METEOROLOGY

METEOROLOGY;

THAT science which investigates the phenomena of our atmosphere* (commonly called meteors), giving an account of the circumstances attending each, and explaining the causes from whence they arise.

In considering this science, we find the objects of it naturally divided into two classes, viz. those which rise high in the heavens, seemingly without any connection with this earth; and others which are more particularly connected with the earth, or are perceptible only in the lower regions of the atmosphere. The former, which may properly be called celestial meteors, are only three in number, viz. the large fire-balls, falling stars, and aurora borealis. The second class is much more numerous; including the phenomena of the ordinary winds, rain, hail, snow, clouds and vapours of all kinds, thunder and lightning, hurricanes, whirlwinds, water-spouts, ignes fatui, and other wandering luminous appearances; not excepting the various changes of the atmosphere itself, with regard to its specific gravity, rarefaction, heat, and moisture, as indicated by the barometer, thermometer, and hygrometer.

To treat of all these in a satisfactory manner, it is plain that we ought to have an intimate acquaintance with the constitution of the atmosphere; with the nature of those powerful agents by which it appears to be principally influenced, viz. fire, light, and electric fluid; and with their peculiar modes of operation and action upon one another and upon the atmosphere, and this in

every possible variety of circumstances. Nor is even all this sufficient. The various phenomena of rain, wind, snow, thunder, heat, cold, &c. are known to depend very much upon the situation of different places on the surface of the earth; and their occasional variations are with great reason suspected to proceed, partly at least, from changes which take place in the bowels of the earth*: whence a meteorologist ought not only to be perfectly well acquainted with geography, but with mineralogy also; and that to an extent at which human knowledge will probably never arrive.

In a science so very difficult, it is not to be supposed that any thing like a certain and established theory can be laid down: our utmost knowledge in this respect goes no farther as yet than to the establishment of a few facts; and in reasoning even from these, we are involved every moment in questions which seem scarcely within the compass of human wisdom to resolve.

In considering the subject of meteorology, it will readily be admitted, that the whole atmospheric phenomena depend some how or other upon the action of the sun upon the earth, and the annual and diurnal revolutions of the latter. As these causes, however, are always invariably the same, why do we not find the same regularity in meteors that we do in other phenomena of nature? The eclipses of the sun and moon, for instance, which depend on the different positions

* See Atmosphere.

1 Meteors, how divided.

2 Difficulties attending the subject.

Meteor, Meteorological.

* See Weather.

5 Causes probably concerned.

positions of the earth and moon with regard to the great luminary, are found to follow a certain and regular course; so that the very same eclipses, both as to quantity and duration, which happened before will happen again. But with meteors the case is quite different. Most of the atmospheric phenomena are so various and uncertain, that no person can pretend to reduce them to any kind of rule. Every succeeding year, for instance, differs in a vast number of particulars from that which preceded it, even in such as are the most similar to one another. Sometimes we find a number of years successively similar to one another, and another set quite different taking place immediately after them; and some have even fancied that this succession took place every 19 years, nearly the time of the revolution of the moon's nodes, though the observations on which this opinion is built are far from being sufficient to establish it: at any rate, the dissimilarity between the phenomena of different years may sufficiently warrant us to conclude, that other causes besides the regular action of the sun and revolution of the earth are concerned. Some of these causes may be supposed to be fermentations and other commotions within the bowels of the earth itself; but as all fermentation is a regular process, and takes place only in certain circumstances, of which heat is a very considerable one, why is there not annually a certain quantity of this fermentation excited, and why are not regular effects observed in proportion? It does not indeed appear, that the immense variety which occurs in meteorological appearances can by any means be accounted for but by the interference of some causes in their own nature irregular; that is, capable of such endless variety, that no assignable space of time is sufficient to exhaust it. These causes, as they cannot be proved to exist either on the surface of the earth or in its internal parts, must be sought for in the celestial expanse itself. Sir Isaac Newton supposed the planets to be influenced by the comets, and that from the tails of the latter some of the finer parts of our atmosphere were produced. He even supposed, that from these bodies a quantity of water, imagined to be wasted in the various operations of nature, might be supplied. But if it is not unreasonable to suppose that comets answer some such purposes in nature, it is as little unreasonable to think that the planets may influence the atmospheres of one another. That this must be the case indeed is very probable, not only on account of the light they reflect upon one another, but also by reason of their spheres of mutual attraction, which extend an immense way, and are so powerful in the planets Jupiter and Saturn, that they disturb the motions of each others satellites as they pass. But besides even these causes, if we allow them to be such, there are others which take place in the immense void betwixt the celestial bodies, and which has with great impropriety been determined an absolute vacuum. That changes do take place in this space, is evident from what is related of the temporary disappearance of some of the satellites of Saturn, and their sudden re-appearance, without any perceptible change in our atmosphere so as to affect our view of other celestial objects. It may appear ridiculous to think, that a change in such distant regions should have any influence upon

the atmosphere of the earth; but we must remember, that if the universe is connected together as one vast system, which we have every reason to believe, it is as impossible that a change can take place in any part without affecting the whole in some degree, as it is impossible to change any part of a clock or watch without in some measure affecting the whole movement.

