21·2 | 24 | 22·2 | 10 | | | 20 | 42 | 16·0 | 43 | 17·1 | 44 | 18·2 | 46 | 19·3 | 47 | 20·4 | 20 | | | 30 | 63 | 13·8 | 64 | 15·0 | 66 | 16·1 | 68 | 17·3 | 69 | 18·4 | 30 | | | 40 | 83 | 11·4 | 84 | 12·6 | 87 | 13·8 | 89 | 15·0 | 91 | 16·2 | 40 | | | 50 | 102 | 8·8 | 104 | 10·1 | 107 | 11·4 | 110 | 12·7 | 112 | 13·9 | 50 | | | 60 | 121 | 6·1 | 124 | 7·4 | 127 | 8·7 | 130 | 10·2 | 133 | 11·5 | 60 | | | 70 | 140 | 2·5 | 143 | 4·1 | 146 | 5·7 | 150 | 7·2 | 154 | 8·7 | 70 | | | 80 | 158 | — 1·3 | 162 | 0·5 | 165 | 2·3 | 169 | 4·0 | 174 | 5·6 | 80 | | | 90 | 176 | — 5·9 | 180 | — 3·8 | 184 | — 1·8 | 188 | 0·2 | 193 | 2·0 | 90 | | | 100 | 193 | — 11·5 | 198 | — 9·0 | 202 | — 6·5 | 207 | — 4·3 | 212 | — 2·0 | 100 | |

| 25° | 24 | 23·3 | 25 | 24·3 | 26 | 25·3 | 26 | 26·4 | 27 | 27·4 | 10 | | | 20 | 48 | 21·4 | 49 | 22·5 | 51 | 23·6 | 52 | 24·7 | 54 | 25·7 | 20 | | | 30 | 71 | 19·5 | 73 | 20·6 | 75 | 21·7 | 77 | 22·8 | 79 | 23·9 | 30 | | | 40 | 94 | 17·4 | 96 | 18·6 | 99 | 19·8 | 102 | 21·0 | 104 | 22·1 | 40 | | | 50 | 116 | 15·2 | 119 | 16·5 | 122 | 17·7 | 125 | 18·9 | 129 | 20·1 | 50 | | | 60 | 137 | 12·8 | 141 | 14·1 | 144 | 15·4 | 148 | 16·7 | 152 | 18·1 | 60 | | | 70 | 158 | 10·1 | 162 | 11·6 | 166 | 13·0 | 171 | 14·3 | 175 | 15·7 | 70 | | | 80 | 178 | 7·2 | 183 | 8·7 | 188 | 10·3 | 193 | 11·7 | 198 | 13·2 | 80 | | | 90 | 198 | 3·9 | 203 | 5·6 | 209 | 7·2 | 214 | 8·9 | 220 | 10·4 | 90 | | | 100 | 218 | 0·0 | 223 | 2·0 | 229 | 3·8 | 235 | 5·6 | 241 | 7·4 | 100 | | | 110 | 237 | — 4·5 | 242 | — 2·3 | 249 | — 0·1 | 255 | 2·0 | 262 | 3·9 | 110 | | | 120 | 255 | — 10·2 | 261 | — 7·4 | 268 | — 4·8 | 275 | — 2·4 | 282 | — 0·1 | 120 | | We may compute, that a cubic mass of air 40 inches of moisture every way, or a little more than the standard of French measures, and of the ordinary density, weighs 20,000 grains. The table now given exhibits, therefore, in grains, the weight of moisture which a metrical cube of air is capable of holding at different temperatures. Thus, at 20 degrees, which corresponds to 68 degrees of Fahrenheit, this body of air could retain 252 grains of humidity. But if a larger scale be preferred, the same numbers will express, in pounds troy, the quantity of water required to saturate a perfectly dry mass of air constituting a cube of twenty yards in every dimension.

It is remarkable how small a portion of the aqueous element is at any time suspended in the atmosphere. Reckoning the mean temperature over the surface of the globe to be nineteen centesimal degrees, the air could only hold 2406 parts of humidity for 20,000 times its whole weight; but this weight is nearly the same as that of a column of water of 400 inches in altitude, and hence, if the atmosphere, from a state of absolute dampness, were to pass into that of extreme dryness, and discharge the whole of its watery store, it would form a sheet of 4-812 inches, or somewhat less than five inches in depth. To furnish a sufficient supply of rain, the air must therefore undergo very frequent changes from dryness to humidity in the course of the year.

But it was requisite to subject theory to the test of accurate experiment. For this purpose a globe or glass balloon was procured, of very large dimensions, containing nearly 4000 cubic inches, terminated by a neck of about three inches wide, having its mouth ground flat. The balloon was supported from the floor by a light circular stand or rim, and a round piece of plate glass perforated through the centre by a hole of about the twentieth part of an inch in diameter, through which passed a slender silver wire suspended from the end of a fine beam placed on the table. This wire was fastened to the top of the scale of a delicate hygrometer, from the lower part of which hung a bit of wet paper nearly three inches in diameter. The whole was balanced by a counterpoise in the opposite scale, so that the instrument occupied the middle of the balloon. As the moisture gradually evaporated from the wet ball of the hygrometer, and still more from the larger surface of the paper attached below it, a loss of weight became visible, while the ascent of the coloured liquid in its tube indicated the corresponding diminution of the dryness of the included air. The progress of humification was observed from fifty to ten degrees of the hygrometer, after each successive grain of water, amounting in all to five, had exhaled and dispersed itself through the medium. Having rectified the hygrometric degrees according to the principle already explained, it was easy to compare them with the weight of the whole mass of air contained within the balloon. The conclusions were perhaps as satisfactory as such a nice and fugacious inquiry will admit; and if they be not absolutely correct, they must, at least, approximate very nearly to the truth.

The table given above is in strictness applicable to air only of the ordinary density. Since this fluid has its capacity for heat enlarged by rarefaction, the same depression of temperature must intimate a proportional augmentation of dryness. Thus, for air at the elevation of three miles and a half, and consequently twice as rare as at the surface, it would be requisite to add the sixteenth part to the numbers in the first column. For the lower altitudes, the correction will be, to multiply those numbers by triple the height in feet, and cut off six decimal places. Thus, suppose, while the thermometer stood at twenty-eight centesimal degrees, that the hygrometer marked 110° on the plains of Mexico, at the elevation of 8000 feet above the level of the sea; then $3 \times 8000 \times 255 = 6120000$, which, being divided by a million, gives 6 to be added to 255, increasing the actual dryness to 261°. In most cases, therefore, this modification may be neglected.

But, in estimating the distance of the point of saturation in rarefied air, a greater correction will be required. The solvent power of that medium is extended about fifty hygrometric degrees each time it has its rarefaction doubled. Hence it may be calculated that our atmosphere would, at the same temperature, become a degree drier for every 360 feet of ascent. Thus, on the preceding supposition, the air on the plain of Mexico would have its distance from the point of saturation enlarged 28½ degrees, its whole range being thus 283½ degrees.

If the papered ball of an hygrometer be suffered to become dry, the instrument, even in that state, will mark, though for a short time only, the different condition of the media into which it is transported. Thus, the air of a room being supposed to have fifty degrees of dryness, on carrying the quiescent hygrometer into another apartment of seventy degrees, the column of liquor will fall near twenty degrees, from the renewed evaporation of that portion of moisture which had still adhered to the coats of paper. But if the same instrument were carried into an apartment of only thirty degrees of dryness, the coloured liquor would actually rise near twenty degrees above the beginning of the scale, the paper now attracting the excess of humidity from the air. This vapour, in combining with it, passes into the state of water, and therefore evolves a corresponding share of heat. The equilibrium, however, unless the coats of paper have a considerable thickness, is again restored in a very few minutes.

Those changes are most readily perceived on immersing the quiescent hygrometer alternately in two receivers containing air drier and damper than that of the room. If a pyroscope, having both its balls covered with goldbeater's skin, be treated in the same way, it will indicate an effect, though momentary indeed, of a similar kind; for, in air which is drier, the pellicle of the naked ball will throw off its moisture more freely than that of the gilt ball; and in damper air it will, on the contrary, imbibe the surplus humidity with greater eagerness, thus losing some portion of heat in the one process, and gaining a minute accession in the other. The quantity of moisture concerned in producing such fleeting alterations may not exceed the thousandth part of a grain.

If a large receiver, having a delicate hygrometer suspended within it, be placed on a brass plate and over a metal cup containing some water, the included air will, from the solution of the moisture, become gradually damper, and this progressive change is marked by the instrument. Yet the mass of air will never reach its term of absolute humidity, and before the hygrometer points at five degrees, the inside of the receiver appears covered with dew. While the humidifying process, therefore, still goes on, the close attraction of the glass continually robs the contiguous air of a portion of its moisture, so that a kind of perpetual distillation is maintained through the aerial medium, the vapour successively formed being again condensed on the vitreous surface. But if, instead of the receiver, there be substituted a vessel formed of polished metal, the confined air will pass through every possible degree of humidity, and the hygrometer will, after some interval, arrive at the beginning of its scale.

The contrasted properties of a vitreous and a metallic surface, in attracting and repelling moisture, may be shown still more easily. In clear calm weather let a drinking-glass and a silver cup be placed empty near the ground on the approach of evening, and, as the dampness begins to prevail, the glass will become insensibly obscured, and next wetted with profuse dew, before the metal has yet betrayed any traces of humidity. The effect is, indeed, augmented by the cold pulses darted from the sky, which act more powerfully on the glass than on the metal.

The hygrometer is an instrument of the greatest utility, Its practical economy, in regulating many processes of art, and in directing the purchase and selection of various articles of produce. It will detect, for instance, the dampness of an apartment, and discover the condition of a magazine, of an hospital, or of a sick ward. Most warehouses require to be kept at a certain point of dryness, which is higher or lower according to the purposes for which they are designed. The printing of linen and cotton is carried on in very dry rooms; but the operations of spinning and weaving succeed best in air which rather inclines to dampness.

The manufacturer is at present entirely guided by observing the effects produced by stoves, and hence the goods are often shrivelled or otherwise injured before he can discern any alteration in the state of the medium. Wool and corn have their weight augmented sometimes as much as 10 or even 15 per cent. by the presence of moisture. But the condition of these commodities could be nicely and readily examined, by heaping them over a small wired cage, within which an hygrometer is placed. V. The Atmometer.—This instrument is an useful auxiliary, and might with some attention serve as a substitute of the hygrometer. It does not mark the mere dryness of the air, but it measures the quantity of moisture exhaled from a humid surface in a given time. The atmometer consists of a thin ball of porous earthenware, two or three inches in diameter, with a small neck, to which is firmly cemented a long and rather wide glass tube, bearing divisions, each of them corresponding to an internal annular section, equal to a film of liquid that would cover the outer surface of the ball to the thickness of the thousandth part of an inch. (Fig. 1, Plate CCCLV.) The divisions are marked by portions of quicksilver introduced, ascertained by a simple calculation, and they are numbered downwards to the extent of 100 to 200; to the top of the tube is fitted a brass cap, having a collar of leather, and which, after the cavity has been filled with distilled water, is screwed tight. The outside of the ball being now wiped dry, the instrument is suspended out of doors, exposed to the free access of the air. In this state of action the humidity transudes through the porous substance just as fast as it evaporates from the external surface, and this waste is measured by the corresponding descent of the water in the stem.

