water which descends from the clouds in form of drops of water.

Rain is apparently the precipitated vapours of watery clouds: thus, when various congeries of clouds are driven together by the agitation of the winds, they mix and run into one body, and by that means dissolve and condense each other into their former substance of water: also the coldness of the air is a great means to collect, compact, and condense clouds into water; which being heavier than the air, must of necessity fall through it in the form we call rain. Now the reason why it falls in drops and not in whole quantities, as it becomes condensed, is the resistance of the air; whereby being broken, and divided into smaller and smaller parts the farther it passes through the air, it at last arrives to us in small drops. Mr. Detham accounts for the precipitation hence, that the vesiculae being full of air, when they meet with a colder air than that they contain, their air is contracted into a less space; and consequently the watery shell rendered thicker, so as to become heavier than the air, &c.

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

According to Rohault, the great cause of rain is the heat of the air; which, after continuing for some time near the earth, is at length carried up high by a wind, and there-thawing the snowy villi, or flocks of the half frozen vesiculae, reduces them into drops; which, coalescing, descend.

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

Now the little vesicles, being once upon the descent, will persist therein, notwithstanding the increase of resistance they every moment meet with. For as they all tend toward the centre of the earth, the farther they fall, the more coalescences will they make; and the more coalescences, the more matter will there be under the same surface; the surface only increasing as the squares, but the solidity as the cubes; and the more matter under the same surface, the less resistance there will 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 coalescences 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, &c. If they neither meet with cold nor wind, they form a heavy, thick, dark sky.

Hence, many of the phenomena of the weather may be accounted for: as, why a cold summer is always a wet one, and a warm a dry one; why we have commonly most rain about the equinoxes; why a settled, thick, close sky scarce ever rains, till it have been first clear. As to the quantity of rain that falls, its proportion in several places at the same time, and in the same place at several times, we have store of observations, journals, &c. in the memoirs of the French Academy, Philosophical Transactions, &c.