ii6 SEVENTEENTH CENTURY. 



CHAPTER XV. 



SCIENCE OF THE SEVENTEENTH CENTURY (CONTINUED). 



Torricelli discovers the reason of Water rising in a Pump — Uses Mer- 

 cury to measure the Weight of the Atmosphere — Makes the First 

 Barometer — M. Perrier, at Pascal's suggestion, demonstrates varia- 

 . tions in the pressure of the atmosphere — Otto Guericke invents the 

 Air-pump — Working of the Air-pump — Guericke proves the 

 Pressure of the Atmosphere by the experiment of the Magdeburg 

 Spheres — He makes the first Electrical Machine — Foundation of 

 Royal Society of London and other Academies of Science. 



Torricelli's Invention of the Barometer, 1644. — We must 

 now turn to quite another subject on which new light was 

 being thrown at this time. Among the many different 

 mechanical experiments which Galileo made during his life, 

 there had been one with a common pump which puzzled 

 him very much, and which he had never been able to 

 explain. 



You know that if you put the mouth of a squirt in water 

 and pull back the handle, the water rises up into the tube. 

 That is to say, as soon as you leave a space inside the squirt 

 quite empty without any air in it, the water rushes in. 

 In the same way, water may be made to rise up a long 

 tube standing with its open end in a pond or basin, by 

 drawing up a tight-fitting stopper a. Fig. 14, called a pis- 

 ton, and so driving the air out at the top and leaving 

 a vacuum inside the tube. But Galileo found that as 

 soon as the water had risen up to the height of about 



