a word generally used to signify the whole mass of fluid, consisting of air, aqueous and other vapours, electric fluid, &c. surrounding the earth to a considerable height.

The composition of that part of our atmosphere properly called air, was till lately very much unknown. In former times it was supposed to be a simple, homogeneous and elementary fluid. The experiments of Dr Priestley discovered, that the purest kind of air, which he called dephlogisticated, was in reality a compound, and might be artificially produced in various ways. His first conjectures concerning its component parts were, that it consisted of earth, nitrous acid, and phlogiston. Subsequent experiments rendered these conjectures dubious; and at last it was supposed that dephlogisticated air is a pure elementary substance, the vivifying principle to animals, and the acidifying principle throughout all nature. This dephlogisticated air, however, is but a small part of the composition of our atmosphere. According to the most accurate computations, the air we usually breathe is composed of only one-fourth of this dephlogisticated air, or perhaps less; the other three or four parts consisting of what Dr Priestley calls phlogisticated, and M. Lavoisier mephitic air. This by itself is absolutely noxious, and exceedingly poisonous to animals: though it seems only to be negatively so; for when mixed in a certain proportion with dephlogisticated air, it may be breathed with safety, which could not be if it contained any ingredient absolutely unfriendly to the human constitution. The other part, viz. the pure dephlogisticated air, seems to stand much in the same relation to plants that phlogisticated air does to animals; that is, it would prove poisonous and destroy them if they were to depend upon it entirely for their subsistence; but as they derive their nourishment partly from the air and partly from the soil, it thence happens, that the plants which are set to grow in dephlogisticated air do not die instantly, as animals do in the phlogisticated kind, but remain for some time weak and sickly.

The other component parts of our atmosphere are so various, and of such heterogeneous natures, that they do not admit of any kind of definition or analysis, one only excepted, namely, the electric fluid. This we know pervades the whole, but appears to be much more copious in the upper than in the lower atmospheric regions. See ELECTRICITY. To measure the absolute quantity of this fluid, either in the atmosphere or any other substance, is impossible. All that we can know on this subject is, that the electric fluid pervades the atmosphere; that it appears to be more abundant in the superior than the inferior regions; that it seems to be the immediate bond of connection between the atmosphere and the water which is suspended in it; and that, by its various operations, the phenomena of hail, rain, snow, lightning, and various other kinds of meteors, are occasioned.

Various attempts have been made to ascertain the height to which the atmosphere is extended all round the earth. These commenced soon after it was discovered, by means of the Torricellian tube, that air is a gravitating substance. Thus it also became known, that a column of air, whose base is a square inch, and the height that of the whole atmosphere, weighs 15 pounds: and that the weight of air is to that of mercury, as 1 to 10,800: whence it follows, that if the weight of the atmosphere be sufficient to raise a column of mercury to the height of 30 inches, the height of the aerial column must be 10,800 times as much, and consequently a little more than five miles high.

It was not, however, at any time supposed, that this calculation could be just; for as the air is an elastic fluid, the upper parts must expand to an immense bulk, and thus render the calculation above related exceedingly erroneous. By experiments made in different countries, it has been found, that the spaces which any portion of air takes up, are reciprocally proportional to the weights with which it is compressed. Allowances were therefore to be made in calculating the height of the atmosphere. If we suppose the height of the whole divided into innumerable equal parts, the density of each of which is as its quantity; and the weight of the whole incumbent atmosphere being also as its quantity; it is evident, that the weight of the incumbent air is everywhere as the quantity contained in the subjacent part; which makes a difference between the weights of each two contiguous parts of air. By a theorem in geometry, where the differences of magnitudes are geometrically proportional to the magnitudes themselves, these magnitudes are in continual arithmetical proportion; there-

fore, if, according to the supposition, the altitude of the air, by the addition of new parts into which it is divided, do continually increase in arithmetical proportion, its density will be diminished, or (which is the same thing) its gravity decreased, in continual geometrical proportion.

It is now easy, from such a series, by making two or three barometrical observations, and determining the density of the atmosphere at two or three different elevations, to determine its absolute height, or its rarity, at any assignable height. Calculations accordingly were made upon this plan; but it having been found that the barometrical observations by no means corresponded with the density which, by other experiments, the air ought to have had, it was supposed that the upper parts of the atmospherical regions were not subject to the same laws with the lower ones. Philosophers therefore had recourse to another method for determining the altitude of the atmosphere, viz. by a calculation of the height from which the light of the sun is refracted, so as to become visible to us before he himself is seen in the heavens. By this method it was determined, that at the light height of 45 miles the atmosphere had no power of refraction; and consequently beyond that distance was either a mere vacuum or the next thing to it, and not to be regarded.

This theory soon became very generally received, and the height of the atmosphere was spoken of as familiarly as the height of a mountain, and reckoned to be as well ascertained, if not more so, than the heights of most mountains are. Very great objections, however, which have never yet been removed, arise from the appearances of some meteors, like large globes of fire, not unfrequently to be seen at vast heights above the earth (see METEOR). A very remarkable one of this kind was observed by Dr Halley in the month of March 1719, whose altitude he computed to have been between 69 and 73½ English miles; its diameter 2800 yards, or upwards of a mile and a half; and its velocity about 350 miles in a minute. Others, apparently of the same kind, but whose altitude and velocity were still greater, have been observed; particularly that very remarkable one, August 18th, 1783, whose distance from the earth could not be less than 90 miles, and its diameter not less than the former; at the same time that its velocity was certainly not less than 1000 miles in a minute. Fire-balls, in appearance similar to these, though vastly inferior in size, have been sometimes observed at the surface of the earth. Of this kind Dr Priestley mentions one seen on board the Montague, 4th November 1749, which appeared as big as a large millstone, and broke with a violent explosion.

