times intermingled with fuch terrible claps, as deprive the hypothefis of all probability; or other inconfiftences therein might be pointed out. The rumbling and repeated echoes, &c. of thunder, ftill remain among the phenomena not yet accounted for.

In general, a courfe of hot weather precedes a thunder-ftorm; and it feldom happens that very hot weather, in the fummer, terminates without a ftorm of thunder. Hence, alfo, in the Eaft and West Indies, where the climate is fo much hotter, thunder and lightning are not only much more frequent, but much more violent, than in this country.

OF WINDS.

Of winds, the obfervation of our SAVIOUR is ftill juft: we hear the found of the wind as it paffes by, but we neither know whence it comes, nor whither it goes; we cannot determine how it originates, or why it ceases. The great Bacon, indeed, was of opinion, that by a close and regular history of the winds, continued for a number of ages together, and the particulars of each obfervation reduced to general maxims, we might at laft come to understand the variations of this capricious element, and be able to foretel the certainty of a wind, with as much ease as we now foretel the return of an eclipfe. Indeed his own beginnings in this arduous tafk feem to fpeak the poffibility of it's fuccefs; but unfortunately this investigation is the work of ages, and we want a Bacon to direct the process.

In the Hifloria Ventorum, Bacon reckons three fources of winds; one by defcent from the fuperior regions of the atmofphere, another from the expanfion of the lower air, and a third by exfpi

ration from the earth of which laft he propofes it as an object of inquiry, What winds blow out of fubterraneous caverns? Whether they come forth in a large body, or blow infenfibly here and there; and then unite in one ftream, like a river formed out of many different fprings? This latter caufe has been but little attended to, though this reciprocation between the earth and air is furely a very interefting part of natural philofophy. In the language of holy writ, God is faid to bring the winds out of his treafures, as if fome hidden ftorehouse were alluded to, fuch as that of the waters and cavities between the earth.

The annual revolution of the fun is doubtlefs a general caufe of winds; but this caufe, confidered alone, fhould produce regular winds, whofe progrefs would correfpond to, and be connected with the seasons; but the phenomena obferved by no means enable us to perceive this connection. There is another caufe, of which we may form an imperfect idea, by which the winds proceeding from the fouth, may be fouth-weft to us, and thofe which come from the north, north-eaft. This cause is the difference in the velocity of the motion of the parts of the earth we inhabit, and that at the equator, or the polar circles. If the air was calm at the equator, that is, moved with the fame velocity as the earth, and that in coming from thence to us in the fame direction, preferving at the fame time a portion of it's acquired motion, it would gain upon the earth in this direction, and would thus become fouth-weft. The fame caufe inverted would change the north for us into a north-east wind. Another caufe, though very inconfiderable, may be found in the different diurnal pofitions of the fun: this, in calm weather, often occafions a gentle eaft wind after fun-rifing, and a weft wind after his fetting.

Of thefe caufes we have fome knowledge; but there must be many, and more powerful ones, to produce those phenomena to which we are continual witnesses, and to which these seem to have little or no affinity. Evaporation and rain have been confidered as caufes; but they are alfo by no means adequate to the purpofe. Evaporation is conftantly operating; it is alfo more abundant in thofe places where the heat is greatest. These places are continually varying; but ftill the variations in evaporation are fo flow, and the differences in heat fo infenfible from one place to another, that it can never occafion any fudden and violent wind. Rain, which is the inverfe of evaporation, operates with more rapidity: but the fame reasons which prove that rain cannot be formed of the immediate product of evaporation, alfo prove that the precipitation of this product, in any ftratum of air, can

not make a fufficient vacuum to caufe the furrounding air to prefs in with violence.

We must then have recourfe to fome other caufe, to explain the winds which accompany the rapid formation and deftruction of clouds; and this may be found in the return of air to a ftate of vapours. It is known from experiment, that in imilar cafes there is a great increase of volume in the new fluid; as in the fudden explofion of inflammable air with vital or common air. When the air is changed into aqueous vapour in the atmofphere, there is probably a confiderable expanfion of the ftratum where this change happens, and the effect is more or lefs extenfive in proportion to the ftrength of the caufe. If the production of the clouds be flow, if it embraces a very great portion of the atmosphere, and if the operation be carried on at a great height, but little agitation will be perceived in. the air under thefe ftrata: the columns thereof extending lengthwife, produce in

diftant

diftant countries winds, of which the inhabitants can no more perceive the caufes, than those near which it originated. But if the clouds are formed rapidly, if they occupy but a fmall space, and are not very high, violent winds may be occafioned by the fudden expanfion of the medium where they are formed. As the quantity of vapour that is the immediate product of evaporation is always very fmall, the formation of drops of rain, on the common system, would only produce infenfible and trifling motions of the air. But in Mr. de Luc's fyftem, the fucceffive production of vapour in the midft of the air is unlimited: their accumulation in the form of veficles may be immense; and when they are refolved into drops, a confiderable vacuum is the natural confequence.

From this view of the origin of winds, we may fee alfo why, in a season of storms and showers, a cold heavy cloud, paffing over the head, with a hafty fall of fnow or hail, is often attended with a fudden violent gust of wind, fuch as failors call a fquall, which fubfides into a calm with the departure of the cloud; till another cloud, coming in the fame direction, brings a fresh blaft. No tempeft, hurricane, or whirlwind, ever happens under a cloudlefs fky. We hence may alfo perceive why a whiftling or howling noife of the wind is the moft infallible prognoftic of rain, indicating the formation of rainy clouds. The facred fcripture feems to agree with this; for the prophet Elijah, before any other fymptom of the weather appeared," feemed to give notice to Ahab from this one: Get thee up, and eat and drink, faid he, for there is a SOUND of abundance of rain. Then it follows, that the heaven was foon black with clouds and wind, and there was a great rain.

OF TRADE-WINDS AND MONSOONS.

There are many parts of the world where the winds, that with us are fo uncertain, pay their stated vifits. In fome places, the winds are found to blow one way by day, another by night; in others, for one half of the year, they go in a direction contrary to their former courfe: but what is more extraordinary, there are fome places where the winds never change, but for ever blow the fame way. This is particularly found to obtain between the tropics, in the Atlantic ocean, and great Pacific fea.

Between the limits of 60 degrees, namely, from 30° of north latitude to 30° of fouth latitude, there is a conftant eafterly wind throughout the year, blowing on the Atlantic and Pacific oceans ; and this is called the trade-wind.

The trade-winds near the northern limits blow between the north and the eaft; and near the fouthern limits, they blow between the fouth and the east.

Thefe general motions of the wind are difturbed on the continent, and near the coafts.

Beyond the northern limit of the general wind, on the Atlantic ocean, the wefterly winds prevail, but not with any certainty of conti

nuance.

In

In the Atlantic ocean, the S. E. trade-wind extends as far as 3 degrees north; and the N. E. trade-wind ceafes at the 5th degree N. the intermediate fpace are found calms with rain, and irregular uncertain fqualls attended with thunder and lightning; but this space is shifted farther to the northward or fouthward, according as the fun's declination is more northerly or foutherly.