174 



HISTORY OF SCIENCE. 



seen in cabinets of physical apparatus. Here the pistons contain 

 valves opening upwards, and the cylinders are of glass, in order to 

 display the action of the valves and pistons. 



FIG. 76. 



The original air-pump was in many ways an extremely imperfect in- 

 strument, but in the hands of its inventor it soon served to demon- 

 strate some important scientific truths. Among these we may name 

 the direct proof of the weight of air, and of the pressure exercised by 

 the atmosphere in consequence of that weight. Guericke's experi- 

 ments in illustration of the pressure of the atmosphere excited the 

 greatest astonishment in the beholders. One of the experiments which 

 most attracted attention has continued* to enter into every course of 