From analogical reasoning, it seems very probable, that the meteors which appear at such great heights in the air are not essentially different from those which, like the fire-ball just mentioned, are met with on the surface of the earth. The perplexing circumstances with regard to the former are, that at the great heights above mentioned, the atmosphere ought not to have any density sufficient to support flame, or to propagate sound; yet these meteors are commonly succeeded by one or more explosions, nay are sometimes said to be accompanied with a hissing noise as they pass over our heads. The meteor of 1719 was not only very bright, insomuch that for a short space it turned night into day, but was attended with an explosion heard over all the island of Britain, occasioning a violent concussion in the atmosphere, and seeming to shake the earth itself. That of 1783 also, though much higher than the former, was succeeded by explosions; and, according to the testimony of several people, a hissing noise was heard as it passed. Dr Halley acknowledged that he was unable to reconcile these circumstances with the received theory of the height of the atmosphere; as, in the regions in which this meteor moved, the air ought to have been 300,000 times more rare than what we breathe, and the next thing to a perfect vacuum.

In the meteor of 1783, the difficulty is still greater, as it appears to have been 20 miles farther up in the air. Dr Halley offers a conjecture, indeed, that the vast magnitude of such bodies might compensate for the thinness of the medium in which they moved. Whether or not this was the case indeed cannot be ascertained, as we have so few data to go upon; but the greatest difficulty is to account for the brightness of the light. Appearances of this kind are indeed with great probability attributed to electricity, but the difficulty is not thus removed. Though the electrical fire pervades with great ease the vacuum of a common air-pump, yet it does not in that case appear in bright well defined sparks, as in the open air, but rather in long streams resembling the aurora borealis. From some late experiments, indeed, Mr Morgan concludes, that the electrical fluid cannot penetrate a perfect vacuum *. If this is the case, it shows that the regions we speak of are not such a perfect vacuum as can be artificially made; but whether it is or not, the extreme brightness of the light shows that a fluid was present in those regions, capable of confining and condensing the electric matter as much as the air does at the surface of the ground; for the brightness of these meteors, considering their distance, cannot be supposed inferior to that of the brightest flashes of lightning.

This being the case, it appears reasonable to conclude, that what is called the density of the air does not altogether keep pace with its gravity. The latter indeed must in a great measure be affected by the vapours, but above all by the quantity of the basis of fixed or dephlogisticated air contained in it: for Mr Kirwan has discovered that the basis of fixed air, when deprived of its elastic principle, is not greatly inferior to gold in specific gravity; and we cannot suppose that of dephlogisticated air to be much less. It is possible, therefore, that pure air, could it be deprived of all the water it contains, might have very little gravity; and as there is great reason to believe that the basis of dephlogisticated air is only one of the constituent parts of water, we see an evident reason why the air ought to become lighter, and likewise less fit for respiration, the higher up we go, though there is a possibility that its density, or power of supporting flame, may continue unaltered.

There are not yet, however, a sufficient number of facts to enable us to determine this question; though such as have been discovered seem rather to favour the above conjecture. Dr Boerhaave was of opinion that the gravity of the air depended entirely on the water it contained; and, by the means of alkaline salts, he was enabled to extract as much water from a quantity of air as was very nearly equivalent to its weight. By the calcination of metals we may extract as much of the basis of dephlogisticated air from a quantity of atmospheric air as is equivalent to the weight of air loft. Were it possible, therefore, to extract the whole of this, as well as all other vapours, and to preserve only the elastic principle, it is highly probable that its gravity would entirely cease. It has been found, by those who have ascended with aerostatic machines, or to the tops of high mountains, that the dephlogisticated air is found to be contained in smaller quantities in the atmosphere of those elevated regions than on the lower grounds. It is also found, that in such situations the air is much drier, and parts with water with much more difficulty, than on the ordinary surface. Salt of tartar, for instance, which at the foot of a mountain will very soon run into a liquid, remains for a long time exposed to the air on the top of it, without showing the least tendency to deliquescence. Nevertheless, it hath never been observed that fires did not burn as intensely on the tops of the highest mountains as on the plains. The matter indeed was put to the trial in the great eruption of Vesuvius in 1779, where, though the lava spouted up to the height of three miles above the level of the sea, the uppermost parts all the while were to appearance as much inflamed as the lowest.

The high degree of electricity, always existing in Gravity of the upper regions of the atmosphere, must of necessity have a very considerable influence on the gravity of any regions of the atmosphere heterogeneous particles floating in it. When we consider the effects of the electric fluid upon light bodies haphazard at the surface of the earth, it will readily be admitted, nighed that in those regions where this fluid is very abundant, the gravity of the atmosphere may be much diminished without affecting its density. We know that it is the nature of any electrified substance to attract light bodies; and that, by proper management, they may even be suspended in the air, without either moving up or down for a considerable time. If this is the case with light terrestrial bodies, it cannot be thought very improbable that the aerial particles themselves, i.e. those which we call the basis of dephlogisticated air, and of aqueous or other vapour diffused among them, should be thus affected in the regions where electricity is so abundant. From this cause, therefore, also the gravity of the atmosphere may be affected without any alteration at all being made in its density; and hence may arise anomalies in the barometer hitherto not taken notice of.

It appears, therefore, that the absolute height of the atmosphere is not yet determined. The beginning and ending of twilight indeed show, that the height at which the atmosphere begins to refract the sun's light is about 44 or 45 English miles. But this may ed. not improbably be only the height to which the aqueous vapours are carried: for it cannot be thought any unreasonable supposition, that light is refracted only by means of the aqueous vapour contained in the atmosphere; and that where this ceases, it is still capable of supporting the electric fire at least as bright and strong as at the surface. That it does extend much higher, is evident from the meteors already mentioned; for all these are undoubtedly carried along with the atmosphere; atmosphere; otherwise that of 1783, which was seen for about a minute, must have been left 1000 miles to the westward, by the earth flying out below it in its annual course round the sun.

It has already been mentioned, that the pressure of the atmosphere, when in its mean state, is equivalent to a weight of 15 pounds on every square inch. Hence Dr Cotes computed, that the preasure of the whole ambient fluid upon the earth's surface is equivalent to that of a globe of lead 60 miles in diameter. Hence also it appears, that the preasure upon a human body must be very considerable; for as every square inch of surface sustains a preasure of 15 pounds, every square foot, as containing 144 inches, must sustain a preasure of 2160; and if we suppose a man's body to contain 15 square feet of surface, which is pretty near the truth, he must sustain a weight of 32,400 pounds, or 16 tons, for his ordinary load. By this enormous preasure we should undoubtedly be crushed in a moment, were not all parts of our bodies filled either with air or some other elastic fluid, the spring of which is just sufficient to counterbalance the weight of the atmosphere. But whatever this fluid may be, we are sure that it is just able to counteract the atmospherical gravity and no more; for if any considerable preasure be superadded to that of the air, as by going into deep water, or the like, it is always severally felt, let it be ever so equable. If the preasure of the atmosphere is taken off from any part of the human body, the hand, for instance, when put in an open receiver from whence the air is afterwards extracted, the weight of the atmosphere then discovers itself, and we imagine the hand strongly sucked down into the glass. See PNEUMATICS.

