AND EXTERNAL RESISTANCE, 



His respiration became difficult, his pulse 

 quick, and he was seized with symptoms of 

 fever; and the report of the pistol was not 

 heard in a greater degree, than would have 

 been produced by the discharge of a child's 

 toy-gun near the surface of the earth. The 

 same effects take place upon the clapper of a 

 bell ; when forcibly struck against the sides, 

 it excites to the ear no more sound, than it is 

 found to do under a receiver, after a great por- 

 tion of the air it contained has been exhausted 

 out of it.* 



* That the expansible force of the air, is the agent by 

 which sound is propagated and conveyed from a sonorous 

 body to distant objects, is decidedly proved by the cessation 

 of all sound in an exhausted medium. If a bell be placed 

 under a receiver full of air, and the clapper made to strike 

 against the sides of the bell ; the sound will\J)e immediately 

 propagated through the receiver, so as to be distinctly heard 

 by the bystanders ; and the sound will be found to increase 

 in degree, in proportion to the condensation which the air has 

 undergone. If a second receirer, however, be put over the 

 first, and the air be exhausted out of it; although the bell 

 within the first receiver be struck the same as before, no sound 

 will be heard ; we are led from thence to conclude, not only 

 that the pulsations of the air, are the agents by which sound 

 is communicated to the auditory nerves, but to understand 

 why it is more perfect in a dry, than in a wet day ; in a calm, 

 than in a storm ; in an atmosphere which is serene and clear, 

 than when it is foggy and wet ; finally, more perfect near the 

 surface of the earth, than at the top of the highest mountain. 



