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putrefaction of the air, the difeafe will then become pefti lential and contagious.

SECT. II.

OF THE THEORY OF THE WINDS.

THE wind is the confequence of the rarefaction of the air, and is no other than the air put into motion by heat, or any other caufe; for when the air is rarefied by heat it will fwell, and thereby affect the adjacent air; and thus, by the degrees of heat being various in different places, there will arise various winds.

When the air is heated to any degree, it will ascend upwards, and the adjacent air will rufh in to fupply its place; therefore, there will be a stream or current of air from all the adjacent parts towards the place where the heat is. This appears evident from the motion with which the air rushes towards any place where there is a great fire, as into a glasshoufe, or through the keyhole of a door in a room where there is a fire.

That wind, called the trade wind, which blows conftantly from eaft to weft about the equator, is a neceffary confequence of this principle. For when the fun fhines perpendicularly upon any part of the globe, the air in that part will be heated, and confequently rarefied, and will therefore afcend upwards; and, when the fun withdraws, the adjacent air, rufhing in to fill the place of the rarefied air, will confequently cause a stream or current of air from all parts to

wards

wards that part which is most heated by the fun. But the courfe of the fun being from east to west, with respect to the earth, the common course of the air which fupplies the place of the rarefied air must be in the fame direction, viz. from east to weft; but on the north fide its course will be directed a little towards the north, and on the fouth fide as much towards the fouth.

This would be the general courfe of the wind about the equator, if it were not affected by other causes, which change its direction: as, 1. By exhalations that arife out of the earth, at different times and different places, occafioned by fubterraneous fires, volcanoes, &c.; 2. by a fudden inundation of rain, which caufes a contraction of the air; 3. by the violent heat of fome burning fands, which cause an extraordinary rarefaction of the contiguous air; 4. by high mountains, which alter the direction of the wind; 5. by the declination of the fun towards the north or fouth, thereby causing a greater heat in the air on the fame fide of the equator.

These are the principal caufes which create fuch a great variety and uncertainty in the winds in moft countries diftant from the equator; as, 1. The variations of the winds in the different parts of Europe; 2. the monsoons which are found in the Indian feas; 3. thofe winds which always blow from west to east, on the western coaft of America, and on the coaft of Guinea; and the sea breezes, which, in họt countries, blow from fea to land in the daytime; and the land breezes, which blow towards the sea in the night; and all those other irregularities in the wind, as ftorms, whirl winds, hurricanes, &c.

VOL. IL

SECT.

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SECT. III.

OF THE CAUSES of thunder, lightning, &c.

THE effluvia and vapours arifing from different bodies meet and unite together in the atmosphere, which is the common receptacle of all vaporous bodies, as the steams from moift bodies, the fmoke from bodies burnt, and the effluvia emitted from fulphurous, nitrous, acid, and alkaline fubftances. And every volatile body rifes to a certain height in the atmosphere, according to its own specific gravity. And when the effluvia which arise from an acid and alkaline body meet each other in the air, there will be a conflict between these two vapours, or what is vulgarly called a fermentation between them. If this fermentation be great, it will produce a fire; and if the effluvia be of a combustible nature, the fire will run from one part of the air to another, following the inflammable matter.

These things may be demonftrated by the following experiment:-Mix fome oil of cloves and Glauber's spirit of nitre together, which will immediatly produce a fudden fermentation, with a fine flame; and if the ingredients be neat, there will be a fudden explosion. These are the effects of the union of an acid and alkaline fluid.

From this experiment, we may account for the effects of thunder and lightning, which is occafioned by the effluvia of fulphurous and nitrous bodies meeting each other in the air, where, affifted by the fun's heat, a fermentation, fire, and explosion enfue. When the inflammable matter is thia and light, it will afcend to the upper parts of the atmo

sphere,

fphere, before the fermentation takes place; but when it is more dense, it will hover near the surface of the earth, where, when an explosion takes place, the fire is vifible, and often dangerous; the explosion also has a violent force; and the heat being great, will rarefy and drive away all the adjacent air, kill men and cattle, fplit trees, rocks, &c.

Lightning differs from all other fires; for it has often been known to pass through wood, leather, cloths, and other fubftances, without hurting them; at the fame time melting iron, fteel, filver, goid, and other hard bodies. It has melted' or burnt afunder a fword, without hurting the fcabbard; and melted money in a man's pocket, without hurting his clothes. So fine are the particles of this fire, that they pafs through foft, loose bodies, without injuring them, and spend their force upon those that are more dense.

Any feel inftruments, as knives, forks, &c. that have been ftruck with lightning, have a ftrong magnetic virtue, which they retain many years. The lightning ftriking the mariner's compafs has often turned it quite round, and made it stand the contrary way, that is, with the north pole towards the fouth.

Thofe explosions which fometimes happen in mines, and called fire-damps, are of the fame nature with lightning, and occafioned by fulphurous and nitrous vapour rifing from the mine, which, mixing with the air, take fire from the lights used in the mine. This fire, when once kindled, continues to run from one part of the mine to another, as the combustible matter happens to be; and as the elasticity of the air is increased by the heat, the air in the mine will fwell confiderably; and, for want of room, will at length explode, with a degree of force equal to the violence of the fire, the quantity of effluvia, and denfity of the vapours. This is fometimes so strong as to blow up the mine; at other times it is fo weak, that when it has taken fire it may be eafily blown out.

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Air that will take fire from the flame of a candle, may be produced thus:Having pumped the air out of the receiver of the air-pump, let the air run into it through the flame of the oil of turpentine; then remove the cover of the receiver, and holding a candle to that air it will take fire.

When combustible vapours are kindled in the bowels of the earth, where there is little or no vent, they produce earthquakes, and violent storms or hurricanes of wind, as soon as they break forth in the open air.

An artificial earthquake may be produced thus:-Take 10 or 15 pounds of fulphur, and as much of the filings of iron, and knead them with common water into the confistence of a pafte: this, being buried under ground, will, in eight or ten hours times, burst out into flames, and caufe the earth to tremble around it to a confiderable distance.

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It is owing to fubstances of this nature that we have vol

canoes,

SECT. IV.

THE CONSTRUCTION AND use of the air-pump, BAROMETER, AND AIR-Gun.

1. Of the Air-pump.

-THE air-pump is a machine to pump the air out of any vessel, and constructed on the fame principle as the water

pump.

The

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