MAMMALIA— MAN. 45 



effect always a vibration, or Vv'ave-lilcc motion, communicated by other bodies 

 to the air, and to our senses, by the air striking on our auditory nerve. 



Every body that strikes against another, produces a sound, Avhich is sim- 

 ple in sucli bodies as are not elastic, but which is often repeated in such as 

 are. If we strike a bell, for instance, a single blow produces a sound, which 

 is repeated by the undulations of the sonorous body, and which is multiplied 

 as often as it happens to undulate or vibrate. These undulations succeed 

 each other so fast, that the ear supposes them one continued sound ; where- 

 as, in reality, they form many sounds. Sounding bodies are, therefore, of 

 two kinds ; those unelastic ones, which being struck return but a single 

 sound; and those more elastic, returning a succession of sounds, which 

 uniting together form a tone. This tone may be considered as a great- 

 number of sounds, all produced, one after the other, by the same body, as 

 we find in a bell which continues to sound for some time after it is struck. 

 A continuing tone may be also produced from a non-elastic body, by repeat- 

 ing the blow quick and often, as when we beat a drum, or when we draw a 

 bow along the string of a fiddle. 



To know the manner in which musical sounds become pleasing, it must 

 be observed, that no one continuing tone^ how loud or swelling so ever, 

 can give us satisfaction ; we must have a succession of them, and those 

 in the most pleasing proportion. The nature of this proportion may 

 be thus conceived. If we strike a body incapable of vibration with 

 a double force, or, what amounts to the same thing, with a double mass 

 of matter, it will produce a sound that will be doubly grave. Music 

 has been said, by the ancients, to have been first suggested by the 

 blows of different hammers on an anvil. Suppose then we strike au 

 anvil with a hammer of one pound weight, and again with a hammer 

 of two pounds, the tAvo pound hammer will produce a sound twice as 

 grave as the former. But if we strike with a two pound hammer, and 

 then with a three pound, the latter will produce a sound one third more 

 grave than the former. If we strike the anvil with a three pound hammer, 

 and then with a four pound, it will likewise follow, that the latter will be 

 a quarter part more grave than the former. Now, in comparing all those 

 sounds, it is obvious that the difference between one and two is more easily 

 perceived than between two and three, three and four, or any numbers 

 succeeding in the same proportion. The succession of sounds will be, 

 therefore, pleasing in proportion to the ease with which they may be dis- 

 tinguished. That sound which is double the former, or, in other words, the 

 octave to the preceding tone, will among all others be the most pleasing 

 harmony. The next to that, which is as two to three, or, iji other words, the 

 third, will be most agreeable. And thus, universally, those sounds whose 

 differences may be most easily compared are the most agreeable. 



Sound has, in common with light, the property of being extensively 

 diffused. Like light, it also admits of reflection. The laws of this reflec- 



