Of Pneumatics.

v effel of quicksilver, without letting in any air :then take away your finger; and the quicksilver^11 remain suspended m the tube 294 inchesabove its surface in the vessel-, sometimes more,a pd at other times less, as the weight of thek varied by winds and other causes. That1 e quicksilver is kept up in she tube by theP re ffure of the atmosphere upon that in the ba-• otl > is evident; for, if the bason and tube beP^t under a glass, and the air be then taken outthe glass, all the quicksilver in the tube willa 'l down into the bason; and if the air be let injl§ a in, the quicksilver will rife to the fameei ght as before. Therefore the air’s pressurethe surface of the earth, is equal to theei ght of 294- inches depth of quicksilver ailPver the earth’s surface, at a mean rate.

A square column of quicksilver, 294 inchesh'gh, and one inch thick, weighs just 15Pounds, which is equal to the pressure of airsipon every square inch of the earth’s surface ;a «d 144 times as much, or 21 60 pounds, upone Very square foot; because a square foot con-[? lns 144 square inches. At this rate, a middle-^ e d man, whose surface may be about 14 squaref 1 "’ lustains a pressure of 30240 pounds,air is of a mean gravity: a pressuretQ lc h w °uld be insupportable, and even fatalc US ’ we re it not equal on every part, and“^balanced by the spring of the air withinand r ^ IS trough the whole body ;

Wardress W ^' a an e q u al force against the out-

^ nce eart ^’ s surface contains (inj nc utttnbers) 200,000,000 square miles,

there Ver ^n^ U3re m ^ e 27,878,400 square feet,must be 5,575,680,000,000,000 square

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