THE CONQUEST OF NATURE 



containing air, which through its elasticity and pressure 

 equalizes the force of the stream that is ejected from 

 the chamber through the hose. 



SUCTION AND PRESSURE 



In the construction of this and various other appara- 

 tus, Ctesibius and Hero were led to make careful 

 studies of the phenomena of suction. But in this 

 they were not alone, since numerous of their predecessors 

 had studied the subject, and such an apparatus as 

 the surgeon's cupping glass was familiarly known 

 several centuries before the Christian era. The cupping 

 glass, as perhaps should be explained to the reader of 

 the present day since the apparatus went out of 

 vogue in ordinary medical practise two or three 

 generations ago consists of a glass cup in which the air 

 is exhausted, so as to suck blood from any part of the 

 surface of a body to which it is applied. Hero describes 

 a method of exhausting air by which such suction may 

 be facilitated. But neither he nor any other philoso- 

 pher of his period at all understood the real nature of 

 this suction, notwithstanding their perfect familiarity 

 with numerous of its phenomena. It was known, 

 for example, that when a tube closed at one end is 

 filled with water and inverted with the open end beneath 

 the surface of the water, the water remains in the tube, 

 although one might naturally expect that it would obey 

 the impulses of gravitation and run out, leaving the 

 tube empty. A familiar explanation of this and allied 

 phenomena throughout antiquity was found in the 