But of all the changes which take place in the celestial regions, those which affect the sun seem most likely to produce changes in our atmosphere, and to be the hidden cause of many meteorological phenomena. That the sun is not exempt from ^S those changes, is evident from the spots which are al-^sun ways or for the most part to be seen on his disk when viewed through a telescope. It has been observed in some years, that the sun has seemed to lose his influence, and even to the naked eye appeared much dimmer than usual. In such cases it is impossible but our atmosphere, and even the whole solar system, must have been affected; and not only must the seasons for the present time have felt the malignant influence of those spots, but the atmosphere itself may have acquired such a disposition as to produce seasons of a peculiar nature for a number of years afterwards. If it be true, according to the hypothesis of some, that the sun is supplied with fuel by comets falling into his body, it is plain that every new accession of this kind must have a proportionable effect upon all the bodies exposed to his light. If the comets do not perform any such office, still it is very probable that they answer some purposes to the planets, because they are never seen without the planetary regions: and though their influence be not immediately perceptible, it is impossible to prove that they have none, nor indeed is it probable that they have not; for we are very certain, that the influence of any object extends as far as its light, and how much farther we cannot tell. Considering the matter in this view, therefore, there is not a spot which can obscure the sun, a comet that can appear in the celestial regions, a planet that can approach the earth, nor perhaps a belt or spot which can take place on Mars, Jupiter, or Saturn, which may not be productive of important changes in our atmosphere, and affect the meteors produced by it in many different ways.

It would no doubt be an error to have recourse to so many obscure causes, were there any plain and obvious ones from whence the phenomena could be deduced. But the endless variety of meteors which occur throughout every part of the globe, plainly show that the causes, whatever they are, must be infinitely varied also. The principal one is no doubt the action ⁶ of the sun upon the earth and atmosphere in its va-^{tion of} rious positions: but this is regular; and, did nothing ^{moon, and} else interfere, would produce regular effects. ^{planets, concerned.} Secondary causes probably are the action of the moon and planets: but these also are regular, though much more diversified than the former: so that we are at last obliged to have recourse to causes still more obscure and remote, as comets, spots on the sun, and changes taking place in the ethereal fluid which pervades the whole celestial expanse. These we must either assign as the remote causes of the phenomena of our atmosphere,

sphere, or admit others equally obscure; or we must be contented to own our ignorance, as indeed must all events be frequently the case.

But though, to satisfy ourselves, such conjectures may occasionally be indulged, it is not from them that we are to derive any of the regular phenomena of nature; for these are evidently owing to the settled and established action of heat, light, and electric matter, which have already been enumerated as the great powers influencing, and indeed in a great measure forming, the substance of our atmosphere. The most remarkable effects of these are,

1. Evaporation. This, which is the principal cause of almost all the meteors of our atmosphere, may be reckoned in a more particular manner the effect of heat. Upon this principle it is explained under the article CHEMISTRY, where vapour is shown to be a compound of water and fire; and such it is supposed to be by M. de Luc, in his Treatise on Meteorology, as well as by other philosophers of the highest rank. In considering this operation, however, as carried on by nature, we will soon find, that it proceeds in a manner very different from what takes place in our chemical operations. In the latter, evaporation is merely the effect of heat; and the process cannot go on without a considerable degree of it, especially if the vessel containing the fluid be close. In the natural way, on the contrary, the process goes on under almost every degree of cold we know; the vapours ascend to an height which has never yet been determined; and, from the extreme cold which they sustain, show evidently that they are connected with our atmosphere by means of some other agent besides heat. From the continual ascent of vapour indeed, if the operations of nature were of the same kind with those of art, the upper parts of our atmosphere would be always involved in a fog, by reason of the condensation of the vast quantity which continually ascends thither; but so far is this from being the case, that in those elevated regions to which the vapours continually ascend, the air is much drier than at the surface of the ground. This was experienced by M. de Saussure and M. de Luc in their journeys up the Alps. The air was there found to be excessively dry, and evaporation to go on much more rapidly than below; so that the surface of their bodies was parched up, and an excessive thirst took place by reason of the great absorption of the moisture. The same dryness was manifest by the hygrometer, which could scarce ever be brought to indicate any moisture, even when our travellers were surrounded with clouds, hail, and rain. From many experiments, indeed, it is evident, that water, after being reduced into a state of vapour, is capable of undergoing a certain change, by which it lays aside its fluidity entirely, and even to appearance its specific gravity; so that it becomes, as far as we can judge, a substance totally different from what it was before. This may be understood from the common operation of slacking lime; for in that case, the water unites with the lime so intimately, that the whole assumes the form of a dry powder, extremely greedy of moisture, and which cannot be reduced to its former state of quicklime without undergoing a much greater degree of heat than the water

