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358. Corol. 5. If the density of the air be increased, preserving the same heat or temperature, its spring or elasticity is also increased, and in the same proportion.

359. Corol. 6. By the pressure and gravity of the atmosphere, on the surface of fluids, the fluids are made to rise in any pipes or vessels, when the spring or pressure within is decreased or taken off.

PROPOSITION LXXIV.

360. Heat Increases the Elasticity of the Air, and Cold Dimi nishes it. Or, Heat Expands, and Cold Condenses the Air.

This property is also proved by experience.

361. Thus, tie a bladder very close with some air in it; and lay it before the fire: then as it warms, it will more and more distend the bladder, and at last burst it, if the heat be continued, and increased high enough. But if the bladder be removed from the fire, as it cools it will contract again, as before. And it was on this principle that the first airballoons were made by Montgolfier: for, by heating the air within them, by a fire beneath, the hot air distends them to a size which occupies a space in the atmosphere, whose weight of common air exceeds that of the balloon.

362. Also, if a cup or glass, with a little air in it, be inverted into a vessel of water; and the whole be heated over the fire, or otherwise; the air in the top will expand till it fill the glass, and expel the water out of it; and part of the air itself will follow, by continuing or increasing the heat.

Many other experiments, to the same effect, might be adduced, all proving the properties mentioned in the pro position.

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363. So that, when the force of the elasticity of air is considered, regard must be had to its heat or temperature; the same quantity of air being more or less elastic, as its heat is more or less. And it has been found, by experiment, that the elasticity is increased by the 435th part, for each degree of heat, of which there are 180 between the freezing and boiling heat of water.

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364. N. B. Water expands about the 이름이 part, with each degree of heat. (Sir Geo. Shuckburgh, Philos. Trans.

1777, p. 560, &c.)

VOL. II.

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Also,

Also, the

Spec. grav. of air 1-201 or 15

water

157 winem the barom... is
and the therum.. is

1000

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mercury 13592) which are their mea:

Or thus, air 1-222 or 14

water 1000

in this country. when the barom. is 3

and thermometer 5

mercury 13600

PROPOSITION LXXV.

$65. The Weight or Pressure of the Atmosphere, on any the Earth's Surface, is Equal to the Weight of a Cor Quicksilver, of the Same Base, and the Height of whi tween 28 and 31 inches.

THIS is proved by the barometer, an instrumen measures the pressure of the air, and which is de below. For, at some seasons, and in some places, sustains and balances a column of mercury, of at inches: but at other times it balances a column of 29 or near 31 inches high; seldom in the extremes 28 but commonly about the means 29 or 30. A v. which depends partly on the different degrees of hea air near the surface of the earth, and partly on the c tions and changes in the atmosphere, from winds an causes, by which it is accumulated in some places, pressed in others, being thereby rendered denser al vier, or rarer and lighter; which changes in its st almost continually happening in any one place. E medium state is commonly about 294 or 30 inches.

366, Corol. 1. Hence the pressure of the atmospl every square inch at the earth's surface, at a medium near 15 pounds avoirdupois, or rather 144 pounds. cubic foot of mercury weighing 13600 ounces nea inch of it will weigh 7.866 or almost 8 ounces, o half a pound, which is the weight of the atmosph every inch of the barometer on a base of a square inc therefore 30 inches, or the medium height, weig near 144 pounds.

367. Corol. 2. Hence also the weight or pressure atmosphere, is equal to that of a column of water f to 35 feet high, or on a medium 33 or 34 feet high. water and quicksilver are in weigh

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Also, the

Spec. grav. of air 1.201 or 17 when the barom. is

water 1000

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29.5, and the therm. is 55° mercury 13592) which are their mean heights in this country. Or thus, air 1.222 or 1) when the barom. is 30, water 1000 and thermometer 55. mercury 13600

PROPOSITION LXXV.

$65. The Weight or Pressure of the Atmosphere, on any Base at the Earth's Surface, is Equal to the Weight of a Column of Quicksilver, of the Same Base, and the Height of which is between 28 and 31 inches.

THIS is proved by the barometer, an instrument which measures the pressure of the air, and which is described below. For, at some seasons, and in some places, the air sustains and balances a column of mercury, of about 28 inches: but at other times it balances a column of 29, or 30, or near 31 inches high; seldom in the extremes 28 or 31, but commonly about the means 29 or 30. A variation which depends partly on the different degrees of heat in the air near the surface of the earth, and partly on the commotions and changes in the atmosphere, from winds and other causes, by which it is accumulated in some places, and depressed in others, being thereby rendered denser and heavier, or rarer and lighter; which changes in its state are almost continually happening in any one place. But the medium state is commonly about 294 or 30 inches.

366. Corol. 1. Hence the pressure of the atmosphere on every square inch at the earth's surface, at a medium, is very near 15 pounds avoirdupois, or rather 144 pounds. For, a cubic foot of mercury weighing 13600 ounces nearly, an inch of it will weigh 7.866 or almost 8 ounces, or nearly half a pound, which is the weight of the atmosphere for every inch of the barometer on a base of a square inch; and therefore 30 inches, or the medium height, weighs very near 144 pounds.

367. Corol. 2. Hence also the weight or pressure of the atmosphere, is equal to that of a column of water from 32 to 35 feet high, or on a medium 33 or 34 feet high. For, water and quicksilver are in weight nearly as 1 to 13.6;

SO

so that the atmosphere will balance a column of water 13.6 times as high as one of quicksilver; consequently

13.6 times 28 inches = 381 inches, or 314 feet,
13.6 times 29 inches = 394 inches, or 325 feet,
13.6 times 30 inches = 408 inches, or 34 feet,
13.6 times 31 inches = 422 inches, or 35 feet.

And hence a common sucking pump will not raise water higher than about 33 or 34 feet. And a siphon will not run, if the perpendicular height of the top of it be more than about 33 or 34 feet.

368. Corol. 3. If the air were of the same uniform density at every height up to the top of the atmosphere, as at the surface of the earth; its height would be about 54 miles at a medium. For, the weights of the same bulk of air and water, are nearly as 1.222 to 1000; therefore as 1.222: 1000::33 feet: 27600 feet, or 5 miles nearly. And so high the atmosphere would be, if it were all of uniform density, like water. But, instead of that, from its expansive and elastic quality, it becomes continually more and more rare, the farther above the earth, in a certain proportion, which will be treated of below, as also the method of measuring heights by the barometer, which depends on it.

369. Corol. 4. From this proposition and the last it follows, that the height is always the same, of an uniform atmosphere above any place, which shall be all of the uniform density with the air there, and of equal weight or pressure with the real height of the atmosphere above that place, whether it be at the same place, at different times, or at any different places or heights above the earth; and that height is always about 54 miles, or 27600 feet, as above found. For, as the density varies in exact proportion to the weight of the column, therefore it requires a column of the same height in all cases, to make the respective weights or pressures. Thus, if w and w be the weights of atmosphere above any places, D and d their densities, and a and b the heights of the uniform columns, of the same densities and weights; Then H X D = w, and

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hx d = w; therefore or H is equal to or b. The

W

D

temperature being the same.

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PROPOSITION

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