CH. XV, 



GUERICKETHE AIR-PUMP. 



121 



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

 exactly 100 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. 1 6 is the 

 simplest kind of air-pump, and FlG I<5 



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 



Air-pump (Knight). 

 Upwards. NOW Suppose We B B . Cylinder, cc, Piston with a valve. 



start with both valves shut *' Valves opening upwards ' 

 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 



