RAIN, the descent of water from the atmosphere in the form of drops of a considerable size. By this circumstance it is distinguished from dew and fog: in the former of which the drops are so small that they are quite invisible; and in the latter, though their size is larger, they seem to have very little more specific gravity than the atmosphere itself, and may therefore be reckoned hollow spherules rather than drops.
It is universally agreed, that rain is produced by the water previously absorbed by the heat of the sun, or otherwise, from the terraqueous globe, into the atmosphere; but very great difficulties occur when we begin to explain why the water, once so closely united with the atmosphere, begins to separate from it. We cannot ascribe this separation to cold, since rain often takes place in very warm weather; and though we should suppose the condensation owing to the superior cold of the higher regions, yet there is a remarkable fact which will not allow us to have recourse to this supposition. It is certain, that the drops of rain increase in size considerably as they descend. On the top of a hill, for instance, they will be small and inconsiderable, forming only a drizzling shower; but at the bottom of the same hill the drops will be excessively large, descending in an impetuous rain; which shows that the atmosphere is disposed to condense the vapours, and actually does so, as well where it is warm as where it is cold.
For some time the suppositions concerning the cause
of rain were exceedingly insufficient and unsatisfactory. It was imagined, that when various congeries of clouds were driven together by the agitation of the winds, they mixed, and run into one body, by which means they were condensed into water. The coldness of the upper parts of the air also was thought to be a great means of collecting and condensing the clouds into water; which, being heavier than the air, must necessarily fall down through it in the form of rain. The reason why it falls in drops, and not in large quantities, was said to be the resistance of the air; whereby being broken, and divided into smaller and smaller parts, it at last arrives to us in small drops. But this hypothesis is entirely contrary to almost all the phenomena: for the weather, when coldest, that is, in the time of severe frost, is generally the most serene; the most violent rains also happen where there is little or no rain to condense the clouds; and the drops of rain, instead of being divided into smaller and smaller ones as they approach the earth, are plainly increased in size as they descend.
Mr Derham accounted for the precipitation of the drops of rain from the vesiculæ being full of air, and meeting with an air colder than they contained, the air they contained was of consequence contracted into a smaller space; and consequently the watery shell rendered thicker, and thus specifically heavier, than the common atmosphere. But, under the article EVAPORATION it has been shown, that the vesiculæ, if such they are, of vapour, are not filled with air, but with fire, or heat; and consequently, till they part with this latent heat, the vapour cannot be condensed. Now, cold is not always sufficient to effect this, since in the most severe frosts the air is very often serene, and parts with little or none of its vapour for a very considerable time. Neither can we admit the winds to have any considerable agency in this matter, since we find that blowing upon vapour is so far from condensing it, that it unites it more closely with the air, and wind is found to be a great promoter of evaporation.
According to Rohault, the great cause of rain is the heat of the air; which, after continuing for some time near the earth, is raised on high by a wind, and there thawing the snowy villi or flocks of half-frozen vesiculæ, reduces them to drops; which, coalescing, descend. Here, however, we ought to be informed by what means these vesiculæ are suspended in their half-frozen state; since the thawing of them can make but little difference in their specific gravity, and it is certain that they ascended through the air not in a frozen but in an aqueous state.
Dr Clarke and others ascribe this descent of the rain rather to an alteration of the atmosphere than of the vesiculæ; and suppose it to arise from a diminution of the elastic force of the air. This elasticity, which, they say, depends chiefly or wholly upon terrene exhalations, being weakened, the atmosphere sinks under its burden, and the clouds fall. Now, the little vesicles being once upon the descent will continue therein, notwithstanding the increase of resistance they every moment meet with. For, as they all tend to the centre of the earth, the farther they fall, the more coalitions they will make; and the more coalitions, the more matter will there be under the same surface; the surface increasing only as the squares, but the
Rain. the solidity as the cubes; and the more matter under the same surface, the less resistance will there be to the same matter. Thus if the cold, wind, &c. act early enough to precipitate the ascending vesicles before they are arrived at any considerable height, the coalitions being but few, the drops will be proportionably small; and thus is formed a dew. If the vapours be more copious, and rise a little higher, we have a mist or fog. A little higher still, and they produce a small rain; if they neither meet with cold nor wind, they form a heavy thick dark sky. This hypothesis is equally unsatisfactory with the others; for, granting that the descent and condensation of the vapours are owing to a diminution of the atmosphere's elasticity, by what is this diminution occasioned? To say that it is owing to terrene exhalations, is only solving one difficulty by another; since we are totally unacquainted both with the nature and operation of these exhalations. Besides, let us suppose the cause to be what it will, if it acts equally and at once upon all the vapour in the air, then all that vapour must be precipitated at once; and thus, instead of gentle showers continuing for a considerable length of time, we would have the most violent water-spouts, continuing only for a few minutes, or perhaps seconds, which, instead of refreshing the earth, would drown and lay waste every thing before them.
Since philosophers have admitted the electric fluid to such a large share in the operations of nature, almost all the natural phenomena have been accounted for by the action of that fluid; and rain, among others, has been reckoned an effect of electricity. But this word, unless it is explained, makes us no wiser than we were before; the phenomena of artificial electricity having been explained on principles which could scarcely apply in any degree to the electricity of nature: and therefore all the solution we can obtain of the natural appearances of which we speak comes to this, that rain is occasioned by a moderate electrification, hail and snow by one more violent, and thunder by the most violent of all; but in what manner this electrification is occasioned, hath not yet been explained.— Throughout the various parts of this work where electricity hath been occasionally mentioned, the principles of artificial electricity, laid down in the treatise appropriated to that subject, have been applied to the solution of the phenomena of nature; those which are necessary to be attended to here, are the following.
