342 HISTORY OF ACOUSTICS. 



parts of the air throiigh which sound is transmitted. 

 It was shown 5 that a body vibrating in an elastic 

 medium, will propagate pulses through the medium; 

 that is, the parts of the medium will move forwards 

 and backwards, and this motion will affect succes- 

 sively those parts which are at a greater and greater 

 distance from the origin of motion. The parts, in 

 going forwards, produce condensation ; in returning 

 to their first places, they allow extension ; and the 

 play of the elasticities developed by these expan- 

 sions and contractions, supplies the forces which 

 continue to propagate the motion. 



The idea of such a motion as this, is, as we have 

 said, far from easy to apprehend distinctly: but a 

 distinct apprehension of it is a step essential to 

 the physical part of the sciences now under notice ; 

 for it is by means of such pulses, or undulations, 

 that not only sound, but light, and probably heat, 

 are propagated. We constantly meet with evidence 

 of the difficulty which men have in conceiving this 

 undulatory motion, and in separating it from a local 

 motion of the medium as a mass. For instance, it 

 is not easy at first to conceive the waters of a great 

 river flowing constantly down towards the sea, while 

 waves are rolling up the very same part of the 

 stream, and which the great elevation, which makes 

 the tide, is travelling from the sea perhaps with a 

 velocity of fifty miles an hour. The motion of such 

 a wave, or elevation, is distinct from any stream, 

 6 Newt. Prin. B. ii. P. 43.' 