If the atmometer had its ball perfectly screened from the agitation of wind, its indications would be proportional to the dryness of the air at the lowered temperature of the humid surface; and the quantity of evaporation every hour, as expressed in thousandth parts of an inch, would, when multiplied by twenty, give the hygrometric measure. For example, in this climate, the mean dryness in winter being reckoned 15°, and in summer 40°, the daily exhalation from a sheltered spot must in winter form a thickness of 0.018, and amount in summer to 0.048 decimal parts of an inch. Suppose a pool for the supply of a navigable canal exposed a surface equal to ten English acres, and that the atmometer sank eighty parts during the lapse of twenty-four hours; the quantity of water exhaled in that time would be

\[ \frac{80}{12000} \times 660 \times 66 \times 10 = 2904 \text{ cubic feet, which corresponds to the weight of 81 tons.} \]

The dissipation of moisture is much accelerated by the agency of sweeping winds, the effect being sometimes augmented five or even ten times. In general, this augmentation is proportional, as in the case of cooling, to the swiftness of the wind, the action of still air itself being reckoned equal to that produced by a celerity of eight miles each hour. Hence the velocity of wind is easily computed, from a comparison of the indications of a hygrometer with an atmometer, or of a sheltered with those of an exposed atmometer. Thus, suppose the hygrometer to mark 40 degrees, or the column of water in a sheltered atmometer to subside at the rate of two divisions every hour, while in one exposed to the current the descent is twelve divisions; then as two is to ten, the superadded effect of the wind, so is eight to forty miles, the distance through which it has travelled in that time.

The atmometer is an instrument evidently of extensive application, and of great utility in practice. To ascertain with accuracy and readiness the quantity of evaporation from any surface in a given time, it is an important acquisition, not only in meteorology, but in agriculture, and the various arts and manufactures. The rate of exhalation from the surface of the ground is scarcely of less consequence than the fall of rain, and a knowledge of it might often direct the farmer advantageously in his operations. On the rapid dispersion of moisture depends the efficacy of drying-houses, which are too frequently constructed most unskilfully, or on very mistaken principles.

It is obvious, that though the atmometer should be exposed to the free air, it must be sheltered from rain, which, by wetting the ball, would derange the proper action of the instrument. This could easily be done, by fixing a small canopy over it; or, in the case of drifting showers, to have a sort of shelved open screen, like Venetian blinds, turned by the wind. The only objection to this atmometer is, that it cannot be used during intense frost, since the expansion of the included water, by a sudden congelation, might burst the ball and even the tube. But the instrument could still be made to act in another way: Let it be emptied, and a certain portion of the water, measured in the stem, be spread over the outside of the ball, by successive layers, to form a coat of ice. The time is to be noted when the whole of this crust has disappeared; or if any portion should remain, it may be deducted from the whole, and thus the hourly quantity of evaporation ascertained.

VI. PHOTOMETER.—This instrument, which was contrived to indicate the power of illumination, by the slight elevation of temperature which it occasions, has been shortly noticed in the article CLIMATE. It consists of a differential thermometer, having one of its balls diaphanous, and the other coated with china ink, or rather blown of deep-black enamel. (See fig. 17 and 18, Plate CCCLIV.) The rays which fall on the clear ball pass through it, without suffering obstruction; but those which strike the dark ball are stopped and absorbed at its surface, where, assuming a latent form, they act as heat. This heat will continue to accumulate, till its farther increase comes to be counterbalanced by an opposite dispersion, caused by the rise of temperature which the ball has come to acquire. At the point of equilibrium, therefore, the constant accessions of heat derived from the action of the incident light are exactly equalled by the corresponding portions of it again abstracted in the subsequent process of cooling. But, in still air, the rate of cooling is, within moderate limits, proportioned to the excess of the temperature of the heated surface above that of the surrounding medium. Hence the space through which the coloured liquid sinks in the stem will measure the momentary impressions of light, or its actual intensity. To prevent any extraneous agitation of the air from accelerating the discharge of heat from the black ball, and thereby diminishing the quantity of aggregate effect, the instrument is always sheltered, and more especially out of doors, by a thin glass case. The addition of this translucent case is quite indispensable. It not only precludes all irregular action, but maintains, around the sentient part of the instrument, an atmosphere of perpetual calm. Under the same force of incident light, the temperature of the black ball must still rise to the same height above that of its encircling medium. The case will evidently have some influence to confine the heat actually received, and hence to warm up the internal air. Therefore, corresponding to this excess, the black ball will acquire a farther elevation of temperature; but the clear ball being immersed in the same fluid, must experience a similar effect, which will exactly counterbalance the former. The difference of temperature between the opposite balls thus continues unaltered; and neither has the size or the shape of the case, nor the variable state of the exterior atmosphere with respect to rest or agitation, any sensible influence to derange or modify the results exhibited by this delicate instrument.

The photometer has, like the hygrometer, two general forms; the stationary, represented by fig. 15, and the portable, delineated in fig. 18. But they are both of them easily transported from one place to another. Their glass cases can be screwed off, and the former instrument, being cemented into a small slip of brass, which slides with a spring into the bottom, may be packed separately, if required, while the latter is protected by an external wooden case, in which it is carried in the pocket as safely as a pencil. This case, if screwed below, serves also as a handle to hold the photometer in a vertical position out of doors.

The photometer, placed in open air, exhibits distinctly the progress of illumination from the morning's dawn to the full vigour of noon, and thence its gradual decline till evening has spread her mantle; it marks the growth of light from the winter solstice to the height of summer, and its subsequent decay through the dusky shades of autumn; and it enables us to compare, with numerical accuracy, the brightness of different countries, the brilliant sky of Italy, for instance, with the murky atmosphere of Holland.

In this climate, the direct impression of the sun about midsummer amounts to about 90 degrees; but it regularly declines as his rays become more oblique. The greatest force of the solar beams with us in the depth of winter reaches only to 25 degrees. At the altitude of 17 degrees, it is already reduced to one half; and at 3 degrees above the horizon, the whole effect exceeds not one minimal degree.

The quantity of indirect light reflected from the sky, though extremely fluctuating in our climate, is often very considerable. It may be estimated at 30 or 40 degrees in summer, and 10 or 15 in winter. This secondary light is most powerful when the sky is overspread with thin fleecy clouds; it is feebler, either when the rays are obstructed by a mass of congregated vapours, or when the atmosphere is clear and of a deep azure tint. On the lofty summits of the Alps or Andes, the photometer, screened from the sun, and only exposed to the dark hue of the broad expanse, would indicate a very small effect. During the solar eclipse which took place on the 7th of September 1820, the sky being completely overclouded, it showed, both before and after the passage of the moon's disc, only 12 degrees of light; but when the obscuration was the greatest, it marked not more than a single degree.

The delicacy of this instrument renders it a valuable auxiliary in various scientific inquiries. It ascertains the diminution which the rays of light suffer in reflection, and during their passage through different transparent substances. By combining it with the transerrer of an air-pump, it likewise detects the comparative powers for conducting heat of the several gases, whether in their ordinary state, or when variously attenuated or condensed. Hence we learn, that air expanded 256 times conducts heat nearly twice as slow, or in the ratio of 7 to 13; but that hydrogen gas transfers it with more than redoubled celerity, or in the ratio of 9 to 4. But the photometer will measure also the conducting powers of different liquids. It is only wanted to remove the case, and plunge the instrument erect in a wide metallic vessel, bright on the outside, but blackened within, containing the liquid to be examined, and exposed to the sun's rays. In this way, it was found that water, which conveys heat about 30 times faster than air of the mean temperature of our climate, transmutes it with still greater rapidity if warmed to a higher pitch. If the photometer, enclosed within its case, be immersed under the surface of water, the impression of the light will be much stronger than when the balls were encircled by the actual contact of this liquid. Yet will the effect be less than if the case had been surrounded externally by a body of air instead of water, which, by its powerful action in drawing off the accumulated heat, hastens the transmission of it through the internal medium, and reduces the elevation of the temperature of the black ball to nearly one third part.

From observations made with this instrument, we likewise discover that, in the clearest and most serene sky, one half only of the sun's light, sloping at an angle of 25°, will reach the ground; and that at an angle of 15°, the proportion is reduced to one third; but with an obliquity of 5°, the length of track being then extended ten times, one twentieth part only of the whole incident light can reach the surface. When the sun has approached within a degree of the horizon, and his rays now traverse a track of air equal in weight to a column of about 905 feet of water, no more than the 212th part of them can penetrate to the ground.

The photometer discovers the relative density of different artificial lights, and even contrasts their force of illumination with that of the solar rays. It may be mentioned as a curious inference, that the light emitted from the sun is 12,000 times more powerful than the flame of a wax-candle; or that, if a portion of the luminous solar matter, rather less than an inch in diameter, were transported to our planet, it would throw forth a blaze of light equal to the effect of 12,000 candles.

To compare the illumination of candles or lamps, and of the flame of coal or oil gas, the best form of the photometer is that of fig. 15, Plate CCCLIV., guarded both in front and behind by a pair of thin spreading plates of mica, set parallel at the mutual interval of about half an inch. It may be sufficient to notice at present, that the flame of coal-gas has more than triple the brilliancy of that of a wax-candle.

A photometer of the branched form is easily adapted to measure the diminution which light suffers in penetrating through a body of water. The scale may then be shortened, and the balls enlarged. A bottom of lead is turned to receive the instrument, with its case, which are cemented to it. Thus loaded, the photometer is suspended vertically by cross silk threads, to which a cord of some definite length is attached, terminating by a small bladder. The sky being clear, and the sun shining bright, the instrument is, by the help of a long pole, stretched from the side of a boat, held a few minutes suspended about four inches below the surface of the water, and then drawn up, and the number of degrees marked. When the direct action of the solar rays amounted to 90 photometric degrees, their enfeebled influence on the instrument, while thus encompassed externally by a dense chilling body of water, was commonly found to be reduced to 32 degrees. From this point, therefore, the subsequent diminution, occasioned by the descent of the instrument, was computed. The photometer, being let down, was left to float near a quarter of an hour at the depth of perhaps three or six fathoms. On drawing it up, the diminished action of the light, occasioned by the length of oblique passage, was at once perceived.

