SCIENTIFIC RECREATIONS. 



45 miles to 212 miles in an attenuated form; but perhaps 100 miles high 

 would be a fair estimate of the height to which our atmosphere extends. 



The pressure of such an enormous body of gas is very great. It has 

 been estimated that this pressure on the average human body amounts to 

 fourteen tons, but being balanced by elastic fluids in the body, the incon- 

 venience is not felt. The Weight of Air can easily be ascertained, though 

 till the middle of the seventeenth century the air was believed to be without 



Fig. 45. Weighing the air. 



weight. The accompanying illustration will prove the weight of air. Take 

 an ordinary balance, and suspend to one side a glass globe fitted with a 

 stop-cock. From this globe extract the air by means of the air-pump, and 

 weigh it. When the exact weight is ascertained turn the stop-cock, the 

 air will rush in, and the globe wall then pull down the balance, thus proving 

 that air possesses weight. The experiments of Torricelli and Otto Von 

 Guerike, however, demonstrated that the air has weight and great pressure. 

 Torricelli practically invented the barometer, but Otto *von Guerike, by the 

 cups known as Magdeburg- Hemispheres > proved the pressure of the outward 



