PROPAGATION OF SOUND. 339 



lie other. It is used to prevent the sound of words 

 poken at the narrower end from spreading out in all 

 lirections and to cause it to be transmitted chiefly in 

 he direction towards which the wide end is turned. 



The difference between the propagation of sound 

 Jirough air and the mere motion of translation of air 

 aay be shown by a small apparatus constructed in the 

 oil owing manner : 



; Bend a piece of strong pasteboard into a cylinder, about 10 cm 

 .-i.'.e, and 15 cm long, or even longer, and glue the edges over one 

 .notlier ; fix upon one end, with glue, a lid, with a circular hole in 

 liu middle, 2 or 3 cm in diameter ; close the other end by a piece of 

 'tout paper (such as is used for packing or drawing), which is drawn 

 eiy tight over the end and then tied with thread ; the paper must 

 rst be wetted and stretched over the end just when the gloss arising 

 rom the wetting has disappeared. Before using the apparatus it 

 nist be perfectly dry. An apparatus made of brass is better and 

 icre durable, and a piece of calf-skin or ox- bladder answers better 

 liun paper. The cylinder must have a projecting rim at the open 

 nd, so as to allow of the cover being tied firmly, otherwise it will 

 ilip off. 



If the elastic cover be tapped moderately strongly 



nth the finger, a small quantity of air is expelled from 



he apparatus with considerable force, and moves through 



ome distance. If the apparatus be filled with smoke, 



jy thrusting a piece of burning tinder stuck upon a 



jdre through the aperture, which is held downwards 



ntil the tinder is consumed, the smoke expelled by 



(nocking with the finger upon the other end issues as a 



jeautiful ring, moving first with great but afterwards 



ith diminishing velocity, and becoming gradually 



rger. Similar rings are often produced on a smaller 



'ale by smokers, by blowing the smoke away in succe*- 



ve jerks. The rings are best seen when the apparatus 



z 2 



