io6 Os Pneumatici

to the fame height as before. Tijeresore the aix's pressure on the fur-face of the earth, is equa! to the weight of 29 -- inches depth of quick-silver all over the earth’s furface, at a mean rate.

A fquare column of quicksilver, 29- inches high, and one inch thick,weighsjust 15 pounds, which is equal to the pressure of air upon eve-ry fquare inch of the qarth’s furface >_and 144 times as much, or 2160pounds, upon every fquarg foot; becaufe 'a fquare foot contains 144fquare inches. At this rate. 4 middje-fized man, whofe furface maybe about 14 fquare feet, fustains a pressure of 30240 pounds, whenthe air is of a mean gravssy : a pressure which would be inlupportable,and even fatal to us, were it„not equal on every part, and counterba-lanced by the spring of thy air within.us, which is diffufed through thewhole body; and redacta with, an equal force against the outwardpressure.

Now, linee the earth’s furface contains (in round numbers)200,000,000 fquare miles, and every fquare mile 27,878,400 fquarefeet, there muss be 5,575,680,009,(300,000 fquare feet on the earth’sfurface; which multiplied by 3160 pounds (the pressure on eachfquare foot) gives 12,043,468,800,000,000,000 pounds for the pressureor weight of the whole atmofphere.

When the end of a pipe is inirperfed in water, and the air is takenout of the pipe, the water will rise in it to the height of 33 feet above' the furface of the water in which it is iromerfed; but will go no higher :for it is found, that a common pump will draw water no higher than33 feet above the furface of the well : and unlefs the bücket goes withinthat distance from the well, the water will never get above it. Now,as it is the pressure of the atmofphere, on the furface of the water inthe well, that caufes the water to afeend in the pump, and follow thepiston or bücket, when the air above it is lifted up; it is evident, that acolumn of water 33 feet high, is equal in weight to a column of quick-lilver of the fame diameter, 29s inches high ; and to as thick a columnof air, reaching from the earth’s furface to the top of the atmofphere.The barome- In serene calm weather, the air has weight enough to fupport a co-ter. lumn of quicksilver 3 1 inches high ; but in tempestuous stormy wea-

ther, not above 28 inches. The quicksilver, thus fupported in a glafstube, is found to be a nice counterbalance to the weight or pressure ofthe air, and to fhew its alterations at disserent times. And being nowgenerally ufed to denote the changes in the weight of the air, and of theweather confequent upon them, it is called the borometer , or weather-glafs.

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