218 ELEMENTARY SCIENCE 



and watched the ripples as they circle outward, getting 

 fainter and fainter until they are imperceptible? Here is 

 another case of the vibrations by which radiant energy is 

 transmitted, and, since it is a case in which waves may 

 actually be seen, it may help us understand the cases in 

 which the waves cannot be seen. We see waves only on 

 the surface of water, but invisible ones also travel down- 

 ward through it. 



We must not think of a wave as the motion of a mass; it 

 is rather the motion of a motion as it travels through a mass, 

 whether this mass be air, or water, or a sheet that you 

 wave in your arms. So a wave on water is not a mass of 

 water which moves forward; it is rather the effect of a 

 vibration of water which produces undulations as it pro- 

 gresses. 



Have you ever had your head under water when some 

 one struck stones together near you ? Then you know that 

 sound-waves travelling through water produce on us an 

 altogether different effect from that produced when they 

 travel through air. 



Now you can better appreciate the kinship between light 

 and heat. For light and heat (and sound, too) are all 

 simply vibrations of wave-like nature, which, considered 

 from the standpoint of physics (that is, apart from the 

 effects they produce on living things), are entirely of the 

 same nature except for the difference in their wave-lengths. 

 So you see that all forms of radiant energy are alike as 

 to their essential nature; they differ only with respect 

 to wave-length. They also differ with respect to the speed 

 at which they travel and with respect to the effects they 

 produce, but both of these things appear to be merely 

 results of differences in wave-length. 



