RAIN

, water that descends from the atmosphere in the form of drops of a considerable size. Rain is apparently a precipitated cloud; as clouds are nothing but vapours raised from moisture, waters, &c. 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 be 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, as vapours, or vesiculæ. These vesiculæ, being specifically lighter than the atmosphere, are buoyed up by it, till they arrive at a region where the air is in a just balance with them; and there they float, till by some new agent they are converted into clouds, and thence either into Rain, snow, hail, mist, or the like.

But the agent in this formation of the clouds into Rain, and even of the vapours into clouds, has been much controverted. Most philosophers will have it, that the cold, which constantly occupies the superior regions of the air, chills and condenses the vesiculæ, at their arrival from a warmer quarter; congregates them together, and occasions several of them to coalesce into little masses: and thus their quantity of matter increasing in a higher proportion than their surface, they become an overload to the thin air, and so descend in Rain.

Dr. Derham accounts for the precipitation, hence; that the vesiculæ being full of air, when they meet with a colder air than that they contain, this is contracted into a less space: and consequently the watry shell or case becomes thicker, so as to become heavier than the air, &c.

But this separation cannot be ascribed to cold, since Rain often takes place in very warm weather. And though we should suppose the condensation owing to the cold of the higher regions, yet there is a remarkable fact which will not allow us to have recourse to this supposition: for 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 half way down the hill it is much more considerable; and at the bottom the drops will be very large, descending in an impetuous Rain. Which shews that the atmosphere condenses the vapours as well where it is warm as where it is cold.

Others allow the cold only a part in the action, and bring in the winds as sharers with it: alledging, that a wind blowing against a cloud will drive its vesiculæ upon one another, by which means several of them, coalescing as before, will be enabled to descend; and that the effect will be still more considerable, if two opposite winds blow together towards the same place: they add, that clouds already formed, happening to be aggregated by fresh accessions of vapour continually ascending, may thence be enabled to descend.

Yet the grand cause, according to Rohault, is still behind. That author conceives it to be the heat of the air, which, after continuing for some time near the earth, is at length carried up on high by a wind, and there thawing the snowy villi or flocks of the half frozen vesiculæ, it reduces them into drops; which, coalescing, descend, and have their dissolution perfected in their progress through the lower and warmer stages of the atmosphere.

Others, as Dr. Clarke, &c, ascribe this descent of the clouds rather to an alteration of the atmosphere than of the vesiculæ; and suppose it to arise from a diminution of the spring or elastic force of the air. This elasticity, which depends chiefly or wholly on the dry terrene exhalations, being weakened, the atmosphere sinks under its burden; and the clouds fall, on the common principle of precipitation.

Now the small vesiculæ, by these or any other causes, being once upon the descent, will continue to descend notwithstanding the increase of resistance they every moment meet with in their progress through still denser and denser parts of the atmosphere. For as they all tend toward the same point, viz, the centre of the earth, the farther they fall, the more coalitions will they make; and the more coalitions, the more matter will there be under the same surface; the surface only increasing as the squares, but the solidity as the cubes of the diameters: and the more matter under the same surface, the less friction or resistance there will be to the same matter.

Thus then, if the causes of rain happen to act early enough to precipitate the ascending vesiculæ, before they are arrived at any considerable height, the coalitions being few in so short a descent, the drops will be proportionably small; thus forming what is called dew. If the vapours prove more copious, and rise a little higher, there is produced a mist or fog. A little higher still, and they produce a small rain, &c. If they neither meet with cold nor wind enough to condense or dissipate them; they form a heavy, thick, dark sky, which lasts sometimes several days, or even weeks.

But later writers on this part of philosophical science have, with greater shew of truth, considered Rain as an electrical phenomenon. Signior Beccaria reckons Rain, hail, and snow, among the effects of a moderate electricity in the atmosphere. Clouds that bring Rain, he thinks are produced in the same manner as thunder clouds, only by a moderate electricity. He describes them at large; and the resemblance which all their phenomena bear to those of thunder clouds, is very striking. He notes several circumstances attending Rain without lightning, which render it probable that it is produced by the same cause as when it is accompanied with lightning. Light has been seen among the clouds by night in rainy weather; and even by day rainy clouds are sometimes seen to have a brightness evidently independent of the sun. The uniformity with which the clouds are spread, and with which the Rain falls, he thinks are evidences of an uniform cause like that of electricity. The intensity also of electricity in his apparatus, usually corresponded very nearly to the quantity of Rain that fell in the same time. Sometimes all the phenomena of thunder, lightning, hail, Rain, snow, and wind, have been observed at one time; which shews the connection they all have with some common cause. Signior Beccaria therefore supposes that, previous to Rain, a quantity of electric | matter escapes out of the earth, in some place where there is a redundancy of it; and in its ascent to the higher regions of the air, collects and conducts into its path a great quantity of vapours. The same cause that collects, will condense them more and more; till, in the places of the nearest intervals, they come almost into contact, so as to form small drops; which, uniting with others as they fall, come down in the form of Rain. The Rain will be heavier in proportion as the electricity is more vigorous, and the cloud approaches more nearly to a thunder cloud: &c. See Lettere dell Elettricismo; and Priestley's Hist. &c of Electricity, vol. 1, pa. 427, &c, 8vo. And for farther accounts of the phenomena of Rain &c, see Barometer, EVAPORATION, Ombrometer, Pluviameter, Vapour, &c. See also the Theory of Rain, by Dr. James Hutton, art. 2 vol. 1 of Transactions of the Royal Society of Edinburgh.

