Pneumatics. 67 



inches and a half high, and one inch thick, 

 weighs just fifteen pounds, which is equal to the 

 pressure of air upon every square inch of the 

 earth's surface ; and one hundred and forty-four 

 times as much, or two thousand one hundred 

 and sixty pounds upon every square foot ; be- 

 cause a square foot contains one hundred and 

 forty-four square inches. At this rate a middle- 

 sized man, whose surface may be about fourteen 

 square feet, sustains a pressure of thirty thou- 

 sand two hundred and forty pounds, when the 

 air is of a mean gravity ; a pressure which would 

 be insupportable, and even fatal to us, were it 

 not equal on every part, and counterbalanced by 

 the spring of the air within us, which is diffused 

 through the whole body, and re-acts with an 

 equal force against the outward pressure. 



Now, since the earth's surface contains, in 

 round numbers, 200,000,000 square miles, and 

 every square mile 27,878,400 square feet, there 

 murst be 5,575,680,000,000,000 square feet oa 

 the earth's surface ; which, multiplied bv 2,160 

 pounds, (the pressure on each square foot) give* 

 12,043,468,800,000,000,000 pounds for the pres- 

 sure or weight of the whole atmosphere. 



The above experiment on the quicksilver, 

 which is called the Torricellian experiment, after 

 its inventor Torricelli, who made it about tin? 

 year 164r5, is the foundation of that instrument 

 which is called the barometer, so useful in fore- 

 telling changes of the weather. In the common 



