314 SOUND. 



ing solid substances, capable of conveying sound, could 

 be entirely cut off. 



This experiment has been tried in the following man- 

 ner. Under the receiver of an air pump, a piece of clock 

 work was placed, to produce sound in a bell, which 

 rested upon a bag stuffed with cotton or wool. A vacuum 

 was produced by working the pump, and then by means 

 of a handle, or stem which went through the top of the 

 receiver, the clock work was set in motion, and the 

 "hammer was then seen to strike the bell continually, but 

 no sound was heard. Another philosopher, to render 

 this experiment still more decisive, placed the bell in a 

 first receiver, which remained full of air, and which was 

 covered by a second receiver, so disposed that a vacuum 

 might be made between the two. Although in this dis- 

 tribution of things, a sound was produced in the interior 

 receiver, when motion was communicated to the hammer, 

 yet the bell remained mute with regard to the observer ; 

 he perceived no sound. 



The loudness of sounds consequently depend in some 

 measure upon the density or rarity of the air. When 

 the air is dense a distant bell is heard more distinctly 

 than usual. A cannon fired on the top of a mountain is 

 not heard so distinctly as it would be in a less elevated 

 situation, on account of the rarity of the air. Saus- 

 sure noticed this when upon the Alps. The pistol 

 which he discharged at that great elevation, produced 

 scarcely anything but a flash. In a deep mine, where 

 the air is denser than at the surface, the sound would be 

 louder than usual. The thundering noise, however, 

 which travellers produce in caverns, is not owing chiefly 

 to this cause, but to reverberations which will be subse- 

 quently explained. 



Sound is affected not only by different degrees of 

 density in the atmosphere, but it varies in the same way, 

 when produced in different^ases of various densities. 



' Sounding bodies vibrate much more rapidly, if 

 placed in hydrogen, than in common air, and more 

 quickly in common air, than in any of the heavier 

 g ases : " because the lighter the air, the less is the 

 resistance to a body moving in it. Thus also a bell will 



