CH. xx. NEWTON TRANSMISSION OF SOUND. 169 



obstacle by which the movement is driven back, and then 

 the sound is reflected. Newton not only pointed out Jww 

 sound travels, he also attempted to ascertain at what rate 

 it travels, and he showed that the more elastic and less 

 dense air is, the more rapidly it will convey sound. A very 

 elastic substance, in the proper sense of the word, is one 

 which will spring back to its original shape whenever it is 

 set free, just as a steel spring does. Now in the case of 

 the train, if it happen to be composed of a set of heavy 

 trucks with no springs between them, they will move slug- 

 gishly, and the waves of compression and rarefaction will 

 take place but slowly. But if the train be composed of 

 light carriages with springs at the end of the buffers, then 

 each carriage will rebound much more quickly, both be- 

 cause it is lighter, and because the elasticity of the springs 

 will help the rebound. Now when air is heated, if it can- 

 not expand it becomes more elastic, and if it can expand 

 it becomes lighter than when it is colder, and therefore in 

 either case sound will travel more quickly along warm air 

 than along cold. 



Newton tried to calculate the rate at which sound travels, 

 but he did not know that the mere fact of the sound- 

 wave passing along makes the air hotter and therefore more 

 elastic, so his results were not strictly accurate. We know 

 now that in air which has a temperature of 32 Fahr., 

 sound travels at the rate of 1090 feet per second, while in 

 air of the ordinary temperature of 52 Fahr., it travels at the 

 rate of 1 1 1 2 feet per second. 



Newton's account of his experiments on sound are 

 to be found in the second Book of his ' Principia.' After 

 this great work was published, in 1687, he next turned 

 his attention to chemistry, but unfortunately all the re- 

 sults of his labour in this science were destroyed by an 



