In talking of light and sound we are apt to confound 

 the sensation with the motion of the medium that ex- 

 cites it. Thus in a deep calm we say, There is no air, 

 because we feel none, though there is really the same 

 quantity of air in equal space as if it hlew a storm. And 

 so in deep darkness we say, There is no light in the 

 room, although there is supposed to be as much light 

 there as there was at noon day : only its rays are qui- 

 escent, and make no impression upon the visive organs. 



Sound is said to move about fourteen miles in a 

 minute, which is performed thus : the stroke given by 

 the sounding body to the contiguous air, is communi- 

 cated to the next, and so on till it reaches the ear. 



The oscillations of the air are required to succeed 

 each other with a certain velocity; and in order to 

 render them audible, they must not be fewer than 

 thirty in a second of time. But the more frequent 

 these sonorous waves are in a given time, the sharper is 

 the sound heard, and the more strongly does it affect us, 

 till we come to the most acute of audible sounds, which 

 have 7520 tremors in a second. 



Acute sounds are, in general, yielded from bodies 

 that are hard, brittle, and violently shook or struck ; 

 grave sounds are from the contrary. Cords JY other 

 bodies, that yield the same number of vibrations in a 

 given time, are said to be tin/son; as those which make 

 double the number of oscillations in that time, yield a 

 tone that is an octave, or eight notes higher ; and other 

 proportions betwixt the number of the vibrations, have 

 ^iiilerent names assigned to them in a musical scale. 

 The shorter cords produce sharper tones, and the reverse 

 in a proportion directly as their lengths ; also lhose > 

 which are more stretched afford sharper sounds. 



The sound, whether acute or grave, strong or weak, 

 is carried through the air nbout 1038 Paris feet in a 

 second, and that with an uniform velocity, without 

 Abating in the larger distances. But a contrary wind, 

 causing the vibrations to extend more slo\vly, retards 



