106 ECHO. [Lesson XVHI 



being interrupted in its passage. For as the air 

 tmdulates forward like a wave, it often meets with 

 various objects, and by striking against them is 

 reflected back to us, and causes new vibrations, 

 xvhich, if the object lie in a proper situation, return 

 to us and repeat the same sound, tone, or word, as 

 was first given : and this, it sometimes happens, 

 not only once but several times. 



From this short account of the nature of echoes, 

 it will not be difficult to conceive that they also 

 may be applied to the measuring of inaccessible 

 distances : thus, Dr. Derham, standing upon the 

 bank of the Thames opposite to Woolwich, ob- 

 served that the echo of a single sound was reflected 

 back from the houses on the opposite bank in 3 

 seconds ; consequently the sum of the direct and 

 reflex rays must have been 1142 multiplied into 3, 

 or 3126 feet, and the half of it, viz. 17 13 feet, the 

 breadth of the river in that place. 



But to treat upon the nature of sounds in such a 

 manner as the importance of the subject demands 



' would fill a volume : I must therefore now desist. 

 From what little has been here mentioned, the 



contemplative young reader will with pleasure 

 draw this inference : that even in the nature and 

 production of sounds the goodness of the CRE- 

 ATOR to his creatures is conspicuously shewn: 

 for, if bodies had no degree of elasticity, or if the 

 air were not so fluid as to be easily put in motion, 

 there would be an end of all the melody and har- 

 mony which now so much delight us. What 

 praise does that benevolent BEING require from 



us, 



