PHYSICAL FORCES 115 



positions as the heads of the wheat do ; but the regular 

 succession of periodic motions gives the appearance of 

 an onward wave motion of the material disturbed. 



All sounds, whatever their nature, travel through the 

 air at the same speed and pass through it in all direc- 

 tions simultaneously, while retaining nevertheless their 

 distinctness as the sounds of birds, bells, cattle and 

 cart-wheels each remain distinct. 



Since sound travels at so very slow a rate compared 

 with light, it is easy to understand how it is we do not 

 hear the sound of a gun till a very appreciable time 

 after we have seen the smoke from it. Even each blow 

 of a man beating carpets some six hundred or seven 

 hundred yards off, will not be heard till after the eye 

 has seen the corresponding movement. 



Sound, like heat and light, diminishes in intensity 

 with the square of the distance, and will be reflected 

 according to the laws of equal angles. It is to this 

 reflexion that all echoes are due, and these may be 

 double or triple or more numerous, according to the 

 arrangement of the surfaces on which the waves 

 impinge. 



Waves of sound which succeed each other at equal 

 intervals and with sufficient frequency, cause us to be 

 aware of a musical sound or note. If the waves do not 

 succeed each other as quickly as sixteen times in a 

 second, we have no such experience, but only a rattling 

 sound is produced. The more rapid the succession, the 

 higher the musical note perceived ; thirty-two vibrations 

 in a second produce almost the deepest note generally 

 audible, and 70,000 vibrations produce the shrillest. As 

 we saw that the human eye can only appreciate what 

 we take to be a certain medium amount of ether vibra- 

 tions, and not lower or more rapid ones, so the range 