by itself could bear. The same thing is manifest from mixing dry plaster of Paris with water; for thus a vast quantity of the water is fixed, and becomes in a manner solid. A still more remarkable instance is in sending the steam of water over red-hot iron; for there the fluid unites in such a manner with the metal, that it cannot be expelled from it even by the heat of a burning-glass. Other instances are mentioned under the article WATER: here we are to consider the changes which the element undergoes after being reduced to the state of vapour. The first of these is, its assuming the appearance of smoke or fog when mixed with the common atmosphere; which smoke, when examined by a microscope, appears to be composed of an infinite number of spherules of water, hollow, and filled with a fluid specifically lighter than air, by which means they ascend in it. As long as the aqueous vapour retains this visible form, it retains also its humidity, and will again become a liquid, and wet whatever comes in its way; and this the more readily, while it retains any sensible degree of heat. As the vapour cools in the atmosphere, it gradually assumes an aerial state, mixing itself with the air so as to be no longer distinguishable from it. In this state the air itself does not by any means appear to become more moist, but continually drier the more water it receives. This, however paradoxical it may seem, is a certain fact: for in summer, though we are assured that evaporation goes on very rapidly from the surface both of the sea and land, yet the air, so far from being moist, is much drier than at any other time; and yet we know that the whole quantity evaporated is some how or other received by the atmosphere. After the water has attained to this state, our inquiries concerning it must in a great measure stop. We know not now, whether it has the form of small hollow spherules, or whether it really becomes part of the atmosphere itself, and assumes the form of what we call dephlogisticated air. From some experiments in which that kind of air is produced from water, we are certain, that part of this element is converted into air: but in these operations, the evaporation of the water is prevented by being carried on in close vessels; so that we cannot tell whether that which would be mere steam in the open air, becomes dephlogisticated air in close vessels or not. From the immense waste of dephlogisticated air, indeed, and the vast quantity which always surrounds the earth, we may suspect that the water, after undergoing the natural process of evaporation, does really become changed into this aerial fluid; and thus we will have a more ample source of it than can be derived from vegetation, or any other cause with which we are yet acquainted.

On this subject M. de Luc has some very curious observations, built principally upon the new doctrine of the composition of water; which, though a position maintained by the antiphlogistians, is by no means inconsistent with the existence of phlogiston, but rather a proof of it. Our author first began to alter his sentiments concerning the aqueous existence of vapour in the atmosphere, from the circumstance already mentioned concerning the great dryness of the upper atmospheric regions already taken notice of.

A very remarkable instance of this was, that the rule of his cane dropped off during his journey up one of the Alpine mountains, which he never had observed it to do before. It is observed likewise, that the air in these elevated regions is somewhat drier in the night than in the day-time; for which M. de Lue gives the following reason, viz. that the air on the plains being condensed by the cold, the superior air must subside, and the air on the mountains of course be replaced by the drier air from above them; though he thinks that this dryness may also be imputed in part to some other cause. This increase of dryness in the night, however, seems less constant than that in the day-time. Our author has often arrived at the tops of mountains before sun-rise, and sometimes found the grass covered with dew; though he never had the good fortune to be able to determine the state of the air for want of an hygrometer; nor indeed could the appearance of dew be any certain indication of the state of the atmosphere, there being strong reasons to believe, that dew is occasioned in great measure by vegetables themselves; for grass, when covered with glass plates, was found to become moist as well as that which had been exposed to the open air. In this case, the plates became moist both on the upper and under sides; but when suspended a foot above the ground, they were found to be covered with dew only on the upper part.

¹⁵ M. Saussure's method of accounting for the dryness of the air.

The dryness of the air on the tops of high mountains was otherwise accounted for by M. Saussure.—When on Mount Blanc, at the height of 7200 feet above the level of the sea, he found, that from six in the evening till half past five next morning, his hygrometer moved 21 degrees (the whole scale containing 100) towards dryness. But this he accounts for by saying, that from sunrise to three or four in the afternoon, the quantity of vapours in the neighbourhood of the earth is continually diminishing, because they ascend in the atmosphere, either in virtue of their own levity, or by means of a vertical wind, which he supposes to be produced by the heat of the sun; that, from the time just mentioned till next morning, their quantity increases in the lower strata, because the upper vapours re-descend in proportion as they condense; and that in the higher regions of the atmosphere, the reverse ought to be the case, as the upper strata are then left drier by the previous descent of the vapours. This argument, however, is contradicted by M. Saussure himself in another part of his work; where he says, that in the middle of the day, when the sun is hottest, the air in the neighbourhood of the earth contains really more water than it does at the moment when a refreshing dew falls. It is besides impossible that a vertical wind can ever be occasioned by the heat of the sun; for this produces only a general expansion of the whole body of the atmosphere, as a condensation of it is occasioned by the action of cold: neither could any considerable quantity of vapour (supposing with M. Saussure that it is a chemical solution of water in air) descend in the night-time; for, according to him, this compound differs very little from common air in its capacity of being expanded and condensed. Neither, according to M. Saussure himself, can any part of the water

with which the air is combined descend at all, until some portion of the former becomes super-saturated with it, that is, till it has received more than it can hold in solution. But if this should happen to be the case, the superfluous quantity would then appear in the form of a mist or cloud, and the hygrometer would indicate extreme humidity; whereas the contrary indication constitutes the difficulty.

It is besides evident, from innumerable instances, that mere cold will not by any means occasion the condensation of aerial vapour. A most remarkable example of this is given by M. de Lue, in an account of a storm in which he was involved on one of the Alps. "Though the hygrometer (says he) was within 33$\backslash frac\{1\}\{2\}$ degrees of extreme dryness, or 66$\backslash frac\{1\}\{2\}$ from extreme humidity, thick clouds formed around us, which obliged us to think of retreating: in a little time the summit of the mountain was surrounded by them: they spread and covered the whole horizon: a premature night surprised us in a very dangerous road; and we suffered one of the most violent tempests I ever experienced, of wind, rain, hail, and thunder. The storm lasted great part of the night, and extended all over the neighbouring mountains and plains; and after it had ceased, the rain continued, with only a few intermissions, till next day at noon. In one of these intervals, I examined the hygrometer on the outside of our cabin; it showed only 1$\backslash frac\{1\}\{2\}$ more humidity than before; and even this increase was no more than what the difference of heat was capable of producing. Nevertheless, new clouds continually rolled around us; and the rain, which presently began again, accompanied us as it were by fits to the bottom of the mountain. When arrived there, we saw the clouds disperse entirely. I observed the hygrometer again in the open air; and though the earth was all drenched with water, and the heat of the sun much less, the hygrometer was 1$\backslash frac\{1\}\{2\}$ drier than it had been two days before, after a course of fine weather. Where was all this water, and all the ingredients of the storm, while the hygrometer showed such a degree of dryness in the very stratum where it was formed?"