In countries at some distance from the equator, the preasure of the atmosphere varies considerably, and thus produces considerable changes on many terrestrial bodies. On the human body the quantity of preasure sometimes varies near a whole ton; and when it is thus so much diminished, most people find something of a listlessness and inactivity about them. It is surprising, however, that the spring of the internal fluid, already mentioned, which acts as a counterpoise to the atmospherical gravity, should in all cases seem to keep pace with it when the preasure is naturally diminished, and even when it is artificially augmented, though not when the preasure is artificially diminished. Thus in that kind of weather when the preasure of the air is least, we never perceive our veins to swell, or are sensible of any inward expansion in our bodies. On the contrary, the circulation is languid, and we seem rather to be oppressed by a weight. Even in going up to the tops of mountains, where the preasure of the atmosphere is diminished more than three times what it usually is on the plain, no such appearances are observed. Some travellers indeed have affirmed, that on the tops of very high mountains, the air is so light as to occasion a great difficulty of respiration, and even violent retching and vomiting of blood. It does not appear, however, that these assertions are well founded. Mr Brydone found no inconvenience of this kind on the top of Mount Aetna; nor is any such thing mentioned by Mr Houel, who also ascended this mountain. Sir William Hamilton indeed says, that he did feel a difficulty of respiration, independent of any sulphurous steam. But, on the top of a volcano, the respiration may be affected by so many different causes, that it is perhaps impossible to assign the true one. The French mathematicians, when on the top of a very high peak of the Andes, did not make any complaint of this kind, though they lived there for some time. On the contrary, they found the wind so extremely violent, that they were scarcely able to withstand its force; which seems an argument for at least equal density of the atmosphere in the superior as in the inferior regions. Dr Heberden, who ascended to the top of Teneriffe, a mountain higher than Aetna, makes no mention of any difficulty of respiration. M. Sauflure, M. Sauflure, however, in his journey to the top of Mont Blanc, the sure'symphighest of the Alps, felt very great uneasiness in this top of Mont Blanc ac- tual way. His respiration was not only extremely difficult, but his pulse became quick, and he was seized with all the symptoms of a fever. His strength was counted also exhausted to such a degree, that he seemed to re-quire four times as long a space to perform some experiments on the top of the mountain as he would have done at the foot of it. It must be observed, however, that these symptoms did not begin to appear till he had ascended two miles and a half perpendicular above the level of the sea. The mountain is only about a quarter of a mile higher; and in this short space he was reduced to the situation just mentioned. But it is improbable that so small a difference, even at the end of his journey, should have produced such violent effects, had not some other cause concurred. A cause of this kind he himself mentions, viz. that the atmosphere at the top of the mountain was so much impregnated with fixed air, that lime-water, exposed to it, quickly became covered with a pellicle occasioned by the absorption of that fluid. Now it is known, that fixed air is extremely pernicious to animals, and would bring on symptoms similar to those above mentioned. There is no reason, therefore, to have recourse to the rarity of the atmosphere for solving a phenomenon which may more naturally be accounted for otherwise.

When the preasure of the atmosphere is augmented, by descending, in the diving-bell, to considerable depths in the sea, it does not appear that any inconvenience follows from its increase. Those who sit in the diving-bell are not sensible of any preasure as long as they remain in the air, though they feel it very sensible in going into the water; yet it is certain, that the preasure in both cases is the same: for the whole preasure of the atmosphere, as well as of the water, is sustained by the air in the diving-bell, and consequently communicated to those who fit in it.

But though artificial compression of the air, as well as natural rarefaction, can thus be borne, it is otherwise with artificial rarefaction. Animals in an air-pump show uneasiness from the very first, and cannot live for any time in an atmosphere rarefied artificially even as much as it appeared to be from the barometer on the top of Mont Blanc.

It is not easy to assign the true reason of the variations of gravity in the atmosphere. Certain it is, however, that they take place only in a very small degree within the tropics; and seem there to depend on the accounted heat of the sun, as the barometer constantly sinks near for half an inch every day, and rises again to its former station in the night time. In the temperate zones the barometer ranges from 28 to near 31 inches, by its various altitudes showing the changes that are about to take place in the weather. If we could know, therefore, the latent causes by which the weather is influenced, we should likewise certainly know those by which the gravity of the atmosphere is affected. In general they may be reduced to two, viz. an emission of latent heat from the vapour contained in the atmosphere, or of electric fluid from the flame, or from the earth. To one or both of these causes, therefore, may we ascribe the variations of the gravity of the atmosphere; and we see that they both tend to produce the same effect with the solar heat in the tropical climates, viz. to rarefy the air, by mixing with it or setting loose a non-gravitating fluid, which did not act in such large proportion in any particular place before. No doubt, the action of the latent heat and electric fluid is the same in the torrid as in the temperate zones: but in the torrid-zone the solar heat and excessive evaporation counteract them; so that whatever quantities may be discharged by the excessive deluges of rain, &c. which fall in those countries, they are instantly absorbed by the abundant fluid, and are quickly ready to be discharged again; while, in the temperate zones, the air becomes sensibly lighter, as well as warmer, by them for some time before they can be absorbed again.

The variations of heat and cold to which the atmosphere is subject, have been the subject of much speculation. In general they seem to depend entirely upon the light of the sun reflected into the atmosphere from the earth; and where this deflection is deficient, even though the light should be present ever so much, the most violent degrees of cold are found to take place. Hence, on the tops of mountains, the cold is generally excessive, though by reason of the clearness of the atmosphere the light of the sun falls upon them in greater quantity than it can do on an equal space on the plain. In long winding passages also, such as the caverns of Ætna and Vesuvius, where the air has room to circulate freely, without any access of the sun, the cold is scarcely tolerable; whence the use of these for cooling liquors, preserving meat, &c.

