GUERICKE—THE AIR-PUMP. 



121 



point of water is o°, and the tube is so divided that there are 

 exactly ioo° degrees between the freezing and the boiling 

 point 



Otto Guericke invents the Air-pump, 1650 The Tor- 

 ricellian 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 experiment. Otto Guericke, 

 a magistrate of Magdeburg, in Prussia, made another step 

 in advance and invented an air-pump^ by which air can be 

 drawn out of a vessel, leaving it almost empty. Fig. 16 is the 

 simplest kind of air-pump, and pj^. ^g 

 the way it works is not difficult 

 to understand. At the bottom 

 is a glass jar which has a 

 round barrel or cylinder, b b, 

 fixed on the top of it. In the 

 cylinder is a tight-fitting piston, 

 c c^ like the one in the suc- 

 tion-tube p. 117, only that this 

 one has in it a valve or door, 

 d. There is also another 

 valve, <?, 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 rest- 

 ing on the valve, e. Then if we pull the piston gradually 

 up, the valve d will be kept shut by the air outside pressing 



7 



Air-pump (Knight). 



B B, Cylinder, c c. Piston with a valve. 

 d e. Valves opening upwards. 