M. de Lue adopts the opinion concerning vapour which has been published in this work, under the articles CHEMISTRY, EVAPORATION, and many others, viz. that it is a combination of fire with water. By vapour, however, he does not mean the visible steam issuing from heated liquids, but that invisible and subtle fluid which is found to be formed even in vacuo, and which of consequence disproves the hypothesis of those who hold that vapour is a solution of water in air. Our author, however, gives a solution of the difference betwixt what he calls fog or mist and vapour, which seems not founded upon any evident principle. According to him, this vapour cannot subsist unless the particles of water united to the fire be at a certain distance from one another. When this distance is lessened, a decomposition takes place by reason of the attraction of the aqueous particles to one another; and they then appear in their proper form of a liquid, the fire dissipating itself through the atmosphere. The smallest distance to which the particles can be brought without any decomposition,

varies according to the temperature; but is always constant in the same degree. When the thermometer stands at temperate, or thereabouts, watery vapours, compressed into the smallest space they can bear, are found to have between $\backslash frac\{1\}\{2\}$th and $\backslash frac\{2\}\{3\}$th of the elasticity of air; but have less than $\backslash frac\{1\}\{2\}$th of its weight. If such vapour, however, be mixed with air, the minimum distance is greatly increased, by reason of the interposition of aerial particles; and thus it can subsist under a much greater pressure than it could otherwise endure. In the heat of boiling water they can, without any mixture of air, bear the pressure of the atmosphere: for ebullition, under any given pressure, cannot take place until the vapour produced in the liquor has acquired a degree of expansive force sufficient to raise the liquor into bubbles under that pressure; and as long as the vapour retains this heat, it must continue capable of resisting the same pressure. As the heat abates, a decomposition begins; hence the opaque steam over boiling water, which, by becoming vapour again by uniting with the fire it meets with in a larger space, is diffused by its expansibility. Thus, vapours are continually undergoing decompositions and new vaporifications. This evaporation of the clouds after they were once formed, M. de Luc observed very evidently; some parts being continually detached, and gradually diminishing and disappearing, while new ones are formed; so that the clouds do not continue the same for two moments together; and the evaporation goes on so fast, that a cloud could not subsist without constant and large supplies.— These phenomena appear to be independent of heat and cold: for sometimes clouds form suddenly in the middle of a hot day; and after they have poured down their water, all is clear again: and sometimes they evaporate after sunset, gradually vanishing in the calmest weather without any change of place.— The appearances are such as would be produced by a large mass of water in violent ebullition, suspended invisibly in the atmosphere; and the similarity of effect naturally points out an analogy in the cause; that is, a source of vapour in the atmosphere itself. It is only when the vapour is produced too abundantly and too rapidly to be dispersed by evaporation that rain is formed; the vesicles in this case running together, and the water falling to the lower part, as it does in soap-bubbles, till they become thin enough to burst.

With regard to this explanation, however, though it may account for the artificial production and decomposition of vapour, it does not seem to answer for that produced in the natural way. That the latter is certainly in a state of dryness, cannot be denied; but it cannot be proved that ever any artificial steam is so, let the heat be what it will. Though the approach of the aqueous particles to one another, therefore, by a diminution of temperature, may occasion the decomposition of artificial steam, it does not seem to be so in the natural way; nor is there any source from which we can reasonably infer a very great and sudden accession of vapour from the earth to the upper regions of the atmosphere in particular places, which might increase the proximity of the aqueous particles, and thus bring on rain according to M. de Luc's hypothesis. It must likewise be remembered,

that, according to the doctrine upon which M. de Luc found his system, water, when decomposed, is not converted into one species of air, but into two, viz. the dephlogisticated and inflammable, each of which contains a quantity of undecomposed water; so that there is still some ambiguity in the experiments; and as the two shrink up into very little bulk by their union, there should seem to be danger of producing vacuums of immense extent by the sudden union of the two airs in the high atmospheric regions.— These vacuums, were they to extend over the whole space occupied by a large cloud, might occasion dreadful concussions by the rushing in of the air to supply them: or, even if we suppose them to be dispersed interstitially, they must certainly affect the barometer very greatly; which does not appear to be the case. M. Saussure, who passed several nights on one of the Alps, at the height of 10,578 feet above the level of the sea, does not mention any considerable variation of the barometer, though he was frequently involved, during that time, in the most violent storms of hail, wind, snow, thunder, and lightning. In the warm climates also, where we should think that the vast deluges of rain would often sink the barometer to an amazing degree, yet we seldom hear of any remarkable variation. M. de Prielong, in his account of Meteorological Observations made at Goree in 1787, informs us, that there were 16 or 18 hurricanes; and that the greater part of these raised the barometer from one twelfth to a sixth part of an inch; others sunk it as much, and some did not at all affect it. Another cause must therefore be concerned, which diminishes the rarefaction, or condenses the air as fast, or nearly so, as the condensation of the vapour would rarefy it: and that another cause really is concerned, we learn from M. Reynier, who has made a great number of observations upon the vapours formed on the Alps, and gives us the following account. "In the morning, the vapours condensed by the coldness of the night rise along the mountains in proportion as the sun rises above the horizon.— When the weather will be fine, they glide uniformly on the brink of the mountain, and rise over it by a regular motion, somewhat slow. When rain impends, the motion is irregular: they are alternately attracted and repelled by the mountain, and rise like elastic bodies rebounding. In a stormy season, particularly when there will be hail, the motions are still more rapid and irregular." "This observation (add the Monthly Reviewers, from whom the above quotation was taken) may be confirmed in the mountainous countries of Great Britain; we have seen it among the mountains of Cumberland, particularly in the neighbourhood of Kefwick. M. Reynier observes, and the observation is sufficiently near the surface not to be overlooked, that the appearance is electrical."