The coldness of the atmosphere on the tops of mountains has been ascribed by M. Lambert and De Luc, to the igneous fluid, or elementary fire, being more rare in those elevated situations than on the plains. M. Lambert is of opinion that it is rarefied above by the action of the air, and that below it is condensed by its own proper weight. He considers fire as a fluid in motion, the parts of which are separable, and which is rarefied when its velocity is accelerated. He does not decide with regard to the identity of fire and light, though he seems inclined to believe it. M. de Luc compares elementary fire to a continuous fluid, whose parts are condensed by being mutually compressed. He denies that fire and light are the same; and maintains that the latter is incapable, by itself, of setting fire to bodies, though it does so by putting in motion the igneous fluid they contain; and that it acts with more force near the earth than at a distance from its surface, by reason of this fluid, which he calls a heavy and elastic one, being more condensed there than at a greater height.

M. Saussure, in treating of this subject in his account of the Alps, does not consider fire as a fluid free and detached as to be able either to ascend with rapidity by its specific levity, or to condense itself sensibly by its proper weight. He supposes it to be united to bodies by so strict an affinity, that all its motions are determined, or at least powerfully modified, by that affinity. As soon therefore as fire, disengaged by combustion or by any other cause, endeavours to diffuse itself, all the bodies that come within the sphere of its activity endeavour to attract it; and they absorb such quantities of it as are in the direct ratio of their affinities with it, or in the inverse ratio of what is necessary for their equilibrium with the surrounding bodies. Now it does not appear that in this distribution the situation of places, with regard to the horizon, has any other influence than what they receive from the different currents produced by the dilatation of the air, and by the levity which that dilatation produces. The ascent of flame, smoke, &c. or of air heated in any way, persuaded the ancients that fire is possessed of absolute levity, by which it had a tendency to mount upwards. "But these effects (says he) are owing either to the levity of the fluid which constitutes flame, or to that of air diluted by heat; and not to the levity of the igneous fluid. I am, however, sufficiently convinced, that this fluid is incomparably lighter than air, though I do not believe that it possesses the power of ascending in our atmosphere by virtue of its levity alone.

"The celebrated Bouguer has demonstrated, by principles the most simple, and most universally adopted, that it is not necessary, in order to account for the diminution of heat on mountains, to have recourse to hypotheses that are at best doubtful. The following is his explanation of what was felt on the mountains of Peru.

"It was proper, in order to explain this subject, to insist on the short duration of the sun's rays, which cannot strike the different sides of mountains but for a few hours, and even this not always. A horizontal plain, when the sun is clear, is exposed at mid-day to the perpendicular and undiminished action of these rays, while they fall but obliquely on a plain not much inclined, or on the sides of a high pile of steep rocks. But let us conceive for a moment an insulated point, half the height of the atmosphere, at a distance from all mountains, as well as from the clouds which float in the air. The more a medium is transparent, the less heat it ought to receive by the immediate action of the sun. The free passage which a very transparent body allows to the rays of light, shows that its small particles are hardly touched by them. Indeed what impression could they make on it, when they pass through almost without obstruction? Light, when it consists of parallel rays, does not by passing through a foot of free atmospheric air, near the earth, lose an hundred thousandth part of its force. From this we may judge how few rays are weakened, or can act on this fluid, in their passage through a stratum of the diameter not of an inch or a line, but of a particle. Yet the subtilty and transparency are still greater at great heights, as was obvious on the Cordilleras, when we looked at distant objects. Lastly, the groser air is heated below by the contact or neighbourhood of bodies of greater density than itself, which it surrounds, and on which it rests; and the heat may be communicated by little and little to a certain distance. The inferior parts of the atmosphere by this means contract daily a very considerable degree of heat, and may receive it in proportion to its density or bulk. But it is evident that the same thing cannot happen at the distance of a league and a half or two leagues above the surface of the earth, although the light there may be something more active. The air and the wind therefore must at this height be extremely cold, and colder in proportion to the elevation.

"Besides, the heat necessary to life is not merely that which we receive every instant from the sun. The momentary degree of this heat corresponds to a very small part of that which all the bodies around us have imbibed, and by which ours is chiefly regulated. The action of the sun only serves to maintain nearly in the same state the sum of the total heat, by repairing through the day the loss it suffers through the night, and at all times. If the addition be greater than the loss, the total heat will increase, as it happens in summer, and it will continue to accumulate in a certain degree; but for the reasons already given, this accumulation cannot be very great on the top of a mountain, where the summit, which rises high, is never of great bulk. The lowest state of the thermometer in every place is always in proportion to the heat acquired by the soil; and that being very small on the top of a mountain, the quantity added to it by the sun during the day must be comparatively greater; and the accumulated heat will be more in a condition to receive increase in proportion to its distance from the degree which it cannot pass.

"Another particular observable on all the high places of the Cordilleras, and which depends on the same cause, is, that when we leave the shade, and expose ourselves to the sunshine, we feel a much greater difference than we do here in our fine days when the weather is temperate. Every thing contributes at Quito to make the sun exceedingly powerful: a single step from an exposed place to the shade gives the sensation of cold: this would not be the case if the quantity of heat acquired by the soil were more considerable. We now also see why the same thermometer, put first into the shade and then in the sun, does not undergo the same changes at all times and in all places. In the morning, upon Pichincha, this instrument is generally a few degrees below the freezing point, which may be reckoned the natural temperature of the place; but when during the day we expose it to the sun, it is easy to imagine that the effect must be great, and much more than double in whatever way it is measured."

This theory is adopted by M. Saussure, who adds the following fact to prove that the action of the sun's rays, considered abstractedly and independent of any extrinsic source of cold, is as great on mountains as on plains; viz. that the power of burning lenes and mirrors is the same at all heights. To ascertain this fact, our author procured a burning-glasf so weak that at Geneva it would just set fire to tinder. This he carried, with some of the same tinder, to the top of the mountain Saleve (a height of 3000 feet); where it not only produced the same effect, but apparently with greater facility than on the plain. Being persuaded, then, that the principal source of cold on the tops of high mountains is their being continually surrounded with an atmosphere which cannot be much heated either by the rays of the sun on account of its transparency, or by the reflection of them from the earth by reason of its distance, he wished to know, whether the direct solar rays on the top of a high mountain had the same power as on the plain, while the body on which they acted was placed in such a manner as to be unaffected by the surrounding air. For this purpose he instituted a set of experiments, from which he drew the following conclusions, viz. that a difference of 777 toises in height diminishes the heat which the rays of the sun are able to communicate to a body exposed to the external air, 14° of the thermometer; that it diminishes the heat of a body partially exposed, only 6°; and that it augments by 1° the heat of a third body completely defended from the air.