From experiments performed in this way last summer, it follows that half of the incident light which might pass through a field of air of the ordinary density and 15½ miles of extent, would penetrate only to the perpendicular depth of 15 feet in the clearest sea-water, which is, therefore, 5400 times less diaphanous than the atmospheric medium. The light is hence diminished four times for every five fathoms of vertical descent; and, consequently, the 64th part only could reach to the depth of 15 fathoms. Supposing the bottom, then, to consist of a clear white sand, the portion of light reflected, and sent back to the surface, would be attenuated more than 64 × 64, or 4096 times, and would therefore hardly be perceptible to the most acute eye. But the water of shallow lakes, though not apparently turbid, betrays a still greater opacity, insomuch that the perpendicular light was diminished one half in descending only through the space of six feet in Loch Leven, or even two feet in a fine artificial sheet of water at Raith, near Kirkaldy. These results, however, are to be considered as mere approximations, the state of the weather having been very unfavourable for such experiments.

It would be easy, by a small modification, to adapt the photometer as a diaphanometer, for measuring the comparative transparency of different collections of water. The black and the clear ball might be blown larger than usual, and the instrument covered with two thin parallel cases of glass, separated by an interval of about three eighths of an inch. The transparency of a lake, or of the sea, would be inversely as the length of passage traversed by the light, when it had suffered a proportional diminution of intensity.

VII. AETHRIOSCOPE.—Such is the name of another very delicate modification of the differential thermometer, intended to measure those frigorific impressions which are showered incessantly from the distant sky. The history of this invention, and of its progressive improvement, has been already given in this work under the article Climate. For about two years few good observations were added, owing to the prevalence of very cloudy and windy weather. The chief object, then, was to render the aethrioscope more portable, in the hope of obtaining, through it, some correct information regarding the state of the atmosphere in other quarters of the globe. The upper ball is now scarcely half an inch in diameter; but to compensate for this diminution, the lower ball has a diameter of about four fifths of an inch. The tube, which exceeds not four inches in length, has its bore contracted, a little above its junction, to the very short cylindrical cavity that holds the coloured liquor. This simple contrivance, augmenting greatly the capillary action, prevents the descent of the column into the ball from any sudden change of temperature, while it only retards the motion of the fluid, without affecting the accuracy of its play. Fig. 10, Plate CCCLV., represents the instrument in this abridged form, and fig. 11 shows the way of packing it, the bottom being merely screwed to the top of the case. The only precaution needed is, not to shake the aethrioscope, or invert it; and as it takes very little room, it may easily be carried by the traveller in his pocket.

In ordinary cases, the hot or cold pulses propagated through the air only assist the energy of the transfer of the different portions of the fluid in promoting an equilibrium of temperature. But the aethrioscope proves that those pulses are incessantly forwarding such a balance, even while the mere transfer and commixture of the medium would not contribute to the effect. In the article Climate, it was shown that the rapid interchange which takes place between the higher and lower strata of the atmosphere maintains an equal distribution in the quantity, and not in the intensity, of heat. Since air has its capacity for heat increased by rarefaction, it must, with the same igneous infusion, indicate a proportionally depressed temperature. But this inequality of temperature, resulting from the internal commotion produced by the sun's rays acting more powerfully near the surface of the earth, is partly corrected by the influence of the cold or hot pulses which are at all times darted, and in every direction, unless obstructed or absorbed by the interposition of the clouds. While the cold pulses from the upper strata of the atmosphere are constantly chilling the lower strata, the warm pulses again, from below are exerted in warming the higher regions. In most cases this mutual influence, indeed, is comparatively feeble; but if the rays of the sun were withdrawn for any considerable time, a great progress would be made by such a mutual interchange of the pulsations towards an equality of temperature through the mass of atmosphere. The lowest strata would become unusually colder, whilst the highest regions would grow warmer, and sparkle with augmented clearness and lustre. Such are some of the effects of the long protracted nights within the arctic circle.

Much yet remains to be explored in the higher strata of our atmosphere. If the differential thermometer, included within the aethroscope, had its position reversed, that instrument would become adapted to measure the hot pulses which are no doubt shot incessantly upwards with various obliquity from the warmer beds incumbent over the surface of the earth. It would be most interesting to obtain the reports of both the erect and the pendant opposite aethroscope, when carried up in the car of a balloon to the elevation of four miles. In that region of mid-air, we might expect the hot and the cold pulses, as they crossed in opposite directions, to act with nearly equal energy. The measures of those effects, compared with the simultaneous indications of the photometer, could not fail to dispel much obscurity, and to open new views of the disposition of the elements, and of the economy of nature.

VIII. CYANOMETER.—This instrument was contrived by M. de Saussure, to measure the variable intensity of the carmine hue which the sky assumes in different climates and elevations, according to the progress of the day or the advance of the season. It consists of fifty-three slips of paper of about a quarter of an inch broad, stained with the successive shades of blue, from the palest sapphire to the deepest azure, which are pasted around the circumference of a circle of pasteboard of about four inches in diameter. The colours were obtained from fine Prussian blue, diluting it with white chalk, or darkening it with a mixture of ivory black. He likewise compared those coloured spaces with the pure tints of a solution of copper in ammonia, which resemble most the soft transparent hues of the atmosphere. To represent the effect of clouds, and diffuse aqueous vapours, he dropped into that liquid a portion of very fine divided argillaceous earth, precipitated by ammonia from a solution of alum.

In observing with the cyanometer, it should be held out of doors, between the eye and the part of the heavens which is to be compared, and, with a little practice, the corresponding tint is easily distinguished.

In this way, Saussure found, that the deepest blue of the zenith on the summit of Mont Blanc, at his station on the Col du Géant, at Chamouni, and at Geneva, corresponded respectively to the shades denoted 39, 37, 34, and 26½. From morning till noon, the colour of the vertical sky darkened, but became lighter again as the evening advanced; and this transition was wider and more rapid in great elevations. On the Col du Géant, the tint of the horizontal air at sunrise was 5, it deepened to 11½ at noon, but again relapsed to 5 towards night. On the 15th of July, which was a very clear day, the atmosphere at the horizon had the 11th shade; at the altitude of 10°, the 20th; at that of 20°, the 31st; at that of 30°, the 34th; at that of 40°, the 37th; and thence with any sensible variation to the zenith. Baron Humboldt, in his voyage from Corunna to Cumana, found the tints of the sky to vary, by the cyanometer, from 13 to 24, and again to 16, while the colour of the ocean fluctuated between 34 and 44.

The misfortune is, that we cannot annex any very distinct ideas to these numbers. We are not informed even of the proportions of the ingredients of the series of colours. The manner of composition likewise will modify the colorific effect; and most of the pigments, and especially the Prussian blue, not only want uniformity of tone, but are subject to great alteration. It would be quite impossible to paint with any water colours two cyanometers that should continue to agree, after being exposed for some time to the action of the air and the sun.

If an accurate method could be devised to discriminate colours, and mark their different tints with a sort of numerical precision, it would prove a valuable acquisition to philosophy and the arts. This was first attempted by the famous painter Leonardo da Vinci. Zahn proposed, in 1702, to accomplish it, by the graduating mixture of primary colours dispersed over the surface of a triangle; but he reckoned five of those colours, including black and white, with red, yellow, and blue. The celebrated Professor Mayer of Göttingen, after various trials, simplified the procedure, in a posthumous work, published by Lichtenberg in 1775. Having distinguished each side of an equilateral triangle into 13 equal parts, he subdivided the whole space into 91 small triangles, which he painted with the successive mixtures of vermilion, ultramarine, and bright orpiment. Lambert assumed three colours, carmine, Prussian blue, and gamboge, to cover a triangular base, upon which he erected a coloured pyramid, having white planted at its apex. But Dr Thomas Young, whose authority in those matters has deservedly great weight, prefers the simple triangle, and adopts red, green, and violet, for the primary colours. Their binary combinations are yellow, formed by mixing red with green; crimson, consisting of red and violet; and blue, produced by blending green with violet. The difficulty, however, is to regulate the intensity of the compounds; nor can the powders be safely mixed except in a dry state, lest some chemical action should be introduced which might alter their tints. But the colours thus combined must evidently want the freshness and brilliancy of those which nature paints, or which the prism reveals.

Air, like water, is, no doubt, by its constitution, a coloured fluid. The former is naturally blue, as the latter is green; but these colours acquire intensity only from the depth of the transparent mass. A small body of limpid water has the appearance of crystal, but, in proportion as it accumulates, it assumes all the successive shades, till it rivals the tints of the emerald and the beryl. This gradation is distinctly seen in the profound lakes of Switzerland, whose lustre is never stained by any vegetable infusion. The same series of colours emerges on receding from our shores and approaching the vast abyss of the Atlantic Ocean. At first, the water on the shelving banks is merely translucent; at the depth of ten fathoms it appears greenish; and the tint, by degrees, becomes more intense, till it passes into a full green at the depth of fifty fathoms; but beyond soundings it darkens almost into azure.

In like manner, the blue shade of the air becomes more intense in proportion to the length of the track of light. Thus we perceive in viewing distant objects, whose colours are always tinted by the deepening hues of the intervening range of atmosphere. The remotest hills seem lost in a cerulean vesture. The mixture of aqueous vapours only diffuses a mist, which tarnishes rather than dilutes the fine blue.

It must be observed, that no substance can disclose its inherent colour, but by a sort of internal secretion or dislocation of the rays of light. The mere reflexion from the surface of a solid body could never betray its tints; for, when rendered most perfect by polish, it would only, like a mirror, send back unchanged the incident beams. To detect the subjacent colour, it is necessary that the particles of light should at least penetrate under the surface, and, after suffering a sort of chemical separation, should be again emitted. In transparent substances, whether solid or fluid, the penetration is greater, but the mode of evolving the native colours must be still the same. The atmosphere, besides dispersing internally the blue rays, likewise reflects in various proportions the white light unaltered. This fact is established by some experiments of polarization, which show that such simple reflexions are the most copious from the portion of the sky which is 90 degrees from the sun, and regularly decline on either side to the opposite points, where they cease altogether.

The white or compound beam of light suffering, in its passage through the air, a continual defalcation of the blue rays, must, as it advances, assume the complementary colour, or the tints of the remaining portions of the spectrum, and therefore merge successively into yellow, orange, red, and crimson. Such, accordingly, are the graduating colours of the solar rays, as they approach to their extreme obliquity. Near sun-setting, the shadow of a pencil along a blank card appears a bright azure on a lilac ground. When a diffuse attenuated vapour reflects the incident light unaltered, the western sky, as the sun declines from his altitude, glows with the successive shades of yellow and orange, which deepen finally into a blush red. These colours again may, under certain circumstances, come to be blended with the natural blue of the atmosphere. Hence the explication of a curious phenomenon, which rarely occurs in this climate, the existence of green clouds. This happens in the mornings and evenings, when a thin cloud is illuminated at once by the yellow rays of the sun and the bright azure of the upper sky, these contrasted colours producing a green by their mixture. For the same reason, sometimes a portion of the bright sky appears, in the finer climates, tinted with violet. This was remarked by Humboldt in his voyage to America, and we have had occasion to observe the same at Avignon. It was no doubt occasioned by the reddish rays of the declining sun dyeing the intense blue of the higher atmosphere.

