HOW LIGHT EXERTS PRESSURE 25 



velocity twice as great as that of a particle in the 

 upper train. But the energy of motion is propor- 

 tional to the square of the velocity. Then the 

 energy in the lower train is four times as great as 

 that in the upper train. Or, again, for waves of 

 the same height the energy in given length is in 

 inverse proportion to the square of the wave-length. 

 Since, then, both kinds of energy follow the same 

 law, the total energy is in inverse proportion to the 

 square of the wave-length, when the height and 

 depth remain the same. 



Now let us see how waves may be crushed up 

 or be opened out. When a source is giving out 

 waves and is at the same time moving, the length 

 of the waves is altered, shortened if it is following 

 them up, lengthened if it is moving backwards 

 from them. This effect was first pointed out by 

 Doppler in 1842. It is easily observed in the 

 sound emitted by a locomotive, whistling as it 

 passes you. The note heard is higher when it is 

 coming towards you than when it has passed and 

 is moving away from you. The effect is notice- 

 able even in the hoot of a motor. The reason is 

 easily seen from fig. 9. Let the upper part of the 

 figure represent a stationary engine sending out 

 waves of sound forward and backward from C. 

 Let us consider four waves in each direction. 

 These will be of the same length and will travel 

 at the same speed, so that an observer at A will 