Before we proceed to give any solution of the decomposition of vapour, it will be necessary to consider a little farther the nature of that fluid or fluids into which water is converted when it assumes a dry state to which in the atmosphere. In the experiments of Lavoisier and others, it seems with great probability to be reduced into the two different kinds of air already mentioned; but M. de Luc does not think that the atmosphere.

mosphere naturally contains any such fluids. Air, when artificially decomposed, does not contain inflammable, but phlogisticated or mephitic, air, mixed in a certain proportion with the dephlogisticated kind. These have different specific gravities; and our author is of opinion, that two fluids of this kind could not mingle uniformly with one another without separating through time; and as the dephlogisticated air has the greatest specific gravity, it thence follows, that the under parts of the atmosphere ought to be almost entirely composed of that kind, and the upper strata of the mephitic or inflammable kind. But this does not appear to be the case; so that M. de Luc concludes, that air is an homogeneous fluid, every particle being similar to every other, and consisting of all the ingredients that we extract from the mass, together, probably, with others yet unknown to us. Though a mixture of vital and mephitic air produces many of the effects of atmospheric air, we cannot thence conclude their absolute identity: the one may suffer a decomposition in order to the production of these effects, while the other produces them immediately. The mixture may support life for a time, but will it equally maintain health also? Though mephitic air by the mixture of one-third of vital air is prevented from being immediately fatal, we are not authorized to conclude that it is altogether innocent. On the whole then, if it is not in the immediate product of evaporation that rain has its source; if the vapours change their nature in the atmosphere, so as to be no longer sensible to the hygrometer or to the eye; if they do not become vapour again till clouds appear; and if, when the clouds are formed, no alteration is observed in the quality of the air; we must acknowledge it to be very probable, that the intermediate state of vapour is no other than air; and that the clouds do not proceed from any distinct fluid in the atmosphere, but from a decomposition of a part of the air itself, perfectly similar to the rest."

This opinion of M. de Luc appears the more probable, that the two ingredients into which water is artificially resolved, by the late experiments do not by any means re-compose atmospheric air by simple mixture; for these explode with extreme violence on the application of flame: the common atmosphere, also, when decomposed, does not resolve itself into dephlogisticated and inflammable air, but into the former, and what is called phlogisticated or the mephitic kind, the difference of which in specific gravity is much less than between dephlogisticated and inflammable airs, though it is probable that even these are connected either by means of a chemical union, or by some other ingredient we do not yet know. By this union the qualities of both may be in some measure changed, and a third kind of substance formed, as neutral salts may be made out of acids and alkalies. This third substance, which we call the common atmosphere, is proper for preserving both animal and vegetable life, which neither of the two ingredients are capable of doing; for plants wither and die in dephlogisticated air, and animals are suffocated in a moment by the mephitic kind; nor, indeed, do we know whether the dephlogisticated kind be altogether proper for the sustenance of animal life for any considerable length of time. It certainly will sustain it much longer than an equal quantity

of atmospheric air, even the purest we are acquainted with; but it is equally certain that animals confined in it die much sooner than according to its apparent purity they ought to do. It is not an unreasonable hypothesis, therefore, that though water may be artificially separated into the two fluids called dephlogisticated and inflammable airs, yet in the natural way the decomposition does not proceed beyond a certain point, which we may, in a subject of such an obscure nature, call a chemical union, or a state in which the two ingredients exist, and are capable of being separated when the air comes into contact with certain substances. Hence, when the atmosphere is taken into the lungs of an animal, some of the dephlogisticated part may enter the blood, and the phlogisticated part combining more intimately with the rest, may form fixed air or part of it appear in its proper form. In like manner, when the common atmosphere comes in contact with a vegetable, it is possible that the phlogisticated part may be absorbed by it, and the dephlogisticated part set free, which in the atmosphere may form new combinations, &c.