The easiest and readiest way of ascertaining the tints of different portions of the sky is perhaps to employ a sharp form of wedge of blue glass, of which the base and the parallel posed sides are painted black and cased with thin brass, and the slanting sides are ground to true planes, and highly polished. To these angular surfaces two slides might be adapted, having each a broad slit, or intermediate opening, to permit the entrance and transmission of white light. Such rays having a greater length of passage to traverse, according to their distance from the top of the wedge, must emerge with a proportional intensity of blue. The scale would hence be determined by dividing the slides into ten or twenty equal parts, which might probably be sufficient.

To examine the orange and crimson tints which gild the east in the morning, or suffuse the western sky on the approach of evening, it would be necessary to combine a series of the complementary or accidental colours. A wedge of glass, stained of a gold-red or deep orange, might answer the purpose; or perhaps a nearer approximation would be obtained by joining two reversed wedges, one of an angle of 8°, and crimson coloured, and another having only 3°, but of a yellow body.

These instruments might be reduced to a very convenient size, not exceeding four inches in length; but they would require to be formed out of the same mass of glass and exactly after the same pattern. With some skill in the execution, they could be made to unite elegance and correctness.

IX. ANEMOMETER.—Various attempts have been made Anemometer. to construct an instrument that should readily indicate the force and velocity of the wind. One method was to employ a very small model of a wind-mill, and either to reckon its revolutions, or to estimate its power by the application of a weight to a conical barrel or axis. But a more direct and accurate procedure consisted in measuring the impulse of wind against a vertical plane, as intimated by the contraction of a spiral spring. All these instruments, however, act with such extreme irregularity, as scarcely ever to furnish any definite results. They are, besides, racked by incessant motion, and soon put out of order.

We may notice, however, a material improvement made in this construction of the machine by Mr Waddell of the Trinity-House, Leith, who, amidst other objects of useful experimental inquiry, has long directed his ingenuity to ascertain the force and velocity of the wind. A circular plate, of four inches diameter, is opposed to the blast; but instead of pushing against a spring, it presses against a fine cylindrical bag, of about an inch long, and the third part of an inch diameter, filled with quicksilver, and joined tight to a vertical tube of glass, a foot or fifteen inches high, but having a bore only the twentieth part of an inch wide. The compression of the bag caused by the impulse of the wind upon the plate squeezes the quicksilver up into the tube, carrying with it a small steel mark, which slightly adheres to the sides of the bore. The height of the mercurial column, diminished in the ratio of the surface of the plate to the section of the bag, must evidently give the measure of the force of the wind. But, in the actual exposure of this anemometer, the quicksilver oscillates excessively, so that the extreme effects only are indicated. The instrument, however, is very sensible, and may continue to act for a long period without being impaired.

The direct action of the wind in supporting a column of water appears to furnish the best and simplest kind of anemometer. This principle was first employed, in 1731, by Pitot, the French engineer, in his recurved tube for estimating the force of the current of a river; and, forty years afterwards, it was applied by Dr Lind to measure the impulse of a stream of air. With some modifications to correct, or at least to diminish, the oscillations of the liquid, this instrument is rendered quite manageable. The tube may consist of two pieces, each about a foot in height, having bores of the fifth and the fifth parts of an inch, the narrow piece being swelled out into a cylinder, perhaps an inch wide and two inches long, near the end where it is joined hermetically to the other piece. The top of the narrow tube is bent horizontally, and cemented into the centre of a vertical circle of plate-glass, of about three inches in diameter; or, instead of this plane, a hollow segment of a sphere of the same expansion, but including only 30° or 40°, is substituted. The top of the wide tube is likewise bent horizontal, and drawn to a point at the same height as the minute central orifice of the cavity, and bent in the opposite direction. A portion of nut-oil, tinged by the alkanet root, had been previously introduced into the cylindrical cistern. On turning the small plate or bason to front the wind, a condensation, corresponding to its force, is immediately produced on the opposing surface, a small portion of air enters the orifice, and continues to press upon the oil, till this rises to form an equiponderant column in the wide tube. As the air can with difficulty penetrate through the very narrow bore, the irregular action of the blast is, in a great measure, corrected, and the oil moves rather tardily.

A scale is adapted, bearing two sorts of divisions, the one indicating the impulse, and the other the velocity, of the wind. Reckoning the weight of the atmosphere equivalent to a column of oil of 400 inches in altitude, this space is subdivided into 10,000 equal portions, each degree thus corresponding to the twenty-fifth part of an inch. It would hence be easy to show, that the pressure of the wind upon every square foot of surface is expressed in pounds avoirdupois, by dividing by five the number of degrees through which the oil ascends.

But we may place an adjacent line of subdivisions, that shall mark the velocity of the wind in miles each hour. Since air will rush into a vacuum at the rate of 1350 feet in a second, it would, under a predominating pressure of the 100th part of an atmosphere, or at 100 degrees, flow with a celerity ten times less, or 135 feet in a second, which corresponds to 92 miles in an hour. Therefore, 25 degrees of the scale of impulse would be marked by a velocity of 46 miles, and 64 degrees by that of 23 miles, in an hour. The subordinate divisions could hence be easily formed.

Such are the velocities which theory would assign to the different altitudes of the columns supported by the force of the wind. But the actual resistance of fluids, owing chiefly to their detention at the obstructing surface, generally exceeds the result of calculation. In the case of water and air, the ratio of excess appears from experiment to be nearly that of eight to five. We may therefore modify the velocities after this proportion above stated. The relations of celerity and impulse will stand thus:

| Celerity in Miles per Hour | Impulse in 10,000th Parts of the Weight of the Atmosphere | |---------------------------|----------------------------------------------------------| | 10 | 1°·9 | | 20 | 7°·6 | | 30 | 17°·1 | | 40 | 30°·4 | | 50 | 47°·5 | | 60 | 68°·4 | | 70 | 93 | | 80 | 121°·5 |

This anemometer, being furnished with a vane to make it always face the wind, might also, by an index, point out the direction. Nor is it absolutely requisite that the instrument should be exposed out of doors. The funnel, with its vane, may have a socket of bell-metal, nicely fitted to the top of a long perforated brass tube, which descends from the roof of the house, and terminates below in the recurved tube and its double scale. The impressions of the wind would thus be conveyed with great regularity and undiminished effect to the surface of the oil. Still, however, it would be impossible to avoid entirely the oscillations of the liquid column. Even the steadiest wind will be found to blow with a reciprocating force, now swelling and again relaxing, and, at certain short intervals, concentrating all its vehemence.

X. OMBROMETER, or RAIN-GAUGE.—A very simple instrument, contrived to indicate the quantity, or rather the depth, of the rain which falls upon any spot. It is likewise named Hyetometer, and has been sometimes called by the barbarous compound Pluviometer. It is composed generally of a circular bason of tinned iron, soldered to the top of a vertical cylinder, which is contracted in some given proportion, and closed below. A small float is introduced, bearing a slender rod, distinguished by the corresponding divisions. In the most ordinary construction,

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1 From ὑπέρ, rain, and μέτρον, a measure. the basin being a foot wide, the attached cylinder has 3½ inches in diameter, and its section is consequently ten times smaller. The inches on the rod are hence marked only tenth parts.

This method of measuring the fall of rain is evidently not susceptible of much accuracy; and it would require the gauge to be very frequently visited, on account of the loss of the water by continual evaporation. The more correct ombrometers have their basin made of brass, and turned to a fine sharp edge; the rain, as it falls, runs through a small orifice into the vertical cylinder, which has only about the fourth part of the diameter, and communicates, by means of a cock, with another perpendicular tube still narrower, and consisting of glass having a scale affixed. The divisions of this scale are determined from the proportion of the joint sections of the cylinder and tube to the horizontal surface of the basin. In making an observation, the cock, being turned, lets the collected water rise to the same level in the glass tube, and thus indicate its quantity, which, by another operation, is now drawn off. In the time of frost or snow, it becomes necessary to warm the instrument gently, and make the water flow.

We may suspect that the measure of the rain, hail, or snowy flakes received by the ombrometer, is not exactly proportioned to the extent of surface which it presents; for, while torrents pour down from the heavens, an eddy plays about the rim of the basin, deranging the regularity of the discharge. A basin of several feet in diameter would perhaps be preferable, or the platform of a roof could be adopted, if it were sufficiently sloped to allow the rain to collect quickly.

But the most perplexing circumstance affecting the ombrometer is, that it has been found to indicate very different quantities of rain as falling upon the very same spot, according to the different elevation at which it was placed. In general, less rain is collected in high than in low situations, even though the difference of altitude should be inconsiderable. Thus it was discovered, that, in the space of a year, while 12½ inches only fell on the top of Westminster Abbey, 18½ inches were collected on the roof of a house sixteen feet lower, and even 22½ inches of rain at the ground. Similar observations have been made at the summit and near the base of hills of no great elevation. In such situations, we can hardly suppose the clouds to stretch down to the surface, or to augment the lower portion of rain. We must hence refer the copious fall near the ground to some other cause. Most of the rain which falls proceeds from drifting showers of short duration. The current moves more slowly along the surface, and allows the drops to descend as fast as they are formed. But being forced to mount a swelling eminence, and thus compressed into a narrower stream, it hurries the mass of vapour along with it, and does not suffer the free or full discharge on the summit. On both sides of the hill, an ombrometer placed near the bottom indicates always a greater fall of rain than on the exposed top.

The observations furnished by this instrument are hence liable to considerable inaccuracy, unless made in an open hampaign country. Thus, a register kept at Keswick gives 57½ inches, which evidently exceeds greatly the annual fall of rain in that district; the quantity at Carlisle, not twenty-five miles distant, being only 20 inches. Again, the measures of rain, being 33 and 34½ inches in the open country about Manchester and Liverpool, are found to amount to 45 and 40 inches at Lancaster and Kendal, which approach the banks of a mountainous range. In general, twice as much rain falls on the western as on the eastern side of our island; and the average annual quantity may be reckoned at 30 inches, or it would form, if all collected, a sheet of water of that depth. According to this estimate, the whole discharge from the clouds in the course of a year, on every square mile of the surface of Great Britain, would, at a medium, be 1,944,648, or nearly two millions of tons. This gives about three thousand tons of water for each English acre.

XI. ELECTROMETER, which detects the electrical state of the lower atmosphere. The best instrument of this meter kind undoubtedly is Bennet's, consisting of two slips of thick gold leaf suspended from a knob within a small cylinder of glass, which is surmounted by a cap of brass. This may be connected with an insulated rod or wire, extending a few feet beyond the window.

The electrometer indicates the condition of the air only in its immediate vicinity. But when thunder-storms prevail, the atmosphere becomes affected to a very considerable extent. Yet the indications of the electrometer are often capricious and evanescent. Whenever clouds are suddenly formed, or melt away, whether the air changes to dryness or humidity, the electrical equilibrium is disturbed. The observations made with the electrometer are hence of much less importance than was once expected, and have been gradually falling into neglect.

