CH. XV. 



GUER TCKE THE A IK- PUMP. 



119 



wish to introduce it into England, because it is so much 

 more simple than Fahrenheit's. It is called ' centigrade/ 

 or a hundred steps, because the tube is so divided that 

 there are exactly 100 between the freezing and the boil- 

 ing point. 



Otto Guericke invents the Air-pump, 16 5 O. The 

 Torricellian vacuum in the barometer was made, as we have 

 seen, by simply filling a glass tube more than 30 inches 

 long with mercury, and then turning it upside down into a 

 basin of the same, so that the mercury in 

 the tube fell to 30 inches, and an empty 

 space was left at the top. But in 1650, 

 a very few years after Torricelli's experi- 

 ment, Otto Guericke, a magistrate of 

 Magdeburg, in Prussia, made another step 

 in advance and invented the air-pump, 

 an apparatus by which air can be drawn 

 out of a vessel, leaving it almost empty. 

 Fig. 17 is the simplest kind of air-pump, 

 and the way it works is not difficult to 

 understand. At the bottom is a glass jar 

 which has a round barrel or cylinder 

 15 B, fixed on the top of it. In the cylin- 

 der is a tight-fitting piston, c c, like the 

 one in the suction-tube p. 115, only that 

 this one has in it a valve or door, d. 

 There is also another valve, e, at the place 

 where the cylinder and glass jar meet, 

 and both these valves open upwards. Now suppose we 

 start with both valves shut and the piston c c down at the 

 bottom of the cylinder resting on the valve, e. Then if we 

 pull the piston gradually up, the valve d will be kept shut 

 by the air outside pressing upon it, and so the piston will 



FIG. 17. 

 Air-pump (Knight). 



B B, Cylinder. 



c c, Piston with valve. 



d e, Valves opening 



upwards. 



