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PROP. 1. -The air is a heavy body, and gravitates on all parts of the surface of the earth.
That the air is a fluid is very plain, as it yields to any the least force that is impressed upon it, without making any sensible resistance. But, if it be moved briskly, by some very thin and light body, as a fan, or by a pair of bellows, we become very sensible of its motion against our hands or face, and likewise by its impelling or blowing away any light bodies, that lie in the way of its motion. Therefore the air being capable of moving other bodies by its impulse, must itself be a boily; and must thereforc be heavy, like all other bodies, in proportion to the matter it con tains; and will consequently press upon all bodies placed under it. And, being a fluid, it will dilate and spread itself all over upon the earth; and like other fluids will gravitate upon, and press every where upon its surface.
The gravity and pressure of the air is also evident from experiments. For, (see fig. 3, p. 723) if water, &c. be put into the tube ABF, and the air be drawn out of the end F, by an air-pump, the water will ascend in the
nd F, and descend in the end A, by reason of the pressure at A, which was taken off or diminished at F. There are numberless experiments of this sort. And, though these properties and effects are certain, yet the air is a fluid so very fine and subtle, as to be perfectly transparent, and quite invisible to the eye.
Cor. 1. The air, like other fluids, will, by its weight and fluidity, insinuate itself into all the cavities and corners within the carth ; and there press with so much greater force as the places are deeper.
Cor. 2. Hence the atmosphere, or the whole body of air surrounding the earth, gravitates upon the surfaces of all other bodies, whether solid or fluid, and that with a force proportional to its weight or quantity of matter.
For this property it must have in common with all other fluids.
Cor. 3. Hence the pressure, at any depth of water, or other fluid, will be equal to the pressure of the fluid, together with the pressure of the atmosphere.
Cor. 4. Likewise all bodies, near the surface of the earth, lose so much of their weight,) as the same bulk of so much air weighs. And, consequently, they are soinething lighter than they would be in a vacuum. But, being so very small, it is commonly neglected; though, in strictness, the true or absolute weight is the weight in vacuo.
PROP. 2.-The air is an elastic fluid, or such a one as is capa. ble of being condensed or expanded. And it observes this law, that its density is proportional to the force that compresses it.
These properties of the air are proved by experiments, of which there are innumerable. If you take a syringe, and thrust the handlo inwards, you will feel the included air act strongly against your hand ; and the more you thrust the further the piston goes in, but the more it resists; and, taking away your band, the handle returns back to where it was at first. This proves its elasticity, and also that air may be driven into a less space and condensed.
Again, fill a strong bottle, fig. 8, half-full of water, and cement a pipe BI closc in it,'going near the bottom: then inject air into the bottle through the pipe BI. Then the water will spout out at B, and form a jet: which proves, that the air is first condensed, and then by its spring Irives out the water, till it becomes of the same density as at first, and ben the spouting ccascs.
If a vessel of glass AB, fig. 13, be filled with water in the vessel CD, and then drawn up with the bottom upwards: if any air is left in the top at A, the higher you pull it up, the more it expands; and, the further the glass is thrust down into the vessel CD, tho more the air is condensed.
Take a crooked glass tube ABD, fig. 14, open at the end A, and close at D ; pour in mercury to the height BC, but no bigher, and then the ; air in DC is in the same state as the external air. Then pour in more
mercury at A, and observe where it rises to in both legs, as to G and H. Then you may always sce that the bigher the mercury is in the leg BH, the less the space GD is, into which the air is driven. And, is the heiglit of the mercury FH be such as to equal the pressure of the atmosphere, thien DG will be half DC; if it be twice the pressure of the atmosphere, DG will be fDC, &c. So that the density is always as the weight or compression. And here the part CD is supposed to be cylindrical.
Cor. 1. The space that any quantity of air takes up, is reciprocally as the force that compresses it.
Cor. 2.-All the air near the earth is in a state of compression by the weight of the incumbent atmosphere.
Cor 3.—The air is denser near the earth, or at the foot of a mountain, than at the top of it and in bigla places; and, the bigher from the carth, the more rare it is.
Cor. 4.- The spring or elasticity of the air is equal to the weight of the atmosphere above it, and produces the same cficets.
For they always balance and sustain cach other.
Cor. 5.—Hence, if the density of the air be increased, its spring or elasticity will likewise be increased in the same proportion.