Granting this to be the case, and we can scarcely hope for a more probable conjecture on the subject, the decomposition of the vapour will be easily accounted for. If by any natural process the water can be converted into air, and if the latter is only water partially decomposed: then, by an inversion of the process, air may be instantly re-converted into water, and will become visible in fog or mist, or be condensed into rain, consisting of greater or smaller drops, according to the degree to which this inverted process is carried. With regard to the means used by nature for carrying on these two opposite processes, we can say very little; because the agents concerned in them are entirely beyond the reach of our senses. From M. Raynier's observation, indeed, of the clouds being attracted by the hills, it would seem probable that electricity was ultimately concerned, but in what manner we cannot determine. On this hypothesis, however, we may explain the phenomenon taken notice of by M. de Luc and others, viz. that even during the time of excessive rains the hygrometer showed scarce any signs of moisture, and that the clouds were in a constant state of evaporation or dissolution in the air. The hygrometer, we know, cannot show signs of moisture, unless it absorbs it: and it cannot absorb, unless the air around it really contains more vapour in an aqueous form than the hygrometer itself does. But in very elevated regions this can scarce ever be the case. So much of the pressure of the atmosphere is then taken off, that the water contained in any substance resolves itself into vapour with the utmost facility. Hence bodies brought from the lower regions into the higher will undoubtedly part with a great deal of the moisture they contained in the lower parts of the atmosphere, and which was kept in it by the superior pressure of the atmosphere in these parts. For the same reason, though the air in the upper regions should be made ever so moist, a body such as the hygrometer can never absorb so much as it would otherwise do, because the water in these regions has a natural tendency to fly away from it. It appears, however, that there was in reality some variation of the hygrometer, though small; and had it been possible to con-

struck an hygrometer of materials found on the top of the mountain, which might be said to be naturalized to the climate, the scale of variation might probably have been larger, though there is no reason to think that it ever would be so large as on the plain. For the same reason, as soon as the process has been inverted, by which water was converted into air, the evaporation instantly takes place in the vapour that has been produced. We are to consider, that the atmosphere is never disposed to let fall the whole of the vapour it contains, for this would amount almost to an annihilation of it. Both processes go on at once; and it is only in particular parts that the reverse process takes place. Thus clouds are formed: but as these seldom continue stationary, they no sooner come into a situation where the contrary operation is going on, than they begin to evaporate; and even in the very same place, as soon as the condensing process has stopped, the other begins; as we see that even in the most damp and moist weather there is a constant evaporation going on; so that during the very time that rain is falling, the atmosphere is taking up what lies upon the ground.—Hence also we may see why the hygrometer indicated a greater degree of dryness in the night than in the day time, viz. because the evaporation from the earth is less during the night than in the day time.

II. Having now in some measure explained the phenomena of natural evaporation, we must next consider those of the Electricity of the atmosphere. Under the article ELECTRICITY, the nature of that fluid is fully discussed, and its identity with the solar light rendered so probable, that there seems no farther occasion for entering into speculations upon the subject. We shall therefore, without taking any notice of the arguments of M. de Luc for its being a compound fluid, proceed to consider, according to the principles laid down in that article, how far electricity is concerned in producing the phenomena of meteorology.

In this inquiry we must observe, that as none of the agents by which those phenomena are produced can act by themselves, but must always be assisted by the rest, so we are not to ascribe any one phenomenon to the influence of a single agent without the rest.—Thus, though in evaporation heat is principally concerned, and though evaporation is the principal cause of the appearance of clouds, &c. yet we do not find that heat is the sole cause of evaporation; neither is evaporation the sole cause of the appearance of clouds. In like manner, though electricity is the principal cause of many of the more grand phenomena of nature, yet electricity does not act by itself, but in conjunction with aqueous vapours; and when the atmosphere ceases to contain any of these vapours, it is highly probable that it ceases to manifest any of the common effects of electricity also.

The general electricity of the earth has, under the article ELECTRICITY, been shown to depend upon the absorption of the rays of the sun by the land and water, especially by the latter. As this absorption must undoubtedly be strongest in those places where the greatest quantity of rays fall upon the surface, it follows, that the emission must be greatest where the fewest are absorbed; that is, at the poles. Hence, were there no obstacles, the electrical fluid would issue

forth in vast quantities at each pole, very little being emitted in the intermediate spaces. By reason of the resistance made by the great body of the earth, and the immense fields of snow and ice with which the polar regions are constantly enveloped, and which are much less perfect conductors than liquid water, the electric fluid, once absorbed, has no free passage through any particular part of the globe, and therefore forces out every where throughout the whole surface. This passage is facilitated by the moisture contained in the atmosphere; and thus the processes of evaporation and electricity assist one another: for where the air has for a long time been very dry, we find that the electric fluid cannot readily pass, and violent thunder and lightning, nay sometimes earthquakes, are ready to ensue. Hence also the common observation relative to thunder, viz. that there is seldom much thunder when it begins to rain before the thunder comes on: The reason is, that the rain, being an excellent conductor, facilitates the passage of the electric matter through the air, and keeps up the equilibrium without any violence or explosion.

As the electric matter gets out of the earth, it is naturally driven upwards to the higher parts of the atmosphere, where it probably assists in the decomposition of air, or resolving air into water, as has been already said. When clouds are formed, it presses strongly upon them by reason of their conducting nature: and hence all clouds, whether high or low, are found to be electrified; as are likewise all fogs, of whatever kind. The fluid getting still higher and higher, at last ascends beyond the regions of our atmosphere, into the unknown spaces which are the residence of those first of all created agents which conduct the planets round the sun, and act as the primum mobile of nature.