XII. DROSOMETER.—An instrument so called was proposed by Weidler, a German professor, in 1727, to measure the quantity of dew which gathers on the surface of a body which has been exposed to the open air during the night. It consisted of a bent balance, which marked in grains the preponderance which a piece of glass of certain dimensions, laid horizontally in one of the scales, had acquired from the settling and adhesion of the globules of moisture.

The main objection to a drosometer of such a construction is, that it would require to be protected from the action of the wind; and being thus screened, it could not receive the whole of the dew which might otherwise have been deposited. The steel beam, too, from continual exposure to the weather, would soon lose its polish, and become unfit for any accurate performance. Besides, it is in general easier to measure than to weigh a portion of liquid.

A simpler and more convenient drosometer could be formed on the principle of the ombrometer, or rain-gauge. Suppose a glass funnel, of about three inches diameter, whose interior surface is very smooth, and slopes towards the centre at an angle of 15 or 20 degrees, to be joined hermetically to a long tube, sealed at the lower end, and having an equable bore not exceeding the quarter of an inch, with an attached scale divided into portions corresponding to the thousandth parts of an inch on the external aperture. The only difficulty is to make the dew which gathers during the night to run down the sides of the funnel into the tube. To facilitate this descent, a coat of delicate salt of tartar may be spread with a hair pencil over the shallow surface, and renewed as often as occasion requires. The dew, instead of settling in minute detached globules, would then be attracted by the alkaline lye, which thus, becoming dilute, would gradually flow into the narrow cavity of the tube. It would be easy at any time to make an allowance for the very small portion of liquid alkali blended with the dew.

Such is the complete apparatus required for keeping a meteorological register. But those instruments are not all of equal importance. The barometer, the thermometer, and the hygrometer, may be considered as indispensable. Next to them deserve to be ranked the photome-

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1 From *littera*, *amber*, and *pitter*, a measure. 2 From *drosa*, *dew*, and *pitter*, a measure. Accurate registers kept in such towering spots are peculiarly interesting.

Light-houses would, from their usual position, be well fitted for observing the force and direction of the wind, and the swell and relapse of the tide. The navigators who traverse the ocean in every latitude might, besides keeping meteorological soundings, record the variation of the needle, and examine the intensity of magnetic attraction.

To promote the science of meteorology, it would be most expedient that the various learned associations planted in different parts of the globe, should institute inquiries into the state and internal motions of the higher strata of our atmosphere. As the ultimate results could not fail to prove advantageous to the public, the several governments might be expected to defray the moderate expense incurred in carrying this plan into effect. Light small balloons might from time to time be launched towards the most elevated regions, to detect by their flight the existence and direction of currents which now escape observation. Barometers, thermometers, hygrometers, and perhaps athrioscopes, in compact forms, and which should register themselves, might be sent up in the car. Observers, furnished with accurate and complete instruments, could likewise be despatched occasionally to the intermediate heights in large balloons. By clasping the various meteorological journals, and combining those ulterior facts, some new lights could not fail to be struck out, which would gradually reveal that simple harmony which no doubt pervades all the apparent complication of the universal frame. Till we obtain such insight, we must content ourselves with the best explications of the phenomena of the atmosphere, which our imperfect and limited knowledge will admit. We shall, therefore, treat in succession of the origin of winds; the generation of clouds and fogs; and their precipitation in the form of rain, snow, or hail. Other collateral objects will be discussed as they present themselves to view.

I. Wind.—It is a curious circumstance, that in all languages the ordinary name of air refers to its mobility, and merely signifies to blow. This impulse alone of the fluid appears to have awakened our sensations; and had the atmosphere continued perfectly still, we might for ages have remained ignorant of the very existence of the fluid which we breathe.

The main cause of wind, or the flow of air, is undoubtedly the variable distribution of heat through the atmosphere, which incessantly affects the local density, and disturbs the equilibrium of the mass. The presence of the sun affects the surface of the terraqueous globe, which again warms and dilates the lower strata of atmosphere. The calorific action of the solar beams is greatly diminished by their obliquity; it rapidly accumulates on the land, but becomes attenuated and diffused in the waters of the ocean.

The alternation of day and night, and the annual revolution of the seasons, are hence the perpetual sources of winds. If the surface of the globe, however, had been wholly covered by the ocean, and not dispersed by land and seas, those winds must have been scarcely perceptible. The daily illumination of the sun does not warm the ground to the depth of an inch; but the same quantity of illumination penetrates, though with a decreasing intensity, many fathoms into water, spreading and dividing its influence. We may reckon the tenth part of the incident light to be intercepted by a superficial stratum of the thickness of one foot; and it will hence follow, that the solar beams communicate every day a hundred times less heat to the surface of a body of water than to an expansion of level ground. The subsequent influence, again, of those contrasted surfaces in warming the incumbent air must be proportionally different, though slightly modified. by the portion of light reflected from the water. In a general view, the diurnal variation of temperature in the atmosphere may be considered as limited to the lowest stratum, not exceeding 2000 feet in height. Such a body of air will intercept commonly the fiftieth part of all the light which traverses it. We may hence conclude, that the range of temperature in the air, caused by the succession of day and night, is, on the whole, about thirty times less above a spacious lake than over the surrounding land.

The current which rushes from all sides towards any heated portion of the atmosphere is easily explained and computed from the diminution of pressure which rarefaction produces at that place. The celerity of the flow is precisely the same as that of the efflux from a small aperture under the pressure of a column equivalent to that diminution, or to the difference between the weight of the warm air and of an equal volume of the exterior fluid. Thus, suppose a chimney 20 feet high were heated up 50 centesimal degrees, or 90 on Fahrenheit's scale; the air in the flue being therefore expanded one fifth-part, or four feet, would be driven upwards by the pressure of a column compensating this difference. The velocity of the discharge would hence be $8 \sqrt{4} = 16$ feet every second, or at the rate of about eleven miles in the hour. If a fire is kindled in an open field, it is evident that the rush of air must proceed from all sides. At the spot itself, therefore, the opposite currents will produce a counterbalance, and no dominant wind can prevail. But if the warm air should cover a very wide extent of surface, its influence may be felt at a great distance, and the several converging winds may have space to blow without any mutual interference.

These views afford a complete explication of the phenomena of sea and land breezes, which are occasionally felt in every latitude, but are constantly observed near the shores of the continent, and of the larger islands within the tropics. In those sultry regions, as the day advances, a refreshing wind blows from the sea, and is succeeded by an opposite current from the interior of the land on the approach of evening, and during a great part of the night. In open seas, and especially near the equator, the thermometer scarcely varies a degree, and very seldom two degrees, by Fahrenheit's scale, in the whole course of a day. But on the land, the change of temperature between the night and day, in similar situations, will rise often higher than 76 centesimal degrees, or 126 Fahrenheit. If we, therefore, conceive a stratum of air 200 feet in altitude, heated only to the mean difference, three centesimal degrees, it would receive an expansion of 24 feet; whence the velocity of the wind produced would be $8 \sqrt{24} = 39$ feet every second, or at the rate of 22 miles in the hour. This is a very moderate estimate; but the celerity of the current must, no doubt, be diminished, from the retardation which it suffers in proportion to the length of track over which it has to sweep.

During the night the lower atmosphere is colder on land than at sea, owing to the descent of the more elevated and colder portions of air which chill the surface of the ground, and partly to those cold pulses which are incessantly darted from every point of the azure sky. If we reckon the reduced temperature of the land only a centesimal degree and a half below the standard of the adjacent ocean, this would give 12 feet for the contraction of the vertical column of air, and, consequently, a stream would flow towards the sea with a celerity of $8 \sqrt{12} = 28$ feet per second, or very nearly 20 miles every hour. In general, the land breeze may be considered not so powerful as what blows from the sea.

The ordinary appearances are clearly and graphically described by that very intelligent and enterprising navigator Captain Dampier.

"These sea-breezes do commonly rise in the morning about nine o'clock, sometimes sooner, sometimes later. They first approach the shore so gently, as if they were afraid to come near it; and oftentimes they make some faint breathings, and, as if not willing to offend, they make a halt, and seem ready to retire. I have waited many a time, both ashore to receive the pleasure, and at sea to take the benefit of it.

"It comes in a fine, small, black curl upon the water, when as all the sea between it and the shore not yet reached by it is as smooth and even as glass in comparison. In half an hour's time after it has reached the shore it fans pretty briskly, and so increaseth gradually till twelve o'clock; then it is commonly strongest, and lasts so till two or three a very brisk gale. About twelve at noon it also veers off to sea two or three points, or more, in very fair weather. After three o'clock it begins to die away again, and gradually withdraws its force till all is spent, and about five o'clock, sooner or later, according as the weather is, it is lulled asleep, and comes no more till the next morning.

"These winds are as constantly expected as the day in their proper latitudes, and seldom fail but in the wet season. On all coasts of the main, whether in the East or West Indies, or Guinea, they rise in the morning, and withdraw towards the evening; yet capes and headlands have the greatest benefit of them, where they are highest, rise earlier, and blow later.

"Land-breezes are as remarkable as any winds that I have yet treated of; they are quite contrary to the sea-breezes; for those blow right from the shore, but the sea-breeze right in upon the shore; and as the sea-breezes do blow in the day and rest in the night, so, on the contrary, these do blow in the night and rest in the day, and so they do alternately succeed each other. For when the sea-breezes have performed their offices of the day, by breathing on their respective coasts, they in the evening do either withdraw from the coast, or lie down to rest. Then the land-winds, whose office is to breathe in the night, moved by the same order of divine impulse, do rouse out of their private recesses, and gently fan the air till the next morning; and then their task ends, and they leave the stage.

"There can be no proper time set when they do begin in the evening, or when they retire in the morning, for they do not keep to an hour; but they commonly spring up between six and twelve in the evening, and last till six, eight, or ten in the morning. They both come and go away again earlier or later, according to the weather, the season of the year, or some accidental cause from the land; for on some coasts they do rise earlier, blow fresher, and remain later, than on other coasts, as I shall show hereafter.

"These winds blow off to sea, a greater or less distance, according as the coast lies more or less exposed to the sea-winds; for in some places we find them brisk three or four leagues off shore, in other places not so many miles, and in some places they scarce peep without the rocks, or, if they do sometimes in very fair weather make a sally out a mile or two, they are not lasting, but suddenly vanish away, though yet there are every night as fresh land-winds ashore at those places as in any other part of the world.

"Indeed, these winds are an extraordinary blessing to those that use the sea in any part of the world within the tropics; for as the constant trade-winds do blow, there could be no sailing in these seas; but, by the help of the sea and land breezes, ships will sail 200 or 300 leagues, as particularly from Jamaica to the Lagune of Trist, in the Bay of Campeachy, and then back again, all against the trade-wind." "The seamen that sail in sloops or other small vessels in the West Indies do know very well when they shall meet a brisk land-wind, by the fogs that hang over the land before night; for it is a certain sign of a good land-wind to see a thick fog lie still and quiet, like smoke over the land, not stirring any way; and we look out for such signs when we are plying to windward. For if we see no fog over the land, the land-wind will be but faint and short that night. These signs are to be observed chiefly in fair weather; for in the wet season fogs do hang over the land all the day, and it may be neither land-wind nor sea-breeze stirring. If in the afternoon, also, in fair weather, we see a tornado over the land, it commonly sends us forth a fresh land-wind.