Cor. 6. — From the gravity and pressure of the atmosphere upon the surfaces of fluids, the fluids are made to rise in pipes or vessels, when the pressure within is taken off. PROP. 3.- The expansion and elasticity of the air is increased by
heat, and decreased by cold: or, heat expands, and cold con. denses, the air.
This is also a matter of experience; for, tie a bladder very close with some air in it, and lay it before the fire, and it will visibly distend the bladder, and burst it, if the heat is continued, and increased high enough.
If a glass vessel AB, (fig. 13,) with water in it, be turned upside down, with a little air in the top A, and be placed in a vessel of water, and lung over the fire, and any weight laid upon it to keep it down; as the water warms, the air in the top À will by degrees expand, till it fills the glass, and, by its elastic force, drive all the water out of the glass; and a good part of the air will follow, by continuing the vessel there. Many more experiments may be produced, proving the same thing. PROP. 4.-The air will press upon the surfaces of all fluids, wito
any force, without passing through them, or entering into them. If this were not so, no machine, whose uso or actions depends apo
the pressure of the atmospherc, could do its business. Thus, the weight of the atmosphere presses upon the surface of water, and forces it up into the barrel of a pump, without any air getting in, which would spoil its working. Likewise, the pressure of the atmosphere keeps mercury suspended at such a height, that its weight is equal to that pressure; and yet it never forces itself through the mercury into the vacuum above, though it stand ever so long: and, whatever be the texture or constitution of that.subtle invisible fluid we call air, yet it is never found to pass through any fluid, though it be made to press ever so strongly upon it. For, though there be some air inclosed in the pores of almost all bodies, whether solid or.Ouid, yet the particles of air cannot, by any force, be made to pass through the body of any fluid, or forced-through the pores of it, although that force or pressure be continued ever so long. And this seems to argue that the particles of air are greater than the particles or pores of other fluids; or, at least, are of a structure quite different from any of them.
PROP. 5.— The weight, or pressure, of the atmosphere, upon any base at the earth's surface, is equal to the weight of a column of mercury of the same base, and whose height is from 28 to 31 inches; seldom more or less.
This is evident from the barometer, an instrument which shows the pressure of the air ; which, at some seasons, stands at a height of 28 incbes, sometimes at 29, and 30, or 31. The reason of this is, not be cause there is at some times more air in the atmosphere than at others, but because the air, being an extremely subtle and elastic fluid, capable of being moved by any impressions, and many miles bigh, it is much disturbed by winds, and by heat and cold; and, being often in a tumultuous agitation, it happens to be accumulated in some places, and consequently depressed in others;. by which•means it becomes denser and heavier where it is higher, so as to raise the column of mercury to 30 or 31 inches; and, where it is lower, it is rarer and lighter, so as only to raise it to 28 or 29 inches: and experience shows that it seldom goes without the limits of 28 and 31.
Cor. 1.--The air in the same place does not always continue of the same weight, but is sometimes heavier, and sometimes lighter; but the mean weight of the atmosphere is that when the quicksilver stands at about 294 inches.
Cor. 2.-Hence the pressure of the atmosphere upon a square inch at the earth's surface, at a medium, is very near 15.pounds, avoirdupois.
l'or an inch of quicksilver weighs 8.102 ounces.
Cor. 3.-Hence, also, the weight or pressure of the atmosphere, in its lightest and heaviest state, is equal to the weight of a commu of water, 32 or 36 feet high; or, at a medium, 34 feet.
For water and quicksilver are, in weight, pearly as 1 to 14.
Cor. 4.-If the air was of the same density, to the top of the aimo. sphere, as it is at the cart.), its height would be about 3f miles, al a medium.
For the weight of air and water are nearly as 12 to 10000.
Cor. 5.-The density of the air in two places, distant from each other but a few miles, on the earth's surface and in the same level, may be looked on to be the same, at the same time.
Cor. 6.- The density of the air at two different altitudes in the same place, differing only by a few feet, may be looked on as the same.
Cor. 7.-If the perpendicular height of the top of a syphon (fig. 11.) from the water be more than 34 feet, at a mean density of the air, the syphon cannot be made to run.
For the weight of the water in the legs will be greater than the pressure of the atmosphere, and both columns will run down, till they be 34 feet high.
