290 OUTLINES OF NATURAL PHILOSOPHY. 



390. There is perhaps, in the economy of na- 

 ture, no contrivance more wonderful, than that by 

 which things apparently so little susceptible of pre- 

 cision as the impulses communicated to an elastic 

 fluid, become the means of conveying to the mind 

 such a multitude of distinct impressions as it re- 

 ceives through the ear ; the finest modulations of 

 harmony, and the nicest distinctions of articulate 

 language. 



391. An Echo is a repetition of sounds produced 

 by the reflection of the aerial pulses that convey 

 sound to the ear. 



A wall, a rock, a grove of trees, may be so placed, as 

 to cause an echo. That an echo may return one 

 syllable as soon as it is pronounced, the reflecting 

 surface should be 80 or 90 feet distant ; for a dissyl- 

 labic echo, 170 feet, &c. The sound, whether di- 

 rect or reflected, appears to proceed nearly at the 

 rate of 1142 feet in a second. 



392. The Speaking Trumpet is an instrument 

 intended to transmit the sound of the voice in some 

 particular direction, to a greater distance than it 

 would otherwise reach. 



The best form of a speaking-trumpet, is found to be 

 that of a hollow cone, with a mouth-piece at the 



narrow 



