THE WORK OF AIR AND WATER 



anism. Under ordinary conditions the hemispheres 

 would fall apart readily, but von Guericke proved, by 

 a famous public demonstration, that when the air was 

 exhausted in the sphere, teams of horses pulling in 

 opposite directions on the hemispheres could not 

 separate them. This is famous as the experiment of 

 the Magdeburg spheres, and it is often repeated on a 

 smaller scale in the modern physical laboratory, to 

 the astonishment of the tyro in physical experiments. 



The first question that usually comes to the mind 

 of anyone who has personally witnessed such an experi- 

 ment, is the question as to how the human body can 

 withstand the tremendous force to which it is subjected 

 by an atmosphere exerting a pressure of fifteen pounds 

 on every square inch of its surface. The explanation 

 is found in the uniform distribution of the pressure, 

 the influence of which is thus counteracted, and by 

 the fact that the tissues themselves contain everywhere 

 a certain amount of air at the same pressure. The 

 familiar experiment of holding the hand over an ex- 

 hausted glass cylinder which experiment is indeed 

 but a modification of the use of the cupping glass above 

 referred to illustrates very forcibly the insupportable 

 difficulties which the human body would encounter 

 were not its entire surface uniformly subjected to the 

 atmospheric pressure. 



AIR IN MOTION 



At about the time when the scientific experiments 

 with the pressure of gases were being made, prac- 

 tical studies of the effects of masses of air in motion 