Thus there is a circulation in the electric fluid as there is in the water. It descends originally from the sun; pervades the whole substance of the globe; and perspiring, as it were, at every pore, ascends beyond the clouds; and, passing the extreme boundaries of our atmosphere, returns to the sun from whence it came. As the sphere of its action in returning, however, must always increase, it follows, that after it has got beyond the bounds of the atmosphere, the signs of its action must continually become less and less, nay, most probably vanish entirely; because it is there opposed by an immense quantity of similar matter, acting in an opposite or different direction from itself.

This last consideration leads into a very curious speculation, and in a very plausible manner answers an objection, the force of which it would otherwise be very difficult to avoid. "If the electric fluid be no other than the light of the sun absorbed by the earth, and emitted from it again by innumerable small vents, how comes it to pass that it is not perpetually drained off from the upper regions as fast as it arrives, without showing any sign of being resisted. The phenomena of thunder, of rain, nay of every meteor, manifestly show that it often meets with very great resistance; but this could not happen, unless there was without the atmosphere something capable of resisting and counteracting the vehement impulse of all the electric fluid with which the earth is filled. This resistance is very evident: for if there were none, there could

could not be any accumulation of electricity in the upper regions of the atmosphere, but a rarefaction in it would take place similar to what there is of air in the same regions. But this is so far from being the case, that all the electrical phenomena are much stronger in the upper than in the lower regions."

²⁸ To solve this objection, Mr Morgan, in a late paper in the Philosophical Transactions, supposes that an absolute vacuum, such as he imagines the celestial spaces to be, is absolutely impenetrable by the electric fluid. But this seems not far from a contradiction. Sir Isaac Newton imagined the celestial spaces to be void of all matter, on account of the apparent facility with which the planets move through them; and we see that the rays of light, the impulse of which is accounted scarce any thing at all, do penetrate them. To suppose, indeed, that a mere non-entity can act either by resistance or any way else, is an absurdity. How can any person imagine, that a perfect vacuum, which even a feather by its weight can cause it pervade from one end to the other, should be impenetrable by a flash of lightning? It is true, indeed, that from some experiments it is found, that when the air is exhausted very perfectly from a receiver, we cannot force an electric spark through it. But this, so far from proving that there is nothing in the glass, plainly shows that there is something in it which makes a greater resistance than we can overcome: and it is very probable that this something is no more than the electric fluid itself; for as we are very certain that the electric fluid can impell, so we are equally certain that it can resist. The truth is, that it is not in our power to move this fluid at all but by lessening in one part the resistance it meets with; in which case it moves very freely of itself: just as we can move the air with great facility, provided we allow the rest to follow; but if we attempt to push a quantity of air before us, without allowing any to follow it to supply the vacuity, we will meet with a most violent resistance. In the case of electric fluid, we can make it circulate from one part of the earth to another by means of conductors: but we cannot force any part of it to a distance from the rest, nor can we cause a small quantity expel a large one from any place, otherwise than by breaking the equilibrium; in which case the quantity which follows is precisely equal to that which went before. In the case of a perfect Torricellian vacuum, we cannot discharge a bottle through it, without setting in motion all that quantity which is contained in the glass, as well as all that is connected with it, which it seems is more than the power of any machine can do. In like manner, the atmosphere of the earth being surrounded by an immense and inconceivable quantity of electric matter, it cannot escape without putting in motion a quantity of that matter equal to what goes out. But this quantity, upon the whole, can never be greater than that which the earth every moment absorbs from the sun. Were a greater quantity to issue forth, it would be resisted by all the rest, even to the utmost boundaries of the universe; a power which no created being could overcome. As matters stand at present, the resistance is inconceivably great: for from the laws of mechanics it is evident, that action cannot exist without reaction; so that where there is no resistance, there can be no effort. In all cases we see that where the

electric matter has a good conductor, it moves silently, and without showing any marks of power whatever. If it meets with a small resistance it makes a small effort, and a greater one if a greater resistance is made, and so in proportion. The violence with which this fluid acts in some cases, shows the strength of the resistance to it all around; for, like other fluids, we are certain that this one also acts with equal force all around it, and the explosion is always made at the least resisting part. The electric fluid of the atmosphere, therefore, is confined by a very great power, which it is not by any means able to overcome, but which yields in a certain degree to its impulse every moment, in proportion to the fresh supplies yielded by it to the earth, and which supplies come every moment from the sun.

²⁹ III. Heat and cold are very powerful agents in producing various meteors: but these are only relatives, cold as and different modifications of the same fluid; the agents. former being its action from a centre, the latter its action from a circumference to a centre. Though we do not know what connection there is between heat, cold, and what we call electricity, yet we know that this last is very much affected by them; for heat makes bodies more pervious to electricity than otherwise they would be, and cold makes them less so.—Hence the most violent electrical phenomena are observed in hot countries; while in the colder regions those which depend on a more moderate electrification, as auroræ boreales, are more frequent. The prevalence of heat and cold in particular places, however, depends upon circumstances which are altogether unknown to us; and therefore we cannot investigate the modes of their operation in such a particular manner as could be wished. From what has been already said, however, about the nature of the different agents concerned in meteorology, both in this article and in other parts of the work, we may take the following view of the causes of meteors in general.

³⁰ 1. Evaporation, combined with electricity, produces all the phenomena of vapour, fog, clouds, rain, &c.; and according as these two are joined to certain degrees of heat or cold, they produce dew, hoar-frost, rain, hail, or snow.

