280 



PHYSIOLOGICAL PHYSICS. [Chap. xxm. 



injurious if it got into the mouth, the suction may 

 be applied by means of a side tube to the syphon, 

 the lower opening being kept closed 

 till the tube is filled, or the tube may 

 be filled before immersion in the 

 liquid. 



It is to be noted that for water the 

 suction tube of a pump, or the ascend- 

 ing limb of a syphon, should not be 34 

 feet above the water level, for beyond 



Fig. 122. 

 byphou. 



that height the water cannot be 



pumped. For other liquids the varia- 

 tion in the height depends on the density. 



The air pump is an instrument for diminishing 

 the atmospheric pressure by removing the air enclosed 

 in a space; it is shown in Fig. 123. It was in- 

 vented by Otto von Guericke, in 1650. It consists 

 of a cylinder fitted 

 with a piston. From 

 the cylinder passes a 

 tube, which opens on 

 a brass plate. The 

 plate supports a bell 

 jar (the receiver), the 

 lower edge of which is 

 carefully ground and 

 smeared with grease, so 

 as to be closely united 

 with the plate. When 

 the piston is raised, 

 air is drawn out of the 

 receiver to occupy the 

 space left void by the piston. A valve opens so 

 as to permit air to pass from the bell-jar. In the 

 piston is an opening guarded by a valve, but its 

 direction of opening is such that the atmospheric 

 pressure keeps it closed during the ascent of the 



Fig. 123. Tlie Air Pump. 