Hence it appears that the atmosphere, though so essentially necessary to the support of fire, is somehow or other the greatest antagonist of heat, and most effectually counteracts the operation of the solar rays in producing it. This power it seems to exert at all distances, at the surface as well as in the higher regions. From some experiments made by M. Picet it appears, that even in places exposed to the rays of the sun, the heat, at five feet distance from the ground, is greater only by one or two degrees than at 50 feet above the very near surface, though the ground was at that time 15 or 20° warmer than the air immediately in contact with it. Inconsiderable as this difference is, however, it does not hold as we go higher up; for if it did, the cold on the top of the mountain of Saleve, which is 3000 feet above the level of the lake of Geneva, would be 65° greater than at the foot of it; whereas in reality it is only 18°. In the night-time the case is reversed; for the stratum of air at five feet from the ground, was found by M. Picet to be colder than at 50. Besides this, different strata of the atmosphere are found to possess very different and variable degrees of cold, without any regard to their situation high up or low down. In the year 1780, Dr. Wilton of Glaigow found a very remarkable cold existing close to the surface of the ground; so that the thermometer, when laid on the surface of the snow, sunk many degrees lower than one suspended 24 feet above it. It has been likewise observed, that in clear weather, though the surface of the earth be then most liable to be heated by the sun, yet after that is set, and during the night, the air is coldest near the ground, and particularly in the valleys. Experiments on this subject were made for a whole year by Mr James Sex, who has given an account of them in the 78th volume of the Philosophical Transactions. He suspended thermometers constructed in such a manner as to show the true maximum and minimum of heat that might take place in the observer's absence) in a shady northerly aspect, and at different heights in the open air. One of these was placed at the height of 9 feet, and the other at that of 220 from the ground; and the observations were continued, with only a few days omission, from July 1784 to July 1785. The greatest variations of heat were in the months of October and June; in the former the thermometers generally differed most in the night, and in the latter mostly in the day. From the 25th to the 28th of October, the heat below, in the night-time, exceeded in a small degree the heat above; at which time there was frequent rain mingled with hail. From the 11th to the 14th, and also on the 31st, there was no variation at all; during which time likewise the weather was rainy; all the rest of the month proving clear, the air below was found colder than that above, sometimes by nine or ten degrees. In the month of June, the greatest variations took place from the 11th to the 15th, and from the 25th to the 30th; and at both these times there appeared to be two currents of wind, the upper from the south-west and the lower from the north-east. Sometimes these were rendered visible by clouds, in different strata, moving in different directions; and sometimes by clouds moving in a contrary direction to a very sensible current of air below. On cloudy nights the lowest thermometer sometimes showed the heat to be a degree or two greater than the upper one; but in the day-time the heat below constantly exceeded that above more than in the month of October.

To determine whether the nocturnal refrigeration was augmented by a nearer approach to the earth, two thermometers were placed in the midst of an open meadow, on the bank of the river near Canterbury. One was placed on the ground, and the other only six feet above it. The thermometer, at six feet distance from the ground, agreed nearly with the former at nine feet; but the nocturnal variations were found to correspond entirely with the clearness or the cloudiness of the sky; and though they did not always happen in proportion to their respective altitudes, yet when the thermometers differed in any respect, that on the ground always indicated the greatest degree of cold.

The difference between these two thermometers, at the small distance of six feet from each other, being found no less than three degrees and a half, the number of thermometers in the meadow was augmented to four. One was sunk in the ground, another placed just upon it, and the third suspended at three feet above it. Three others were placed on a rising ground where the land was level with the cathedral tower, and about a mile distance from it. One of these was likewise sunk in the ground, another placed just upon it, and a third suspended fix feet above it. With these seven thermometers, and the two first mentioned, which were placed in the city, he continued his observations for 20 days; but as the weather happened to be cloudy during the whole of that space, excepting for seven or eight days, no considerable variation happened excepting on these days. The result of the experiments was, that the cold was generally greater in the valley than on the hill; but the variations between the thermometers on the ground and those fix feet above them, were often as great on the hill as in the valley.

Thus it was perceived that a difference of temperature took place at the distance of only three feet from the ground; but the length of the thermometers hitherto made use of rendered it impossible to make any experiment at a smaller distance. Two new ones, therefore, were formed by bending down the large tube, the body or bulb of the thermometer, to a horizontal position, while the stem remained in a vertical one; by which method the temperature might be observed to the distance of a single inch. Sometimes, in clear weather, these two horizontal thermometers were placed in the open air, one within an inch of the ground, and the other nine inches above it. When the variation among the other thermometers was considerable, a difference was likewise perceived between these; the lower one sometimes indicating more than two degrees less heat than the upper one, though placed so near each other.

From these experiments, Mr Sex concludes, that a greater diminution of heat frequently takes place near the earth in the night-time than at any altitude in the atmosphere within the limits of his inquiry, that is, 220 feet from the ground; and at such times the greatest degrees of cold are always met with nearest the surface of the earth.

This is a constant and regular operation of nature under certain circumstances and dispositions of the atmosphere, and takes place at all seasons of the year; and this difference never happens in any considerable degree but when the air is still, and the sky perfectly unclouded. The moistest vapour, as dews and fogs, did not at all impede, but rather promote, the refrigeration. In very severe frosts, when the air frequently deposits a quantity of frozen vapour, it is commonly found greatest; but the excess of heat which in the day-time was found at the lowest station in summer, diminished in winter almost to nothing.

