CH. xx. NEWTON TRANSMISSION OF SOUND. 167 



a vibration of the air which we feel when it acts upon the 

 drum of our ear ; but none of these men had shown what 

 kind of vibration this is. Newton was the first to suggest 

 that it is caused by the particles of air moving backwards 

 and forwards in the direction in which the sound travels ; 

 the first particle being set in motion by some impulse given 

 to it, and thus striking the second ; the second striking the 

 third ; and so on until the particle nearest the drum of our 

 ear hits it, causing it also to oscillate backwards and for- 

 wards, and producing in us the sensation of sound. 



Many persons find it difficult to understand how waves 

 of sound can travel for long distances, while the air-particles 

 forming the waves ordy move to and fro within very small 

 spaces. One of the best ways to realise how this can be 

 is to watch a railway train which has been bumped by a 

 luggage van being joined on to it The first push to the 

 stationary train will have been given by the van, which 

 bumping against the first carriage, will cause it to bump 

 against the second ; this in its turn will bump against the 

 third, and the movement will thus pass on to the end of 

 the line, so that if you were standing there the last carriage 

 would knock you down. Yet each carriage will at the 

 most have only stirred a foot or two from its place, and in 

 the end they may all settle down nearly in their original 

 positions. If you are standing by the side of the train 

 which is being shunted, and watch it carefully, the reason 

 of this will be clear. You will see that though the first car- 

 riage drives on the second, and the second the third, yet 

 be tote many carriages have been so driven, the first carriage 

 will have rebounded, and be moving away from the second, 

 which, in its turn, will move away from the third ; so that 

 while the fourth, fifth, and sixth carriages are sending on 

 the bump and are huddled together, i, 2, and 3 are moving 