The phenomena of dew and hoar-frost seem to proceed from a quantity of aqueous and undecomposed vapour which always exists in the atmosphere; and which, being raised by mere heat, is condensed by mere cold, without undergoing that process by which water is changed into air. Hence it both ascends and descends; for if we cover a small space of ground with plates of glass, they will be wetted both above and below. The reason of this is, that the evaporation from the ground does not stop immediately after the air begins to cool, especially if it be covered with anything which prevents the access of the cold air, as the glass plates do in this case. The cold air, therefore, acting upon the glass, condenses the vapour below it, in the same manner that the head of a still or the receiver of a retort condenses the vapours which rise from the matter to be distilled. If the cold be very intense, hoar-frost appears instead of dew; which is nothing more than the dew frozen after it falls upon the ground, in the same manner that the vapour in a warm

warm room congeals on the inside of the windows in a frosty night. As this seems to be the whole process, it has not been observed that any electricity is concerned in the production of dew.

When the vapour has been thoroughly decomposed, and become invisible, it very frequently returns back to its pristine state, so far as to assume the appearance of mist or fog. In this case, electricity appears evidently to be concerned; for Mr Cavallo has observed that all fogs are electrified. When the process has advanced farther, and the water begins to collect into drops, the electricity is still more remarkable; and it is with great reason supposed that it is by means of electrical repulsion that the drops of rain keep at a regular distance from one another. When the cold is intense, and the electricity strong, the drops of water are frozen, and hail is produced: but snow indicates a more moderate degree of electricity; and a very violent cold, accompanied with a strongly electrical atmosphere, produces that excessively disagreeable vapour in the polar regions called frost-smoke, which is a general congelation of all the aqueous moisture contained in the atmosphere.

2. By violent electricity alone are produced the phenomena of thunder, lightning, fire-balls, ignes fatui, and the aurora borealis. In the phenomena of thunder, evaporation and the other agents by which rain and hail are produced are also concerned; though electricity is most remarkably so, and thunder and lightning of the most violent kind frequently occur without any rain. The ignis fatuus, aurora borealis, large fire-balls, and the smaller ones called falling-stars, seem to depend upon electricity alone, without any aid from evaporation, or from heat or cold. Aurora borealis, indeed, is most common in the northern and southern parts of the world, where the cold is intense; though this seems to be owing, not to the cold, but to the natural emission of the electrical fluid from the polar regions in much greater quantities than from others. The fire-balls commonly appear collected on the very extreme boundaries of the atmosphere, where, from the violent resistance already mentioned, the fluid is confined as it were in a concave shell, which it cannot by any means penetrate in great quantities in any particular place. Though these fire-balls, therefore, contain an immense quantity of this fluid, they can only proceed in an horizontal direction, and never fly perpendicularly up from the earth, as those will sometimes do which are formed nearer the ground. The ignis fatuus seems to depend on the strong electricity of a certain portion of atmosphere, the cause of which is not well understood.

3. By the action of heat and electricity combined, are produced the phenomena of hurricanes, whirlwinds, and water-spouts. It is not, indeed, known in what manner those agents combine themselves to produce

such tremendous effects; but it seems evident that electricity is concerned in them, as the sea-water becomes unusually clear before an hurricane, and many signs of electricity are likewise observed in the heavens.

4. The winds are supposed to proceed mostly from the heat of the sun rarifying the atmosphere, and occasioning a continual influx of fresh air to fill up the vacuum; but very violent winds are frequently observed where no such cause can be supposed to exist.— Thus, on the tops of high mountains the winds are commonly very violent; and mountainous countries, especially when cold, are for the most part also subject to high winds. As the tops of mountains, however, are known to be strongly electrified by their attracting and repelling the clouds, we must suppose that this electricity has a considerable share in producing the winds which are generally so violent on their tops.— This will appear the more probable, when we consider that frequently storms of wind, and those of the most violent kind, seem to be brought along with clouds; as, for instance, that mentioned under the article MALTA, in which a dreadful tempest, brought along with a cloud, almost destroyed the whole town.

Thus we have endeavoured to give a general sketch of the doctrine of METEORLOGY: a more particular detail of the causes by which meteors are produced is given under the names of each of them as they occur in the order of the alphabet. With regard to their uses, those of the more magnificent and tremendous kind seem to be destined to preserve the balance of the electric fluid in the atmosphere, the want of which would be productive of the most fatal effects to the world in general. The effects of the inferior ones are more confined, and are of use only to particular districts, scarcely ever extending their influence over a whole country. Thus the clouds, which produce rain for the purposes of vegetation, do not extend themselves over a whole country at once, but transitorily fly over different parts of it; so that when it is rain, for instance, in one place, it may be sunshine in another, thunder in a third, &c. It is, however, surprising to observe how equally these act over the whole of a very large tract of land; so that though there is never precisely the same weather in two places twenty miles distant from one another, yet vegetation goes on without any perceptible difference in the one as well as the other; neither, unless there be some very remarkable difference in the weather of one year from that of another, will there be any perceptible difference in the crop. For a more particular investigation of this point, however, see the articles VEGETATION and WEATHER.

For the application of meteorology to the foreknowledge of the weather, see the article WEATHER.

METEOROMANCY, a species of divination by meteors, principally by lightning and thunder. This method of divination passed from the Tuscans to the Romans, with whom, as Seneca informs us, it was held in high esteem.

METESSIB, an officer of the eastern nations, who has the care and oversight of all the public weights and measures, and sees that things are made justly according to them.

METHEGLIN, a species of mead; one of the most