It has been observed, that a thermometer, included in a receiver, always sinks when the air begins to be rarefied. This has been thought to arise, not from any degree of cold thus produced, but from the sudden expansion of the bulb of the thermometer in consequence of the removal of the atmospheric pressure: But from some late experiments related, Phil. Trans. vol. lxviii., by Mr Darwin, it appears that the atmosphere always becomes warm by compression, and cold by dilatation from a compressed state. These experiments were,

1. The blast from an air-gun was repeatedly thrown upon the bulb of a thermometer, and it uniformly sunk it about two degrees. In making this experiment, the thermometer was firmly fixed against a wall, and the air-gun, after being charged, was left for an hour in its vicinity, that it might previously lose the heat it had acquired in the act of charging; the air was then discharged in a continued stream on the bulb of the thermometer, with the effect already mentioned.

2. A thermometer was fixed in a wooden tube, and fo applied to the receiver of an air-gun, that, on discharging the air by means of a screw pressing on the valve of the receiver, a continued stream of air, at the very time of its expansion, passed over the bulb of the thermometer. This experiment was four times repeated, and the thermometer uniformly sunk from five to seven degrees. During the time of condensation there was a great difference in the heat, as perceived by the hand, at the two ends of the condensing syringe: that next the air-globe was almost painful to touch; and the globe itself became hotter than could have been expected from its contact with the syringe. "Add to this (says Mr Darwin), that in exploding an air-gun the stream of air always becomes visible, which is owing owing to the cold then produced, precipitating the vapour it contained; and if this stream of air had been previously more condensed, or in greater quantity, so as not instantly to acquire heat from the common atmosphere in its vicinity, it would probably have fallen in snow.

3. A thermometer was placed in the receiver of an air-pump, and the air being hasty exhausted, it sunk two or three degrees; but after some minutes regained its former station. The experiment was repeated with a thermometer open at the top, so that the bulb could not be affected by any diminution of the external pressure; but the result was the same. Both during exhaustion and re-admission of the air into the receiver, a steam was regularly observed to be condensed on the sides of the glass; which, in both cases, was in a few minutes re-absorbed, and which appeared to be precipitated by being deprived of its heat by the expanded air.

4. A hole, above the size of a crow-quill, was bored into a large air-vessel placed at the commencement of the principal pipe of the water-works of Derby. There are four pumps worked by a water-wheel, the water of which is first thrown into the lower part of this air-vessel, and rises from thence to a reservoir about 35 or 40 feet above the level; so that the water in this vessel is constantly in a state of compression. Two thermometers were previously suspended on the leaden air-vessel, that they might affume the temperature of it, and as soon as the hole above mentioned was opened, had their bulbs applied to the stream of air which issued out; the consequence of which was, that the mercury sunk some degrees in each. This sinking of the mercury could not be ascribed to any evaporation of moisture from their surfaces, as it was seen both in exhausting and admitting the air into the exhausted receiver mentioned in the last experiment, that the vapour which it previously contained was deposited during its expansion.

5. There is a curious phenomenon observed in the fountain of Hiero, constructed on a very large scale, in the Chemnitzcian mines in Hungary. In this machine the air, in a large vessel, is compressed by a column of water 260 feet high: a stop-cock is then opened: and, as the air issues with great vehemence, and in consequence of its previous condensation becomes immediately much expanded, the moisture it contains is not only precipitated, as in the exhausted receiver above mentioned, but falls down in a shower of snow, with icicles adhering to the nose of the cock. See Phil. Trans. vol. lli.

From this phenomenon, as well as the four experiments above related, Mr Darwin thinks "there is good reason to conclude, that in all circumstances where air is mechanically expanded, it becomes capable of attracting the fluid matter of heat from other bodies in contact with it.

"Now (continues he), as the vast region of air which surrounds our globe is perpetually moving along its surface, climbing up the sides of mountains, and descending into the valleys; as it passes along, it must be perpetually varying the degree of heat according to the elevations of the country it traverses: for in rising to the summits of mountains, it becomes expanded, having so much of the pressure of the superincumbent atmosphere taken away; and when thus expanded, it attracts or absorbs heat from the mountains in contiguity with it; and, when it descends into the valleys, and is compressed into less compass, it again gives out the heat it has acquired to the bodies it comes in contact with. The same thing must happen to the higher regions of the atmosphere, which are regions of perpetual frost, as has lately been discovered by the aerial navigators. When large districts of air, from the lower parts of the atmosphere, are raised two or three miles high, they become so much expanded by the great diminution of the pressure over them, and thence become so cold, that hail or snow is produced by the precipitation of the vapour: and as there is, in these high regions of the atmosphere, nothing else for the expanded air to acquire heat from after it has parted with its vapour, the same degree of cold continues, till the air, on descending to the earth, acquires its former state of condensation and of warmth.

"The Andes, almost under the line, rests its base on burning sands; about its middle height is a most pleasant and temperate climate covering an extensive plain, on which is built the city of Quito; while its forehead is encircled with eternal snow, perhaps coeval with the mountain. Yet, according to the accounts of Don Ulloa, these three discordant climates seldom encroach much upon each other's territories. The hot winds below, if they ascend, become cooled by their expansion; and hence they cannot affect the snow upon the summit; and the cold winds that sweep the summit, become condensed as they descend, and of temperate warmth before they reach the fertile plains of Quito."

Notwithstanding all these explanations, however, several very considerable difficulties remain with regard to the heat and cold of the atmosphere. That warm air should always ascend; and thus, when the source of heat is taken away by the absence of the sun, that the stratum of atmosphere lying immediately next to the earth should be somewhat colder than that which lies a little farther up; is not at all to be wondered at. We have an example somewhat similar to this in the potter's kiln; where, after the vessels have been intensely heated for some time, and the fire is then withdrawn, the cooling always begins at bottom, and those which stand lowermost will often be quite black, while all the upper part of the furnace and the vessels next to it are of a bright red. It doth not, however, appear why such degrees of cold should take place at the surface of the earth as we sometimes meet with. It is, besides, no uncommon thing to meet with large strata in the upper regions of the atmosphere, remarkable for their cold, while others are warmer than those at the surface; as we have been assured by the testimony of several aerial navigators. It is also difficult to see why the air which has once ascended, and become rarefied to an extreme degree, should afterwards descend among a denser fluid of superior gravity, though indeed the atmospherical currents by which this fluid is continually agitated may have considerable effect in this way. See the article WINDS.

