58 GREAT MEN OF SCIENCE 



inasmuch as they always expanded when the air pressure 

 was removed. A pig's bladder with a small quantity of air 

 sealed in it, burst of itself when in a vacuum. It was thus 

 clear that during pumping, the air passes quite by itself from 

 the vessel to be evacuated into the empty pump, in so far 

 as it is not prevented from doing so by the back pressure of 

 the valve. Guericke eliminated this by the employment of a 

 special arrangement for opening the valves. He also replaced 

 the latter by stop-cocks. He convinced himself that the 

 automatic equalisation of pressure also takes place through a 

 long tube, when the pump and the vessel were situated on 

 different storeys of the house. He noticed especially the 

 energy with which the equalisation of pressure takes place 

 under suitable conditions, so that small stones and nuts may 

 be hurled about by the air, and concludes from this that 

 winds and storms can only be caused by differences of 

 pressure in the atmosphere; he was once able to prophesy 

 quite correctly a destructive storm from the particularly low 

 pressure of the air. He also noticed the formation of mist 

 when moist air is expanded, and supposed that there is a 

 causal connection between the reduction in pressure of the 

 atmosphere and the formation of clouds. 



The hard clash produced by two masses of water meeting 

 in a vacuum was also very remarkable; 'This could not be 

 produced by anyone in any other way than in an empty 

 vessel of this kind.'^ Guericke also remarked that after some 

 time air-bubbles always again appeared under water on the 

 walls of the vessel, unless it has been pumped out for a long 

 time previously. Since at that time, and even for many years 

 later, the material character of gases, which the air-pump 

 first taught us to regard as something that could be taken 

 hold of, had never been properly grasped apart from the 



^ Commonly demonstrated by the 'water hammer,' a glass tube con- 

 taining some water, which is boiled to expel the air, the tube then being 

 sealed. [Trans.] 