"These land-winds are very cold, and though the sea-breezes are always much stronger, yet these are colder by far. The sea-breezes, indeed, are very comfortable and refreshing; for the hottest time in all the day is about nine, ten, or eleven o'clock in the morning, in the interval between both breezes; for then it is commonly calm, and then people pant for breath, especially if it is late before the sea-breeze comes, but afterwards the breeze allays the heat. However, in the evening again, after the sea-breeze is spent, it is very hot till the land-wind springs up, which is sometimes not till twelve o'clock or after." (Voyages, vol. ii.)

The Trade-Winds, which, within the tropics, at all times constantly blow from the east, but somewhat vary their force, and decline a little to the north or the south, according to the latitude and the season, are the most remarkable of all the aerial currents, and of signal importance in navigation. These steady breezes favoured the voyage of Columbus, and conducted him to the discovery of the Mexican archipelago. The same powerful stream afterwards drew the Portuguese from their southern course, and carried them to the shores of the Brazils. Since the character and extent of those winds have become perfectly known, the navigator reckons safely on their aid, and shapes his voyage in such a way as to reduce its performance almost to a calculation.

The cause of the trade-winds, however, is not obvious, or very easily traced. Various attempts have been made to explain the phenomenon, yet seldom on any solid or accurate principles. It would form an interesting discussion to examine the different hypotheses advanced; but we can afford room to notice, very briefly, the more considerable only of those opinions.

Descartes and his followers imputed the trade-winds to the inertia of the atmosphere, which they conceived to prevent this fluid from acquiring the full rotation of the earth, especially near the equator. The air being thus left behind as the globe rolled from the west, would have an apparent motion in the contrary direction, and seem to blow from the east. But it may be urged, that as passengers almost insensibly gain the celerity of the ship which carries them, so every portion of the incumbent atmosphere, though more loosely adherent to the terraqueous surface, must soon acquire the peculiar motion corresponding to the parallel of latitude. Nor would the inequality of such combined movements in the air at all disturb the order and arrangement of its general mass.

Dr Halley gave a different explication of the origin of hypothesis, trade-winds, which seems very plausible, and has long been deemed quite satisfactory. This able philosopher and experienced navigator supposed, that the spot where the sun's vertical rays exert their utmost heating energy, being in the lapse of a day successively transferred from east to west round the circumference of the globe, must, as a centre of confluence, draw in its train a current of air. The current thus formed would result from the excess of streaming from the east above that from the west; and it would therefore advance with a tardy pace, following at a distance the powerful energy of the sun. The same easterly wind might incline towards the north or the south, according as the great luminary appears to approach to the northern or the southern tropic.

But it should be observed, that the torrid zone stretches mostly over the ocean, and includes only a narrow portion of land. The heat excited in succession through that liquid track, by the diurnal passage of the sun, is hence extremely small, and hardly sufficient to produce the aspiration of the gentlest air. Nor could even this feeble current have a decided and constant direction. It would only tend towards the heated part of the surface of the ocean. In the morning, it would breathe from the west; about noon, it would become neutral, and die away; and, in the evening, it would again spring up, and flow from the east. Near midnight this current would sink into a perfect calm. The hypothesis will, therefore, not bear any strict examination. It is neither adequate to the production of such effects, nor accordant with the actual phenomena of the trade-winds. It casts a false glare over the subject, without elucidating its real bearings.

The first who succeeded in taking a correct view of the question was George Hadley, in a short paper inserted in the Philosophical Transactions for 1735. By combining in some measure the idea of Descartes with the opinion of Halley, he produced a clear and simple account of the cause of trade-winds, which appears entirely consistent, and free from every objection. Though the daily variation of temperature be very inconsiderable within the tropics, yet the annual accumulation of heat renders the equatorial regions much warmer than the higher latitudes, and consequently maintains a perpetual current of air from either side. If those aerial motions were not modified by the figure and rotation of the globe, there would always be two opposite winds blowing directly from the north and from the south to the equator. But the stream which perhaps originates at the northern tropic, in advancing to the equator, must seem gradually to deflect towards the west, in consequence of the increasing velocity with which the successive parallels of latitude are carried eastwards. During the time this current takes to perform its journey, it is apparently transported to the west, through a space equal to the excess of the arc described by the equator above the corresponding arc traced by the tropic. The current from the southern tropic is equally bent towards the west. When both of them meet at the equator, their opposite impulsions from the north and the south are extinguished, and they flow directly west in a single united stream, and with accumulated force. The apparent motions of the different streamlets which from both hemispheres conspire to constitute the trade-wind, is represented in fig. 18, Plate CCCLV.

But it is not enough to connect the general facts; a complete theory should harmonize in all the subordinate details. An easy calculation, accordingly, is conducted to those precise results which are commensurate and exactly congruous with the actual phenomena. The trade-wind may be reckoned to begin about the latitude of 25 degrees. At this parallel, the mean temperature is four centesimal degrees colder than immediately under the equator, which difference of heat may graduate through the atmosphere to the altitude of 10,000 feet. Wherefore, the expansion of the air at the equator, which draws to it a meridional wind, will amount to a column of 100 feet. The velocity of the current hence produced must be $8\sqrt{100}$, or 80 feet every second, which corresponds to fifty-four miles in the hour. But each point on the parallel of 24° is carried eastwards by its revolution about the earth's axis seven miles faster every hour than on the parallel of 25°. Consequently, when the wind arrives at the parallel of 24°, it will seem to have acquired a tendency of seven miles an hour to the west. As it reaches the successive parallels of 23°, 22°, 21°, &c., it will gain continual, though decreasing, additions to its apparent westerly course, which at the equator will have augmented to 104 miles in the hour.

In this calculation we have made no deduction for the resistance which the streams of air must experience in creeping over the surface of the globe, because no experiments have been made to ascertain the effect of such retardation. It is no doubt less on the ocean than on the land, and must evidently be diminished in proportion to the depth of the mass of fluid which is borne along. Still, however, this obstruction, joined to this impediment of internal motion, must be very considerable; and we may safely reduce the numbers before stated to one third, which would give eighteen miles an hour for the celerity of the primary meridional wind, and thirty-five miles for that of the oriental or trade-wind, resulting from the influence of the figure and rotation of the earth.

Our northern hemisphere, presenting to the action of the solar beams a larger surface of land than the southern, on the whole, rather warmer. Hence the parallel of greatest heat runs not exactly through the equator, but about three degrees farther north. This circle is therefore strictly the mean path of the aggregate easterly streams of air.

But though the hottest part within the torrid zone, taking the average of a whole year, occupies the parallel of three degrees north latitude, it must, to a certain extent, lift its position with the seasons. In the summer months the sun shines twice vertical upon the tropic of Cancer, and consequently raises the temperature of the northern half of the zone. During winter, again, this effect is transferred to the southern half of the torrid region. In the progress of summer, therefore, the trade-wind gradually tends about a point towards the north; but as winter advances, it declines as much to the south.

Such is the character of that general wind which encircles the globe, flowing with slight deviation constantly from the east, and spreading over a zone of more than 50° in breadth. It sweeps the Atlantic Ocean from the coast of Africa to Brazil, and the Pacific from Panama to the Philippine Isles and New Holland, and again the Indian seas partially from Sumatra to Zanguebar.

The trade-wind undergoes an essential modification, however, where the continent stretches into the torrid zone. The sun acting more powerfully upon the land than upon the surface of the sea, the accumulated warmth is much greater, and shifts with the revolution of the seasons to either side of the equator. The centre of heat approaches in summer to the northern, and in winter to the southern tropic. Instead of the great eastern stream, those regions have two opposite periodic winds alternating towards the north and the south, and called the monsoons. Then these winds advance to the equator, they conjoin a apparent easterly velocity; but when they recede from the equator, they carry their excess of velocity from the west. A diagonal motion results from the combined tendencies. In the Arabian and Indian Seas, on the north side of the equator, the monsoon blows north-west during the summer months, from April to October; and in the opposite direction, or south-east, during the winter. But on the south side of the equator, near Java and Sumatra, the course of the monsoon is north-east in summer and south-west in winter.

The primary winds, which blow from the parallels of 25° to 30° to the equator, must evidently give rise to opposite currents that flow in the higher atmosphere towards the poles. These streams, after they have travelled beyond the tropics, may descend to the surface, transporting the velocity of equatorial rotation. They will appear, therefore, to blow from the western quarter, with the excess of their previous velocity above that of the parallel which they reach. Hence a westerly breeze, of considerable force and regularity, prevails in either hemisphere above the latitude of 30°. The same winds cross the Atlantic from Newfoundland to Cornwall, and traverse the Southern Ocean from the Plata to the Cape of Good Hope, and thence to New Holland. Any wind which blows from the quarter inclining to the south of the west comes really from the equatorial region, and is therefore relatively warm. Such is the disposition of our westerly winds, which commonly prevail for nine months in the year.

On the same principle, a wind which blows directly from the arctic pole, and impregnated with intense cold, must, in consequence of the rotation of the globe, appear to arrive from some point to the north of the east. In passing through the first degree of latitude, it will suffer a deflection of eighteen miles in the hour towards the west; in a short space, therefore, it will seem to flow with impetuous force, and almost directly from the east. Hence our easterly and north-easterly winds have a polar origin, and are always bitterly cold.

Local winds could be explained if the different circumstances which affect them were distinctly known. The latitude and temperature of the place, its relative position, the figure and contour of the surrounding country, would all enter into the calculation. We shall content ourselves with a concise notice of some peculiar winds. The Bise is a cold piercing wind which blows from the ridge of the Jura, and the frozen summits of the Pyrenees. The Sirocco is a hot, moist, and relaxing wind, which visits Naples and the south of Italy from the opposite shores of the Mediterranean. The Harmattan seems to be a cold and dry wind, of a very parching quality, which is frequent in Africa and some of the eastern countries. The Saimel, or Simoom, is a burning pestilential blast, extremely arid, which springs up at times in the vast deserts of Arabia, and rushes with tremendous fury, involving whole pillars of sand.

II. CLOUDS AND RAIN.—Their formation and dissolusion produce all the varied train of the meteorological phenomena. The humidity suspended in the atmosphere is derived by exhalation partly from the land, but ultimately from the vast expanse of the ocean. A surface of lake, of pasture, corn fields or forest, supports a continual evaporation, augmented only by the dryness of the air, and the rapidity of its successive contacts. Even ploughed land will supply nearly as much moisture to the exhaling fluid as an equal sheet of water. It is only when the ground has become quite parched that it obstinately retains its latent store.