For the quantity of water contained in the atmosphere, see the articles HYGROMETER, CLOUDS, VAPOUR, FOUR, &c. For the cause of the elasticity of the atmosphere, see ELASTICITY; and for an explanation of its various operations, see METEOROLOGY.

The uses of the atmosphere are so many and so various that it is impossible to enumerate them. One of the most essential is its power of giving life to vegetables, and supporting that of all animated beings. For the latter purpose, however, it is not in all places equally proper: we shall therefore conclude this article with some remarks on

The Salubrity of the ATMOSPHERE.—The air on the tops of mountains is generally more salubrious than that in pits. Dense air indeed is always more proper for respiration than such as is more rare; yet the air on mountains, though much more rare, is more free from phlogistic vapours than that of pits. Hence it has been found, that people can live very well on the tops of mountains where the barometer sinks to 15 or 16 inches. M. de Saufure, in his journey upon the Alps, having observed the air at the foot, on the middle, and on the summits of various mountains, observes, that the air of the very low plains seems to be the least salubrious; that the air of very high mountains is neither very pure, nor, upon the whole, seems so fit for the lives of men, as that of a certain height above the level of the sea, which he estimates to be about 200 or 300 toises, that is, about 430 or 650 yards.

Dr White, in the 68th volume of the Phil. Trans., giving an account of his experiments on air made at York, says, that the atmospherical air was in a very bad state, and indeed in the worst he had ever observed it, the 13th of September 1777; when the barometer stood at 30.30, the thermometer at 69°; the weather being calm, clear, and the air dry and fultry, no rain having fallen for above a fortnight. A flight shock of an earthquake was perceived that day.

The air of a bed-room at various times, viz. at night, and in the morning after sleeping in it, has been examined by various persons; and it has been generally found, that after sleeping in it the air is less pure than at any other time. The air of privies, even in calm weather, has not been found to be so much phlogiticated as might have been expected, notwithstanding its disagreeable smell.

From this and other observations, it is thought that the exhalations of human excrements are very little if at all injurious, except when they become putrid, or proceed from a diseased body; in which case they infect the air very quickly.

Dr Ingenhoutz, soon after he left London, sent an account of his experiments made in the year 1779 upon the purity of the air at sea and other parts; which account was read at the Royal Society the 24th of April 1780, and inserted in the 70th vol. of the Phil. Trans. His first observations were made on board a vessel in the mouth of the Thames, between Sheerness and Margate, where he found that the air was purer than any other sort of common air he had met with before. He found that the sea-air taken farther from the land, viz. between the English coast and Ostend, was not so pure as that tried before; yet this inferior purity seems not to take place always. The Doctor's general observations, deduced from his numerous experiments, are, "That the air at sea, and close to it, is in general purer, and fitter for animal life, than the air on the land, though it seems to be subject to some inconsistency in its degree of purity with that of the land: That probably the air will be found in general much purer far from the land than near the shore, the former being never subject to be mixed with land air."

The Doctor in the same paper transcribes a journal of experiments, showing the degree of purity of the atmosphere in various places, and under different circumstances; which we shall insert here in an abridged manner.

The method used in those experiments was to introduce one measure of common air into the eudiometer tube, and then one measure of nitrous air. The movement that these two sorts of elastic fluids came into contact, he agitated the tube in the water-trough, and then measured the diminution, expressing it by hundredths parts of a measure; thus, when he says, that such air was found to be 130, it signifies, that after mixing one measure of it with one of nitrous air, the whole mixed and diminished quantity was 130 hundredths of a measure, viz, one measure and 30 hundredths of a measure more.

"The different degrees of salubrity of the atmosphere, as I found it in general in my country house at Southal-Green, ten miles from London, from June to September, lay between 103 and 109. I was surprised when, upon my return to town to my former lodgings in Pall Mall Court, I found the common air purer in general in October than I used to find it in the middle of summer in the country; for on the 22d of October, at nine o'clock in the morning, the weather being fair and frosty, I found that one measure of common air, and one of nitrous air, occupied 100 subdivisions in the glass-tube, or exactly one measure. That very day, at two o'clock in the afternoon (it being then rainy weather), the air was somewhat altered for the worse. It gave 102. October the 23d, it being rainy weather, the air gave 102. October the 24th, the weather being serene, the air at nine o'clock in the morning gave 100. October the 25th, the sky being cloudy at 11 o'clock in the morning, the air gave 102. At 11 o'clock at night, from five different trials, it gave 105. October the 26th, the weather being very dark and rainy, the air gave 105, as before."

The air at Ostend was found by the Doctor to be generally very good, giving between 94 and 98. At Bruges, the air taken at seven o'clock at night gave 103. November the 8th, the air at Ghent at three in the afternoon gave 103.

November the 12th, the air of Brussels at seven o'clock P. M. gave 105½. The next day, the air of the lower part of the same city gave 106; that of the highest appeared to be purer, as it gave 104: which agrees with the common popular observation. November the 14th, both the air of the highest and that of the lowest part of the city appeared to be of the same goodness, giving 103. The weather was frosty.

November the 22d, the air of Antwerp in the evening gave 109½; the weather being rainy, damp, and cold. November the 23d, the air of Breda gave 106. The next day about 11 o'clock the air gave 102; the weather being fair, cold, and inclining to frost. At seven o'clock it gave 103. Next day, being the 25th, the air gave 104; the weather being cold and rainy. The 26th it gave 103; the weather being very rainy, cold, and stormy. November the 27th, the air at the Moordyke close to the water gave 101\( \frac{1}{2} \); the weather being fair and cold, but not frosty. This spot is reckoned very healthy. November the 28th, the air of Rotterdam gave 103; the weather being rainy and cold. November the 29th, the air of Delft gave 103; the weather being stormy and rainy.

November the 30th, the air of the Hague gave 104; the weather being cold, and the wind northerly. The first of December the weather underwent a sudden change; the wind becoming southerly and stormy, and the atmosphere becoming very hot. The day after, Fahrenheit's thermometer stood at 54°; and the common air being repeatedly and accurately tried gave 116; and that preserved in a glass phial from the preceding day gave 117; and that gathered close to the sea gave 115.