Cor. 8.-Hence, also, the qnicksilver rises bigher, in the barometer, at the bottom of a mountain than at the top, and at the bottom of a coalpit than at the top of it.
SCHOLIUM. Hence the density of the air may be found at a: y height from the earth, as in the following table:
Miles. Density. Milcs. Density.
-C, &c. where A, B, C,
The first and third columns are the height in miles from the surface of the earth; and the second and fourth columns show the density at that height, supposing the density at the surface of the earth to be 1.
The density at any height is easily caleulated by this series. Put r = radius of the earth, h= height from the surface, both in feet. Then the density at the height, h, is the number belonging to the logarithm, de
h h h noted hy this series
68444 &c. are the preceding terms. The terms here will be alternately negative and affirmative: but the first term alone is sufficient when the height is but a few miles.
By the weight and pressure of the atmosphere, the operations of pneumatic engines may be accounted for and explained.
Fig. 15, is a common pump. AB the barrel or body of the pump, being a hollow cylinder, made of wood or lead. CD the handle, movable about the pin E. DF an iron rod, moving about a pin D: this rod is hooked to the bucket, or sucker, FG, which moves up and down within the pump. The bucket FG is bollow, and has a valve, or clack, L, at the top, opening upwards. H, a plug fixed at the bottom of the barrel, being
likewise hollow; and a vaive at I, opening also upwards. BK, the bot. tom, going into the well at K: the pipe below B need not be large, being only to convey the water out of the well into the body of the pump. The plug H must be fized close, that no water can get between it and the barrel; and the sucker FG is to be armed with leather, to fit close, that no air or water can get through between it and the barrel.
When the pump is first wrought, or any time, in dry weather, when the water above the sucker is wasted, it must be primed, by pouring in some water at the top A, to cover the sucker, that no air get through. Then, raising the end C of the handle, the bucket F descends, and the water will rise through the bollow GL, pressing open the valve L. Then putting down the end C raises the bucket F, and the valve shuts by the weight of the water above it; and, at the same time, the pressure of the atmosphere forces the water up through the pipe KB, and, opening the valve I, it passes through the plug into the body of the pump. And, when the sucker descends again, the valve I shuts, and the water cannot return, but, opening the valve L, passes the sucker GL. And, when the sucker is raised again, the valve L shuts again, and the water is raised in the pump. So that, by the motion of the piston up and down, and the alternate opening and shutting of the two valves, water is conti. nually raised into the body of the pump, and discharged at the spout M.
The distance KG, from the well to the bucket, must not be above 32 feet; for the pressure of the atmosphere will raise the water no higher, and, if it is more, the pump will not work. It is evident a pump will work better when the atmosphere is heavy than when it is light, there being a twelfth or fifteenth part difference, at different times; and, when it is lightest, it is only equal to 32 feet: wherefore the plug H must als ways be placed so low, as that the sucker CL may be within that compass.
A Barometer is an instrument to measure the elasticity of air. It consists of a bollow glass cone, filled with mercury, and hermetically sealed at the end, so that no air be left in it. When it is set upright, the mercury descends, down the tube, into the bubble, wbich has a little opening at the top, that the air may have free ingress and egress At the top of the tube, there must be a perfect vacuum. The instrument is fixed in a frame, and hung perpendicular against a wall. Near the top, on the frame, is placed a scale of inches, showing how high the mercury is in the tube, above the level of it in the bubble, which is gencrally from 28 to 31 inches, but mostly about 29 or 30. Along with the scale of inches, there is also placed a scale of such weather as bas been observed to answer the several beights of the quicksilver. lp dividing the scale of inches, care must be taken to make proper allowance for the rising or falling of the quicksilver in the bubble, which ought to be about half full when it stands at 29į, which is the mean height; for, wbilst the quicksilver rises an inch, it descends a little in the bubble ; and that descent must be deducted, which makes the clivisions be something less than an inch. These inches must be divided into tenth parts, for the more exact measuring the weight of the atmo. sp here: for the pillar of mercury in the tube is always equal to the weiglit of a pillar of the atmosphere of the saine thickness; and, as the height of the quicksilver increases or decreases, the weight of the air increases or de creases accordingly. The tube must be near 3 feet long, and the bore not less than or of an inch in diameter, or else the quicksilver will not move freely in it.
By help of the barometer, the height of mountains may be measured, by tbe following table: in which the first column is the height of the