If the whole of the waters which fall from the heavens were to return again, the evaporation from the ground might be sufficient alone to maintain the perpetual circulation. But more than one third of all the rains and melted snows are carried by the rivers into the ocean, which must hence restore this continued waste. The commerce of land and sea is thus a necessary part of the economy of nature.

The air, in exhaling its watery store, is rendered quite damp; but it may afterwards become dry on being transported to a warmer situation. Such is the case of the sea-breeze, particularly in summer. It arrives on the shore cold and moist; but as it advances into the interior of the continent it grows milder and drier. The same principle accounts for the disposition of different winds in respect to humidity. At Colombo, in the island of Ceylon, as we gather from some remarks of Dr Davy, the north-east monsoon, with a temperature of only 68 on Fahrenheit's scale, has yet a dryness of 75 hygrometric degrees; but the opposite monsoon, from the south-west, though at 82 by the thermometer, is so damp as to indicate scarcely 30 degrees. The cold wind, coming from the north, was rendered warmer and drier in its progress; while the hot wind, flowing from the equator, was somewhat chilled and made damper as it approached Ceylon.

Since air in mounting upwards has its capacity for heat enlarged, and becomes colder, it will hence likewise grow proportionally damper. But a continual intercourse being maintained between the lower and the higher atmosphere, the middle region must, from its chillness, be soon charged with moisture. If this tendency were to act, therefore, without control, the heavens would have been shrouded with perpetual clouds and darkness, and never could the cheering rays of the sun have visited the surface of the earth. A principle of conservation happily occurs to restrain, and finally to overpower, the effect of cold, in disposing air to part with its moisture. By expansion, this fluid is made capable of holding, at the same temperature, a larger share of humidity. Each portion of air, in rising vertically, grows, from the predominance of cold, constantly damper; but after having reached a certain altitude, it again becomes gradually drier, from the influence of its wide dilatation. Every time the air has its volume doubled, it acquires an additional dryness corresponding to fifty hygrometric degrees. Hence one degree would be the effect of the rarefaction of only the 72d part. This small variation again answers to a depressed temperature of $1\frac{1}{9}$ on the centesimal scale, which, near the surface, will occasion an increase of humidity equal to the actual range of the solvent power of the air divided by 314. Suppose the thermometer to mark 15° centesimal at the ground, the air would, for each ascent of about 390 feet, be $\frac{200}{314}$ or 8 hygrometric degrees damper, which would be reduced to 7° by the influence of dilatation. Had the temperature at the surface been as low as — 25°, which answers to a solvent power of 314°, the opposite agencies of cold and rarefaction would evidently have produced a perfect balance, and the same dryness would have continued to a moderate height.

It would be easy to show that $d$, expressing the density of the air at any altitude, and $h$ the corresponding indication of the hygrometer, $\frac{h}{628} \left( \frac{1}{d} + d \right)$ will denote the increment of humidity occasioned by depressed temperature, while the corresponding decrement resulting from expansion is one degree. Hence at the pole the position of the maximum humidity in the atmosphere must occur at an elevation of 13,300 feet; where the density is -6, the temperature would stand at 26°7', and the hygrometric range only 29°. Under the equator, that limit would attain a much greater altitude, and yet not rise so far above the curve of perpetual congelation. It is probable, however, that the canopy of clouds descends considerably lower, being warmed by the hot pulses darted from the ground and the inferior strata of the atmosphere. We shall not err much if we estimate the position of extreme humidity at the height of two miles at the pole, and four miles and a half under the equator, or a mile and a half beyond the limit of congelation. This range is represented in fig. 18, Plate CCCLV., running nearly parallel to the curve of perpetual congelation, but bending nearer in approaching the equatorial parts. It marks the mean height of the clouds in different latitudes, and intimates the shading into the fine etherial expanse.

The moisture deposited by a body of air in minute globules, which remain suspended, or subside slowly in the atmosphere, constitutes a Cloud. When it comes near us, whether it hovers on the tops of the hills, or spreads over the valleys, it receives the name of a Fog. The cold occasioned by the ascent or transfer of air may be sufficient to form thin clouds, but a more powerful and extended energy is required for the production of Rain. The subject has from the earliest times engaged the attention of philosophers, who have made numerous unavailing attempts to explain it. At length the very ingenious Dr James Hutton subjected the problem to a correct analysis, and succeeded in deducing a most satisfactory solution. His fine Theory of Rain, which first appeared in the Transactions of the Royal Society of Edinburgh for the year 1787, constitutes an epoch in meteorological science. Its merits, however, have been slowly perceived by the public, because the author, full of his original conception, satisfied himself with merely sketching the general outline. But it was not enough that the operation of the principle advanced should always cause rain; it was farther requisite that the results arising from its application should quite accord with the actual phenomena. We shall, therefore, endeavour to render this theory more definite and more complete.

Air in cooling becomes ready, we have seen, to part with its moisture. But how is it cooled in the free atmosphere, unless by the contact or commixture of a colder portion of the same fluid? Now, the portion of the air which is chilled must in an equal degree warm the other. If, in consequence of this mutual change of condition, the former be disposed to resign its moisture, the latter is more inclined to retain it; and, consequently, if such opposite effects were balanced, there could, on the whole, be no precipitation of humidity whatever. The separation of moisture, on the mixing of two masses of damp air at different temperatures, would therefore prove, that the dissolving power of air suffers more diminution from losing part of the combined heat, than it acquires augmentation from gaining an equal measure of it; and, consequently, this power must, under equal accessions of heat, increase more slowly at first than it does afterwards, thus advancing always with accumulated celerity.

The quantity of moisture which air can hold thus increases in a much faster ratio than its temperature. This great principle in the economy of nature was traced by Dr Hutton from indirect experience. It is the simplest of the accelerating kind, and perfectly agrees with the law of solution which the hygrometer has established. Suppose equal bulks of air in a state of saturation, and at the different temperatures of 15° and 45° centesimal degrees, were intermixed, the compound arising from such union will evidently have the mean temperature of 30°. But since at these temperatures the one portion held 200 parts of humidity, and the other 800, the aggregate must contain 1000 parts, or either half of it 500; at the mean or resulting temperature, however, this portion could only suspend 400 parts of humidity, and, consequently, the difference, or 100 parts, amounting to the two hundredth part of the whole weight of air, must be precipitated from the compound mass.

As another illustration, let air of 15° be mixed with air at the temperature of 35°, in three different proportions, all at the point of saturation; one part being combined with three parts, two with two, and three with one. The temperatures arising from the commixture would be 20°, 25°, and 30°; the corresponding parts of moisture precipitated from the mass being derived from the intermediate proportions of 200 and 504, are, 352, 317-5 or 34-5; 276, 252 or 24; 352, 317-5 or 34-5; and 428, 400 or 28. These depositions are represented in fig. 8, Plate CCCLV., by the several intervals between the logarithmic curve and the oblique line which connects the summits of the ordinates of 15° and 35°.

In these examples we have assumed the portions of differently heated air to be quite charged with moisture be- fore mixing; but it is only required that they should approach to the point of humidity. The effect, however, of simple commixture would, in most cases, be very small. To explain the actual phenomena, we must have recourse to the mutual operation of a chill and of a warm current, driving swiftly in opposite directions, and continually mixing and changing their conterminous surfaces. By this rapidity, a larger volume of the fluid is brought into contact in a given time. Suppose, for instance, the one current to have a temperature of 50°, and the other that of 70 degrees, by Fahrenheit's scale; the blending surfaces will, therefore, assume the mean temperature of 60°. Consequently, the two streams throw together 200 and 334-2 parts of moisture, making 567-1 parts for the compound, which, at its actual temperature, can hold only 258-6 parts; the difference, or 8-6 parts, forms the measure of precipitation, corresponding to the 2325th of the whole weight of the commixed air. It would thus require a column of air 30 miles in length to furnish, over a given spot, and in the space of an hour, a deposit of moisture equal to the height of an inch. If the sum of the opposite velocities amounted to 60 miles an hour, and the intermingling influence extended but to a quarter of an inch at the grazing surfaces, there would still, on this supposition, be produced in the same time a fall of rain reaching to half an inch in altitude.

These quantities come within the limits of probability, and agree sufficiently with experience and observation. But, in the higher temperatures, though the difference of the heat between the opposite strata of air should remain the same, the measure of aqueous precipitation is greatly increased. Thus, while the mixing of equal masses of air, at the temperatures of 40° and 60°, is only 6-6, that from a like mixture at 80° and 100° amounts to 19°. This result is entirely conformable to observation, for showers are most copious during hot weather and in the tropical climates.

The quantity of moisture precipitated from the atmosphere thus depends on a variety of circumstances; on the previous dampness of the commixed portions of the fluid, their difference of heat, the elevation of their mean temperature, and the extent of the combination which takes place. When this deposition is slow, the very minute aqueous globules remain suspended, and form clouds; but if it be rapid and copious, those particles conglomerate, and produce—according to the state of the medium with regard to heat—rain, hail, or snow.

The profuse precipitation of humidity is caused by a rapid commixture of opposite strata. The action of swift contending currents in the atmosphere brings quickly into mutual contact vast fields of air over a given spot. The separation of moisture is hence proportionally copious. In temperate weather this deposition forms rain; but, in the cold season, the aqueous globules, freezing in the mid-air into icy spiculae, which collect in their slow descent, become converted into flakes of snow. Hail is formed under different circumstances, and generally in sudden alternations of the fine season, the drops of rain being congealed during their fall, by passing through a lower stratum of dry and cold air.

The drops of rain vary in their size perhaps from the twenty-fifth to a quarter of an inch in diameter. In parting from the clouds, they precipitate their descent, till the increasing resistance opposed by the air becomes equal to their weight, when they continue to fall with an uniform velocity. This acquired or terminal velocity is therefore in the sub-duplicate ratio of the diameters of the aqueous globules. A thunder-shower hence pours down much faster than a drizzling rain. In general, if \(d\) express the diameter of a drop in parts of an inch, the terminal velocity, according to theory, will be denoted by \(78\sqrt{d}\), or, if the usual correction be made for the discrepancy in fluids, it will be \(67\sqrt{d}\). Thus a drop of the twenty-fifth part of an inch, in falling through the air, would only gain a celerity of 11\(\frac{1}{2}\) feet, while one of a quarter of an inch would acquire a celerity of 33\(\frac{1}{2}\) feet. A flake of snow, being perhaps nine times more expanded than water, would descend thrice as slow. But hailstones are often of considerable dimensions, exceeding sometimes an inch in length. They may hence fall with a velocity of 70 feet each second, or at the rate of about 50 miles in the hour. Striking the ground with such impetuous force, it is easy to conceive the extensive injury which a hail-shower may occasion in the hotter climates. The destructive power of those missiles in stripping and tearing the fruits and foliage increases, besides, in a faster ratio than the momentum, and may be estimated by the square of their velocity multiplied into mass. This fatal energy is hence as the fourth power of the diameter of the hailstone.