December the 4th, the air of Amsterdam gave 103; the weather being rainy, windy, and cold. The day after, the weather continuing nearly the same, the air gave 102. December the 10th, the air of Rotterdam gave 101; the weather being rainy. December the 12th, being in the middle of the water between Dort and the Moordyke, the air gave 109; the weather being remarkably dark, rainy, and windy. December the 13th, the air of Breda in the morning gave 109; the weather continuing as the day before. And in the afternoon, the air gave 106\( \frac{1}{2} \); the weather having cleared up. December the 16th, the air of the lower part of the city of Antwerp gave 105, that of the higher part 104; the weather being rainy and temperate. December the 17th, the air of Antwerp gave 107; the weather continuing nearly as in the preceding day. December the 19th, the air of Brussels gave 109; the weather being rainy, windy, and rather warm. December the 21st, the air of Brussels gave 106; the weather being dry and cold. The next day the air and the weather continued the same. December the 23d, the air of Mons gave 104; the weather being rainy and cold. December the 24th, the air near Bouchain gave 104\( \frac{1}{2} \); the weather being cloudy and cold. December the 25th, the air of Peronne gave 102\( \frac{1}{2} \); the weather being frosty. December the 26th, the air of Cuvilli gave 103; the weather frosty. December the 27th; the air of Senlis gave 102\( \frac{1}{2} \); the weather frosty. December the 29th, the air of Paris gave 103; the weather frosty. 1780, January the 8th, the air of Paris gave 100; the weather frosty. January the 13th, the air of Paris gave 98; hard frost.

Thus far with Dr Ingenhouz's observations. His apparatus was a very portable one, made by Mr Martini, which in reality is the eudiometer-tube and measure as used by Mr Fontana before he made his last improvement. "The whole of this apparatus (says Dr Ingenhouz) was packed up in a box about ten inches long, five broad, and three and a half high. The glass-tube or great measure, which was 16 inches long, and divided into two separate pieces, lay in a small compass, and could be put together by brass screws adapted to the divided extremities. Instead of a water trough, such as is used commonly, I made use of a small round wooden tub," &c.

The abbe Fontana, who has made a great number of very accurate experiments upon this subject, gives his opinions on the subject in the following words: "I have not the least hesitation in afftering, that the experiments made to ascertain the falsity of the atmospherical air in various places in different countries and situations, mentioned by several authors, are not to be depended upon; because the method they used was far from being exact (A), the elements or ingredients for the experiment were unknown and uncertain, and the results very different from one another.

"When all the errors are corrected, it will be found that the difference between the air of one country and that of another, at different times, is much less than what is commonly believed; and that the great differences found by various observers are owing to the fallacious effects of uncertain methods. This I advance from experience; for I was in the same error. I found very great differences between the results of the experiments of this nature which ought to have been similar; which diversities I attributed to myself, rather than to the method I then used. At Paris I examined the air of different places at the same time, and especially of those situations where it was most probable to meet with infected air, because those places abounded with putrid substances and impure exhalations; but the differences I observed were very small, and much less than what could have been suspected, for they hardly arrived at one-fiftieth of the air in the tube. Having taken the air of the hill called Mount Valerian, at the height of about 500 feet above the level of Paris, and compared it with the air of Paris taken at the same time, and treated alike, I found the former to be hardly one-thirtieth better than the latter.

"In London I have observed almost the same. The air of Illington and that of London suffered an equal diminution by the mixture of nitrous air; yet the air of Illington is esteemed to be much better. I have examined the air of London taken at different heights (for instance, in the street, at the second floor, and at the top of the adjoining houses), and have found it to be of the same quality. Having taken the air at the iron gallery of St Paul's cupola, at the height of 313 feet above the ground, and likewise the air of the stone gallery, which is 202 feet below the other; and having compared these two quantities of air with that of the street adjoining, I found that there was scarce any sensible difference between them, although taken at such different heights.

"In this experiment a circumstance is to be considered, which must have contributed to render the above-mentioned differences more sensible: that is, the agitation of the air of the cupola; for there was felt a pretty brisk wind upon it, which I observed to be stronger and stronger the higher I ascended; whereas

(A) It is plain that Dr Ingenhouz's method is not implied in this remark; since the Doctor's experiments were made long after, and the method used by him was properly that of Mr Fontana. in the street, and indeed in all the streets I passed through; there was no sensible wind to be felt. This experiment was made at four in the afternoon, the weather being clear. The quicksilver in the barometer at that time was 28.6 inches high, and Fahrenheit's thermometer stood at 54°."

A few lines after, Mr Fontana proceeds thus:—

"From this we clearly see, how little the experiments hitherto published about the differences of common air are to be depended upon. In general, I find that the air changes from one time to another: so that the differences between them are far greater than those of the airs of different countries or different heights. For instance, I have found that the air of London in the months of September, October, and November, 1778, when treated with the nitrous air, gave II, I, 1,90, and II, II, 2,25, which is a mean result of many experiments which differed very little from each other. The 26th day of November last, I found the air, for the first time, much better; for it gave II, I, 1,80, and II, II, 2,20; but the 14th of February 1779, the air gave II, I, 1,69 and II, II, 2,21; from whence it appears, that the air of this 14th of February was better than it had been six months before. There can be no doubt of the accuracy of the experiments, because I compared the air taken at different times with that which I had first used in the month of September, and which I had preserved in dry glass bottles accurately stoppered."

This difference in the purity of the air at different times, Mr Fontana farther remarks, is much greater than the difference between the air of the different places observed by him: notwithstanding this great change, as he observed, and as he was informed by various persons, no particular change of health in the generality of people, or facility of breathing, was perceived.

Mr Fontana lastly concludes with observing, that "Nature is not so partial as we commonly believe. She has not only given us an air almost equally good everywhere at every time, but has allowed us a certain latitude, or a power of living and being in health in qualities of air which differ to a certain degree. By this I do not mean to deny the existence of certain kinds of noxious air in some particular places; but only say, that in general the air is good everywhere, and that the small differences are not to be feared so much as some people would make us believe. Nor do I mean to speak here of some vapours and other bodies which are accidentally joined to the common air in particular places, but do not change its nature and intrinsical property. This state of the air cannot be known by the test of nitrous air; and those vapours are to be considered in the same manner as we should consider so many particles of arsenic swimming in the atmosphere. In this case it is the arsenic, and not the degenerated air, that would kill the animals who ventured to breathe it."