332 PROPAGATION OF SOUND. 



colour ; the piece of paper is then placed upon A, so thj 

 the slit is exactly along the dotted line. The book is no 

 slowly drawn along in the direction of the arrow, tl 

 piece of paper being held in the same position. At fir 

 the lower extremity of the curved line on Am seenthroug 

 the slit ; but as the book is drawn along, the portions 1 

 the right, and those to the left come successively in vie\\ 

 the small white dot which is the only visible portion 

 the curved line appears as a point which moves first 

 the right and then to the left, and imitates closely tl 

 motion of a vibrating particle of air, the rate of moti( 

 being, however, much slower. If now the slit 1 

 placed over the dotted line in (7, fig. 210, and the bo< 

 drawn along underneath it in the direction of the arro , 

 a representation is obtained of the motion of a seri; 

 of particles of air which are acted on by a number ; 

 successive equal undulations or waves. Each partic 

 merely moves a little right and left, and always com 

 back again to its starting-point ; but the condensatic * 

 and rarefactions, represented by the lines being :- 

 spectively closer together or farther apart, are gradua, 

 transmitted through the whole series of air-partic> 

 from one end to the other. 



The propagation of sound through the air is exce< 

 ingly rapid. If sound is produced at a short distaii 

 no time apparently elapses between the production 

 the sound and its perception by the ear. At a grea 

 distance, and when the production of sound can . 

 ascertained by the eye, we perceive easily that ti 

 elapses before the sound reaches our ear. The flash 

 a pistol-shot, the column of steam issuing from ' 

 whistle of a locomotive, is seen perceptibly earlier tl i 



