314 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH, 



of the air and sky, which would otherwise be dark except in the direc- 

 tion of the sun, moon, and stars. The beauty of land and sea and of 

 atmospheric effects would be vastly reduced if the reflecting particles 

 were absent, and houses not facing the direct sunskiue would be incon- 

 veniently dark. Ozone and oxygen molecules, in some state probably 

 of aggregation, are concerned in the reflection of blue rays, so that an 

 elimination of the coarser dust would not entirely darken the atmos- 

 phere. A complete removal of reflected rays would slightly diminish 

 the terrestrial warmth derived from the incidence of light rays from 

 the general atmosphere, and slightly increase that derived from the 

 direct rays of the sun. Invisible, or barely visible, vapor particles are 

 probably still more efficacious in producing similar effects. 



SUNLIGHT AND THE EARTH'S ATMOSPHERE — ABSORPTION AND 



REFLECTION. 



The light of the sun which reaches the earth has passed through two 

 atmospheres, one of the sun and one of the earth, and each of these 

 atmospheres robs the light emitted from the sun's body of some of its 

 brilliancy and an unequal proportion of color, so that the original color 

 of the sun is modified by the successive subtractions from parts of the 

 spectrum before it reaches our eyes. The sun's atmosphere arrests 

 more blue rays than red, and the light from the middle of the sun's 

 disk is more blue than that which reaches us from the limbs, for it has 

 to traverse less of the solar atmosphere. Prof. S. P. Langley has 

 shown that the effect of the invisible solar atmosphere is so important 

 that its diminution by a third part would cause the temperature of the 

 British Isles to rise above that of the torrid zone. The earth's atmos- 

 phere, also, has the effect of scattering many rays, and principally 

 those waves which form the most refrangible end of the visible spec- 

 trum and gives the impression of blue. By the use of an exceedingly 

 delicate instrument, at a height of 15,000 feet, Professor Langley was 

 able to show that at this elevation, where nearly one-half of the absorb- 

 ing mass of the air was got rid of, the ray 60, near D, had grown in 

 brightness in the proportion 2 to 3, that the blue end of the spectrum 

 had grown in intensity out of all proportion to the rest, and that a very 

 great length of invisible spectrum became recognizable beyond the 

 visible rays below the red. The amount of energy in this invisible 

 extension is much less than that of the much shorter visible end. The 

 conclusions to which Professor Langley arrived as the result of his 

 investigations on the solar light was that the sun is blue, that the solar 

 heat is greater than was supposed, and that the total loss by absorption 

 in the atmosphere is nearly double what had been estimated. The sun 

 he calculates to be competent to melt a shell of ice 60 yards thick over 

 the whole earth annually, or to exert 1 horsepower for each square 

 yard of the normally exposed surface. The existence of life on the 

 planet, and especially of the human race, must clearly be dependent 



