ON LIGHT. . 309 



water, the " wave-length," or " length of an entire undu- 

 lation," is the linear distance between two consecutive 

 crests or two consecutive troughs. This is its sim- 

 plest conception, and it will suffice for our immediate 

 purpose. The waves being equal and similar will all run 

 on with the same velocity, which may be ascertained by 

 noticing how long any one takes to run over a measured 

 distance on the surface, or the distance run over in a 

 determinable time, suppose a second. And if at the 

 same time we note the number of waves whose crests 

 pass a fixed point (a float, for instance) in the surface, in 

 a second of time the interval between two consecutive 

 crests will of course become known. And vice versa, if 

 this interval be known, and the velocity of the waves ; 

 the number of undulations passing the float per second 

 is easily calculated. Now this number is necessarily 

 identical with that of the periodically reciprocating move- 

 ments or vibrations of the first mover (whatever it be) by 

 which the waves are originally excited. This continu- 

 ing the same, the same number of waves will pass the 

 float in the same time, whatever be their velocity of pro- 

 pagation. Of these three things the velocity of propa- 

 gation, the number of alternating movements, waves, or 

 pulses per second, and the linear interval between two 

 consecutive ones any two being given, the third is 

 easily calculated. For example, a string sounding a 

 certain note C in the musical scale makes 256 complete 

 oscillations to and fro, per second. As each of these 

 sends forward an air-wave consisting of a semi-wave of 

 compression by which the particles of air advance, and 



