ON SHIP WAVES. 451 



at least a million times as far from us as the sun is. 

 Think of ninety-three million million miles, and 

 think of waves of light coming from stars known 

 to be at as great a distance as that ! So much for 

 the distance of propagation or progression of waves 

 of light. But there are two other magnitudes con- 

 cerned in waves : there is the wave-length and 

 there is the amount of displacement of a moving 

 particle in the wave. Waves of light consist of 

 vibrations to and fro, perpendicular to the line of 

 progression of the wave. The length of the wave 

 I shall explain the meaning of "wave-length" 

 presently : it speaks for itself in fact, if we look at 

 waves of water the length from crest to crest in 

 waves of light is from one thirty-thousandth to one 

 fifty-thousandth or one sixty-thousandth of an inch ; 

 and these waves of light travel through all known 

 space. Waves of sound differ from waves of light 

 in the vibration of the moving particles being 

 along the line of propagation of the wave, instead 

 of perpendicular to it. Waves of water agree more 

 nearly with waves of light than do waves of sound ; 

 but waves of water have this great distinction from 



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