THE BOOK OF THE LANTERN. 



67 



natural tendency is to remain compressed. This top is 

 furnished with a collar O l , having a screw-thread in which 

 the screw pillar F can easily turn. It will be noticed 

 that the thread of the screw is coarse, so we may call 

 it an Archimedean screw. At its lower part is a fine, and 



d* 



Fig. 28. 



therefore slow-motion screw, which works in the collar c. 

 Now, let us see how beautifully this double-screw motion is 

 applied to regulating the delivery of the highly-compressed 

 gas. As soon as the gas emerges from the bottle it passes 



