PNEUMATICS. 



167 



The relation between the compressibility of air, and its elasticity and dens- 

 ity, also obeys a certain law whicli may thus be expressed : — 



374. The density and elasticity of air are 

 directly as the force of compression. 



Tliis relation is clearly exliibited by the following table : — 

 With the same amount of air, occupying the space of 



What relation 

 exists between 

 the compressi- 

 bility of air 

 and its elastic- 

 ity and density ? 



1, 



2) 3> 4; 3) t^) I on> 



the elasticity and density will be 1, 2, 3, 4, 5, 6, 100. 



Hence by comprcssinsr air into a yery small space, by means 

 What are il- „ •' ^ " •, i 5 r * u 



lustrations of of a proper apparatus, we can mcrease its elastic lorce to sucn 



the elastic force ^^ extent as to apply it for the production of very powerful 



effects. The well-known toy, tlie pop-gun, is an example of 



Ihe application of this power. The space A of a hollow cylinder, Fig. 156, is 



inclosed by the stopper, p, at one end, and by the end of the rod, S, at the 



other end.' This rod being pushed further into the cylinder, the air contained 



in the space. A, is compressed until its elastic force becomes so great as to 



drive out the stopper, p, at the other end of tlie cyhnder with great force, 



Fig. 156. 



accompanied with a report, 

 BimUar principle. 



375. The laws of Mariotte may be 

 lustratc the illustrated and proved by the following 

 laws ofMariotte. experiment : let A B G D be a long, 

 bent glass tube, open at its longer extremity, and fur- 

 nished with a stop-cock at the sliortcr. Tlie stop-cock 

 being open so as to allow free communication with 

 the air, a quantity of mercury is poured into the 

 open end. The surfaces of the mercury will, of course, 

 stand at tlie same level, E F, in both legs of the 

 tube, and will both sustain the weight of a col- 

 umn of air reaching from E and F to the top of 

 the atmosphere. If we now close the stop-cock, D, 

 the effect of the weight of the whole atmosphere 

 above that point is cut off, so tliat the surface, F, can 

 sustain no pressure arising from the weight of the 

 atmosphere. Still, the level of the mercury in both 

 legs of the tube remains the same, because the elas- 

 ticity of the air inclosed in F D is precisely equal, and 

 BuEQcient to balance the weight of the whole column 



The air-gun is constructed and operated on a 

 Fig. 157. 



