64 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



stratum of air. Now, experiment proves that the adherence of this 



gaseous pad to the surface of the 

 piston is such, that it replaces the 

 oily substance with which the 

 piston is generally covered ; in a 

 word, its presence is sufficient to 

 intercept all communication be- 

 tween the spaces of the pump- 

 barrel above and below the 

 piston. M. Deleuil at first gave 

 to the latter a height double 

 its diameter, and he obtained a 

 vacuum from 8 to 1 8 milli- 

 metres according to the capacity. 

 Since his first experiment, al- 

 though he has given to the 

 diameter of the piston a value 

 equal only to its height, he has 

 been able to obtain a vacuum 

 of 2 to 3 millimetres in a 

 capacity of 14 litres; in a 

 quarter of an hour, he has ob- 

 tained a vacuum of 10 milli- 

 metres in a receiver of 250 litres. 



IV. ATMOSPHERIC RAILWAYS. 



One word now on the industrial application of air-pumps. One 

 of the most important has been the use that has been made of them 

 on some railways to obtain motion without the help of locomotives. 



The principle of this application is very simple. Along the 

 whole length of the railway a tube or metallic pipe is fixed, in the 

 interior of which a piston can move. By the aid of an air-pump, a 

 vacuum is made in the tube on one side of the piston, the 

 atmospheric pressure being exerted on the other side on its surface ; 

 this causes the piston to which the train is attached to move. 



The idea of making use of atmospheric pressure as motive-power 



