PNEUMATICS. 129 



nature's " abhorrence of a vacuum." Torricelli, 

 also, pursuing this idea, by using a column of 

 mercury instead of water, produced what has since 

 been called the Torricellian vacuum, for which see 

 the description of the barometer. But from the 

 nature of the apparatus producing this vacuum or 

 space absolutely free from air, it was very difficult 

 to try what effect would be produced on various 

 bodies placed in it. In 1654, Otto von Guericke, 

 of Magdeburgh, invented the air pump, and made 

 numerous experiments with it. His air pump, 

 however, was of a very clumsy construction, being 

 obliged to place the vessels under water, in order 

 to be exhausted. Boyle, assisted by Hooke, con- 

 trived the present pump with two barrels, and the 

 vacuum so made has been frequently called the 

 Boylean vacuum, in contradistinction to the Torri- 

 cellian. 



As it is by means of the air pump that all the 

 mechanical properties of air are demonstrated, it 

 will be necessary to describe its construction, and 

 the manner of using it. 



Plate 6. fig. 1. is the air pump that is most in 

 use. A A are two brass barrels, each containing a 

 piston or short cylinder of metal, that fits the in- 

 side exactly. A hole passes through this piston 

 which is covered by a flap or valve that can only 

 open upwards. The pistons are made to move up 

 and down alternately in the barrel, by means of 

 the winch B, which has a pinion that fits into the 

 teeth of the racks C C, which are made upon the 

 ends of the pistons. 



On the square wooden frame D E, there is 



placed a brass plate G, ground perfectly fiat, and 



also a brass tube let into the wood communicating 



with the two cylinders and the cock I, and open- 



vol. i. K 