1. The electric fluid and solar light are the same substance in two different modifications.
2. Electricity is the motion of the fluid when running, or attempting to run, in a continued stream from one place to another: heat is when the fluid has no tendency but to vibrate outwards and inwards to and from a centre; or at least when its streams converge to a point or focus.
3. The fluid acting as electricity, like water, or any other fluid, always tends to the place where there is least resistance.
On these three principles may the phenomena of atmospheric electricity, and the descent of rain by its means, be explained as follows.
1. The light or heat of the sun, acting in that peculiar manner which we call heat, unites itself with the moisture of the earth, and forms it into vapour,
which thus becomes specifically lighter than air, and of consequence ascends in the atmosphere to a certain height.
2. Besides the quantity of light which is thus united to the water, and forms it into vapour, a very considerable quantity enters the earth, where it assumes the nature of electric fluid.
3. As the earth is always full of that fluid, every quantity which enters must displace an equal quantity which is already there.
4. This quantity which is displaced must escape either at a distance from the place where the other enters, or very near it.
5. At whatever place a quantity of electric matter escapes, it must electrify the air above that place where it has escaped; and as a considerable quantity of light must always be reflected from the earth into the atmosphere, where it does not combine with the aqueous vapour, we have thence another source of electricity to the air; as this quantity must undoubtedly assume the action of electric fluid, especially after the action of the sun has ceased. Hence the reason why in serene weather the atmospheric electricity is always strongest, and rather more so in the night than in the day.
6. From these considerations, we see an evident reason why there must commonly be a difference between the electricity of the earth, and that of the atmosphere, excepting when an earthquake is about to ensue. The consequence of this must be, that as the action of the solar light continues to bring down the electric matter, and the earth continues to discharge an equal quantity of it into the atmosphere, some part of the atmosphere must at last become overloaded with it, and attempt to throw it back into the earth. This attempt will be vain, until a vent is found for the electricity at some other place; and as soon as this happens, the electrified atmosphere begins to throw off its superfluous electricity, and the earth to receive it. As the atmosphere itself is a bad conductor, and the more so the drier it is, the electric matter attacks the small aqueous particles which are detained in it by means of the latent heat. These, being unable to bear the impetus of the fluid, throw out their latent heat, which easily escapes, and thus makes a kind of vacuum in the electrified part of the atmosphere. The consequences of this are, that the aqueous particles being driven together in large quantity, at last become visible, and the sky is covered with clouds; at the same time a wind blows against these clouds, and, if there is no resistance in the atmosphere, will drive them away.
7. But if the atmosphere all round the cloud is exceedingly electrified, and the earth is in no condition to receive the superfluous fluid excepting in that place which is directly under the cloud; then the whole electricity of the atmosphere for a vast way round will tend to that part only, and the cloud will be electrified to an extreme degree. A wind will now blow against the cloud from all quarters, more and more of the vapour will be extricated from the air by the electric matter, and the cloud will become darker and thicker, at the same time that it is in a manner stationary, as being acted upon by opposite winds;
Rain. winds; though its size is enlarged with great rapidity by the continual supplies of vapour brought by the winds.
8. The vapours which were formerly suspended invisibly by means of the latent heat, are now suspended visibly by the electric fluid which will not let them fall to the earth, until it is in a condition to receive the electric matter descending with the rain. It is easy to see, however, that thus every thing is prepared for a violent storm of thunder and lightning as well as rain. The surface of the earth becomes electrified from the atmosphere: but when this has continued for some time, a zone of earth considerably below the surface acquires an electricity opposite to that of the clouds and atmosphere; of consequence the electricity in the cloud being violently pressed on all sides will at last burst out towards that zone where the resistance is least, as explained under the article LIGHTNING.—The vapours now having lost that which supported them, will fall down in rain, if there is not a sufficient quantity of electric matter to keep them in the same state in which they were before: but if this happens to be the case, the cloud will instantly be charged again, while little or no rain will fall; and hence very violent thunder sometimes takes place without any rain at all, or such as is quite inconsiderable in quantity.
9. When the electricity is less violent, the rain will descend in vast quantity, especially after every flash of lightning; and great quantities of electric matter will thus be conveyed to the earth, inasmuch that sometimes the drops have been observed to shine as if they were on fire, which has given occasion to the reports of fiery rain have fallen on certain occasions. If the quantity of electric matter is smaller, so that the rain can convey it all gradually to the ground, there will be rain without any thunder; and the greater the quantity of electricity, the more violent will be the rain.
From this account of the causes of rain, we may see the reason why in warm climates the rains are excessive, and for the most part accompanied with thunder; for there the electricity of the atmosphere is immensely greater than it is with us. We may also see why in certain places, according to the situation of mountains, seas, &c. the rains will be greater than in others, and likewise why some parts of the world are exempted from rain altogether; but as a particular discussion of these would necessarily include an explanation of the causes and phenomena of THUNDER, we shall for this reason refer the whole to be treated of under that article.