Quantity of Rain. As to the general quantity of Rain that falls, with its proportion in several places at the same time, and in the same place at different times, there are many observations, journals, &c, in the Philos. Trans. the Memoirs of the French Academy, &c. And upon measuring the rain that falls annually, its depth, on a medium, is found as in the following table:

See also Philos. Trans. Abr. vol. 4, pt. 2, pa. 81, &c<*> and vol. 10 in many places; also the Meteorological Journal of the Royal Society, published annually in the Philos. Trans. and the article Pluviameter or Ombrometer.

It is reasonably to the expected, and all experience shews, that the most Rain falls in places near the sea coast, and less and less as the places are situated more inland. Some differences also arise from the circumstances of hills, valleys, &c. So when the quantity of Rain fallen in one year at London, is 20 inches, that on the western coast of England will often be twice as much, or 40 inches, or more. Those winds also bring most Rain, that blow from the quarter in which is the most and nearest sea; as our west and south-west winds.

It is also found, by the pluviameter or Rain-gage, that, in any one place, the more Rain is collected in the instrument, as it is placed nearer the ground; without any appearance of a difference, between two places, on account of their difference of level above the sea, provided the instrument is but as far from the ground at the one place, as it is from the ground at the other. These effects are remarked in the Philos. Trans. for 1769 and 1771, the former by Dr. Heberden, and the latter by Mr. Daines Barrington. Dr. Heberden says, “A comparison having been made between the quantity of Rain, which fell in two places in London, about a mile distant from one another, it was found, that the Rain in one of them constantly exceeded that in the other, not only every month, but almost every time that it rained. The apparatus used in each of them was very exact, and both made by the same artist; and upon examining every probable cause, this unexpected variation did not appear to be owing to any mistake, but to the constant effect of some circumstance, which not being supposed to be of any moment, had never been attended to. The Rain-gage in one of these places was fixed so high, as to rise above all the neighbouring chimnies; the other was considerably below them; and there appeared reason to believe, that the difference of the quantity of Rain in these two places was owing to this difference in the placing of the vessel in which it was received. A funnel was therefore placed above the highest chimnies, and another upon the ground of the garden belonging to the same house, and there was found the same difference between these two, though placed so near one another, which there had been between them, when placed at similar heights in different parts of the town. After this fact was sufficiently ascertained, it was thought proper to try whether the difference would be greater at a much greater height; and a Rain-gage was therefore placed upon the square part of the roof of Westminster Abbey. Here the quantity of Rain was observed for a twelvemonth, the Rain being measured at the end of every month, and care being taken that none should evaporate by passing a very long tube of the funnel into a bottle through a cork, to which it was exactly fitted. The tube went down very near to the bottom of the bottle, and therefore the Rain which fell into it would soon rise above the end of the tube, so that the water was no where open to the air except | for the small space of the area of the tube: and by trial it was found that there was no sensible evaporation through the tube thus fitted up.

The following table shews the result of these obsertions.

By this table it appears, that there fell below the top of a house above a fifth part more Rain, than what fell in the same space above the top of the same house; and that there fell upon Westminster Abbey not much above one half of what was found to fall in the same space below the tops of the houses. This experiment has been repeated in other places with the same result. What may be the cause of this extraordinary difference, has not yet been discovered; but it may be useful to give notice of it, in order to prevent that error, which would frequently be committed in comparing the Rain of two places without attending to this circumstance.”

Such were the observations of Dr. Heberden on first announcing this circumstance, viz, of different quantities of Rain falling at different heights above the ground. Two years afterward, Daines Barrington Esq. made the following experiments and observations, to shew that this effect, with respect to different places, respected only the several heights of the instrument above the ground at those places, without regard to any real difference of level in the ground at those places.

Mr. Barrington caused two other Rain-gages, exactly like those of Dr. Heberden, to be placed, the one upon mount Rennig, in Wales, and the other on the plane below, at about half a mile's distance, the perpendicular height of the mountain being 450 yards, or 1350 feet; each gage being at the same height above the surface of the ground at the two stations.

“The inference to be drawn from these experiments, Mr. Barrington observes, seems to be, that the increase of the quantity of Rain depends upon its nearer approximation to the earth, and scarcely at all upon the height of places, provided the Rain-gages are fixed at about the same distance from the ground.

“Possibly also a much controverted point between the inhabitants of mountains and plains may receive a solution from these experiments; as in an adjacent valley, at least, very nearly the same quantity of Rain appears to fall within the same period of time as upon the neighbouring mountains.”

Dr. Heberden also adds the following note. “It may not be improper to subjoin to the foregoing account, that, in places where it was first observed, a different quantity of Rain would be collected, according as the Rain-gages were placed above or below the tops of the neighbouring buildings; the Rain-gage below the top of the house, into which the greater quantity of Rain had for several years been found to fall, was above 15 feet above the level of the other Rain-gage, which in another part of London was placed above the top of the house, and into which the lesser quantity always fell. This difference therefore does not, as Mr. Barrington justly remarks, depend upon the greater quantity of atmosphere, through which the Rain descends: though this has been supposed by some, who have thence concluded that this appearance might readily be solved by the accumulation of more drops, in a descent through a great depth of atmosphere.”