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the pressure of the atmosphere, 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 sus. pended 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 poies of almost all bodies, whether solid or (uid, 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. Fig. 13.
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 io 31 inches; seldom more or less.
This is evident from the barometer, an instrument which shows the pressure of the air ; whiclı, at some seasons, stands at a height of 28 inches, 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 columu of wattt, 32 or 36 feet high; or, at a medium, 34 feet.
For water and quicksilver are, in weight, early 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 cartl, its height would be about of miles, at 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 tbe 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 sec! bigh.
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. Miles. Density.
The first and third columns are the height in miles froin 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 calculated by this series. Putra 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 noted hy this series
A B C, &c. where A, B, C,
68444 &c. are the preceding terms. The terms here will he 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 pomp, being a hollow cylinder, made of wood or lead. CD the handle, movable about the pin E. DH 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 npwards. 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 fixed close, that no water can get between it and the barrel; and the sucker FG is to be armned 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 always be placed so low, as that the sucker GL 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 Jittle 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 bow high the mercury is in the tube, above the level of it in the bubble, which is generally from 28 to 31 inches, but mostly about 29 or 30. Along with the scale of inches, there is also płaced a scale of such weather as bas been observed to answer the several heights of the quicksilver. lo 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, whilst the quicksilver rises an inch, it descends a little in the bubble; and tbat descent must be deducted, which makes the divisions be something less than an inch. Tbese 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 weight of a pillar of the atmosphere of the same 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 the following table: in which the first column is the height of the
mountain, &c. in feet or miles; the second, the lieight of the quicksilver ; and the third, the descent of the quicksilver in the barometer; and this at a mean density of the air.
Feet. High Barom. Descent. Feet. High Barom. | Descent
The Table continued, in Miles. Miles. High Barom. | Descent. Miles. High Barom. Descent.
This table is made from a table of the air's density, made as in Scho. Jium; and, then multiplying all the numbers thercof by 29.5, the mean density of the air. For the density of the air, at any height above the earth, is as the weight of the atmosphere anyo it, and that is as the height of the mercury in the barometer. Fig. 15.
There is another sort of barometer, which shows the ascent and descent of the mercury at the bottom.
ABC (fig. 16,) is a recurve tube, close at the top, where the bucket C is, and open at the end A. The length of CB is 32 or 33 inches, and of AB 6 or 7. The bucket C should contain about as much as the end AB; and the bucket and end EB must be quite filled with mercury, as far as B, a little beyond the turn. The wider the bucket Cis, the better. The scale set to the end AB must be graduated downwards; for the mercury falls in this, when it rises in the other sort. This being placed against a wall, will show the height of the mercury, as in the common ones: and this way is more commodious, as it saves the labour of clambering up upon chairs to see it, as one must do, in the common sort, to see exactly.
A barometer may also be made of water, as in fig. 17, wbich is a water. barometer. AB is a glass tube open at both ends, and cemented close in the month of the bottle EF, and reaching very near the bottom; then, warming the bottle at the fire, part of the air will fly ont; then the end A is put into a vessel of water mixed with cochineal, which will go through the pipe into the bottle as it grows cold. Then it is set upright; and the water may be made to stand at any point C, by sucking or blowing at A. And, if this barumeter be kept to the same degree of heat, by putting it in a vessel of sand, it will be very correct for taking small altitudes; for a little alteration in the weight of the atmosphere will make the water at Crise or fall in the tube very sensibly: but, if it be suffered to grow warmer, the water will rise too high in the tube, and spoil the use of it; so that it must be kept to the same temper. If a barometer was to be ade of water put in an eshausted tube,