WEATHER PROVERBS 433 



wave to the corresponding point of its nearest neighbor, as, for instance, 

 from crest to crest. 



Of all colors violet light has the shortest wave-length, and red the 

 longest. Blue is next to violet, yellow next to red, and green about an 

 average of all. The wave-length of red light is less than twice that of 

 the violet, and yet it would take more than 30,000 of the longest waves 

 to which the eye is sensitive to span a single inch. 



Turning, now, our attention to the atmosphere, we find that at 

 nearly all times, and everywhere within two miles of the surface, and 

 probably much higher still, it contains, in every cubic inch, thousands 

 of dust particles coming from fires, from plants, from the dry earth as 

 caught up by winds, and from still other sources. Much of this dust is 

 excessively fine and settles down with extreme slowness. It serves, as 

 already explained, as nuclei about which the mjrriads of cloud droplets 

 are formed. 



In addition to this important function, extremely fine particles of 

 dust, and even single molecules, but not the coarser portions, as shown 

 many years ago by Lord Eayleigh, both scatter and absorb light of all 

 colors according to the laws: (1) that the amount both of absorption 

 and of scattering decreases in the same proportion that the fourth 

 power of the wave-length increases; (2) that both increase with the 

 number of particles per unit volume, and with the average square of the 

 volume of the individual particle. 



The refractive index of the air and of the foreign substances it con- 

 tains, together with certain numerical terms, also enter into the com- 

 plicated equations that deal quantitatively with atmospheric absorption 

 and scattering of light. These latter facts, since they are not essential 

 to what follows, are mentioned here only for the sake of completeness. 



Now scattering and absorption, acting according to the above laws, 

 combine to give us the colors of the sky, because sky light is only the 

 residual, after absorption, of that portion of sunlight which was scat- 

 tered by the molecules of the atmosphere and by the foreign substances 

 floating in it. 



Since, according to the first law, but little light of very long wave- 

 length is scattered while nearly all of exceedingly short wave-length is 

 absorbed, it follows that the light of maximum intensity, or the prevail- 

 ing color, must have some intermediate wave-length. Hence the sky 

 overhead is neither red (long wave-length) nor violet (short wave- 

 length). Also, from the second law, we see that different parts of the 

 sky at the same time, and the same parts of the sky at different times, 

 will have different colors owing to the amount, aggregation and distri- 

 bution of atmospheric dust. 



When these particles are relatively few and small the prevailing 

 color is blue. On the other hand, where the dust motes increase in size 



VOL. liXxvni. — 30. 