III. OPTICAL PHENOMENA.—It remains for us to explain the general optical appearances of the sky. When the rays of the sun strike upon a cloud, they are copiously reflected, but partly absorbed by the minute suspended globules. In working their progress through the mass of vapour, they suffer a great diminution from the multiplied acts of absorption. The quantity of light thus finally detained depends on the density of the cloud, and its thickness. But the portion which penetrates through the nebulus medium is always much less than what traverses an equal body of air. In extreme cases, perhaps, the solar beams will suffer greater defalcation by repeated reverberations within a congregated cloud, than from passing through fifty times the same extent of a clear aerial expanse. Hence such clouds always appear dark and black, by their scanty transmitted light. Whiteness, being produced by the copious emission of intermingled rays, can belong only to very thin clouds. The depth of shade indicates the mass of floating vapour.

Owing to the excessive minuteness of the aqueous globules, the particles of light are only reflected or absorbed at their external surface, without entering them. But when they collect into large drops, the luminous pencil which strikes at a certain angle converges by refraction to a point of the posterior surface, and, after suffering one or more interior reflections, it emerges dissected into its primitive colours. Hence the glorious vision of the rainbow, which was reduced to mathematical calculation by Descartes, but only received its complete explication from the optical discoveries of Newton. The phenomena occur whenever the sun shines upon the falling drops of rain behind the spectator, the coloured arch being a portion of a circle whose centre is a point in the sky directly opposite to the sun. The primary or interior bow is formed by a single reflection, and lies 45° beyond that centre; but the secondary or superior bow, produced by a double reflection, appears with inverted tints at the distance of 56°. A ternary bow may exist, but being so extremely faint from the repeated reflections, it is scarcely ever perceived. It hence follows, that rainbows are only visible when the altitude of the sun is below 45° and 56°. In summer, accordingly, they are not seen in this climate about the middle of the day. For the same reason, they generally appear less than a semicircle; but viewed from the top of a spire, or any lofty pinnacle, they embrace nearly the whole circumference. Lunar rainbows may be frequently observed, only the faintness of their colours makes them far less conspicuous.

The coloured rings or halos which are often seen surrounding the moon and sun, are evidently occasioned by very thin vapour diffused through the atmosphere. They are supposed chiefly to encircle the moon; but, in this climate, hardly a day passes with light flecked clouds, when at least portions of halos may not be perceived near the sun. It is only necessary to remove the glare of light which makes the delicate colours appear white. Thus, if we examine the reflection from a smooth surface of water, we shall perceive that the sun gilds the fleecy clouds with segments of beautifully coloured rings. This effect is still more distinctly seen if the rays from a hazy or mottled sky be received upon a sheet of white paper, held before a small hole in the window-shutter of a dark room. But even when the sun shines from an azure firmament, circles of the richest tints may be produced by experiment. Holding a hot poker below, and a little before, the small hole of the shutter, throw a few drops of water upon it, and the sun will be painted upon the paper like the glowing radiations of the passion-flower. The appearance is exactly similar to what the traveller, in awakening from a short slumber, perceives, in a winter's morning, on opening his wearied eyes to a burning candle—concentric rings of violet, green, yellow, and red.

The explication formerly given of the cause of halos, even that proposed by Newton himself, is inadmissible; since it would confine them, like the rainbows, to certain definite limits, whereas they appear with every possible degree of extension. Our earliest inquiries led us to refer the origin of halos to the deflection of light, or that property of the rays to bend and divide as they pass near the edge of a body. Thus the light admitted through a very narrow slit in a card, or a bit of tinfoil, spreads into bright coloured fringes. The finer also is the slit, the broader are the fringes. A similar appearance is obtained by looking at the elongated flame of a candle through the delicate fibres of a feather, or even through the streaks of grease rubbed by the finger along a piece of glass. But if a very small round hole be substituted for the slit, the fringes will change into coloured rings. Thus, if a piece of tinfoil, punctured with the point of a needle, be held close to the eye, the sun will appear through it surrounded by a halo very near his disc, but spreading more in proportion as the hole is contracted. That ingenious artist Mr Troughton constructed for us a slide of brass, and afterwards another of platina, perforated with a series of the finest conical holes, which were measured by his delicate micrometer. The purpose was to compare the angle subtended by the coloured ring with the diameter of the perforation, it being inferred that an aqueous globule of the same dimension might, by the exterior deflection of the solar rays, produce a similar halo. But our variable sky is very seldom fit for any refined optical experiment, and many delays happening to intervene, we could arrive at no very precise or certain result. We may state, however, as at least an approximation, that the globules of the diffuse vapour which occasions the appearance of coloured circles about the sun and moon, vary from the 5000th to the 50,000th part of an inch in diameter. When the halo approaches nearest to the luminous body, the largest globules are floating, and therefore the atmosphere is surcharged with humidity. Hence the justness of the vulgar remark, that a dense halo close to the moon portends rain.

Nearly the same theory has been struck out by Dr Thomas Young, to whose profound ingenuity and most extensive information we are glad to bear honourable testimony. By a skilful application of the principle of deflected light, he has likewise constructed the Eriometer, a curious instrument for measuring the size of the fibres of wool and other filamentous substances. But we cannot at present enter into the details.

The same acute philosopher has given perhaps the only true account of the origin of the parhelia or mock suns, which are frequently seen in the arctic regions during certain dispositions of the atmosphere. This gorgeous appearance of intersecting luminous arches, studded with opposite and transverse images of the sun, he ascribes to the combined reflections of the rays from the natural facets of the snowy spicula floating abundantly in the air.

Another most remarkable optical deception occurs in a Phenomena peculiar state of the atmosphere on the verge of the horizon in various countries, and especially in the warmer climates, whether on the level plains, or on the margin of rivers or lakes, and near the sea-shore. In such situations the remote objects often appear with extraordinary elevation, and in double or inverted images. This singular phenomenon is obviously caused by the irregular refractions which the rays of light occasionally suffer by passing through the different strata of the lower atmosphere. When the effect is confined to the apparent elevation of an object, our common sense calls it looming; but if inverted images be formed, the French and Italians give to this play of vision the appellations of mirage and fata morgana. The shipping and range of buildings on the shore of Naples have from Messina sometimes appeared floating inverted in the air. In the autumn of 1798 the coast of France was distinctly seen raised above the sea, from the beach of Hastings; the appearance lasted about an hour, and then sunk beneath the horizon. In the following year, Prof. Vince watched the phenomena at Ramsgate with a large telescope. On the afternoon of the 1st of August he first described the sails of a ship, but as she came fuller in view, he perceived another inverted image just below the surface of the water. Fig. 16, Plate CCCLV., represents the successive appearances; A is the ship when first observed, C after she had approached, and B the inverted image. Fig. 17 exhibits a variation of the effect; a marks the ship entering the horizon, C and B the double image of the ship when near, a portion of the sea lying between its opposite traces. To produce the appearances now represented, it was quite requisite that the rays of light, in traversing the lower strata of the air, should describe curves, whose final tangents point in the visual directions of the objects. These curves or trajectories are delineated in fig. 13, where E marks the place of the observer, z the horizon, b the hull and a the mast of the ship; the lowest image is formed by the curves bsE and arE; above this the curves bsE and arE give an inverted image, and the curves bsE and arE exhibit the highest erect image.

In ordinary cases, a ray of light, in crossing different layers of the atmosphere, describes a trajectory, which is nearly the portion of a circle, having six times the diameter of the earth. The rate of inflection is proportional to the tangent of obliquity, and to the difference of the refractive power of the conterminous strata, which may be assumed as the same as their difference of density. This variation in a stratum of air twenty-eight feet thick is only the thousandth part of the whole refraction, and is hence equal to the effect of the quarter of a centesimal degree. But since a much greater difference of heat often occurs within that limit, the incursion of the trajectory must be proportionally increased. Humidity, by dilating the air, will produce a similar effect, though in a much inferior degree, unless in very high temperatures.

Dr W. H. Wollaston, whose acuteness in devising philosophical experiments was unrivalled, exhibited on a small scale the various appearances occasioned by irregular atmospheric attraction. His paper inserted in the Philosophical Transactions for the year 1800 contains a very clear exposition of the phenomena. On looking lengthwise over the side of a red-hot poker at a distant object, besides the ordinary image, another inverted one was seen within the edge of the streaming air, and a larger erect one still nearer the poker, as represented in fig. 14. The deviation was here only half a degree; but along a red-hot bar of iron, the separation of the images was increased to a degree and a quarter. On directing the eye over the surface of a green board, which had been heated by exposure to the sun, a double image was distinctly seen. Another board, merely wetted with water, betrayed a similar disposition, though it was very difficult to distinguish the irregular image, its elevation being only 3°. But on moistening the surface of the board with alcohol, the images were easily seen at a separation of 15°. Still more conspicuous was the appearance on spreading a little ether over a plate of glass, for the irregular image showed itself 7½° raised.

It was easy to imitate and examine the natural phenomena by means of a square parallelopiped composed of plate glass. Having filled the half of this with cold water, let hot water be gently poured over it; and the cover placed upon the top. (See fig. 15, Plate CCCLV.) On standing a few feet behind the parallelopiped, and looking at distant objects through the stratum where the cold and hot water have become blended, the double and inverted images will for a short time be clearly perceived. But a more durable effect is obtained by filling one third of the glass case, as represented in fig. 15, with syrup or a strong solution of white sugar, another third with distilled water, and the rest with pure alcohol. A mutual penetration slowly takes place between the contiguous surfaces of the alcohol A and the water B, and between this and the syrup C; and after the lapse of perhaps two or three days, the blending spaces become sufficiently broad for observation. The appearances then are nearly such as those that have been sketched in the plate.

This mode of experiment is at once simple and convincing. The theory which Dr Wollaston draws from it is equally ingenious, but not so demonstratively established as its author presumes. It rests chiefly on the supposition, that the stratum occupied by the penetration of two fluids of different densities, graduates in refractive intensity more slowly at its boundaries, and most rapidly in the middle. Or, to borrow the elucidation of geometry, if the successive densities be denoted by ordinates, their summits will form a line of double curvature, having consequently a point of contrary flexure. This principle may be sound, though we can perceive no cogent reason why the curve of intermediate refraction should be convex or concave, or should combine both sorts of incurvation. But admitting this double curvature to be a constituent law, it will very satisfactorily explain the phenomena. The refraction will be the same as if the light had traversed an uniform medium bounded by a surface of that reverted curvature, the convex portion diverging, and the concave portion converging, the parallel rays. The different effects are exhibited in fig. 12; the distant object O is viewed in its true position by the pencil that arrives at O; it is seen inverted at I, by the rays which enter at m; the rays which fall on r make it appear again erect at A. This explication is certainly very plausible, and may probably approximate to the truth. But though much has been already achieved, the subject of irregular refraction needs to be revised, and still more closely investigated.

For some further illustrations of the subject of this article, see Barometer, Climate, Cold, and Weather.