134 LIGHT SCIENCE FOR LEISURE HOURS. 



being the height of a goodly prominence, the latter too 

 small a height to be measurable at the sun's distance. 

 The height of the sierra in 1871 averaged some 6,000 

 miles. But it had been noticed that this envelope had 

 been shrinking as the sun spots diminished in number. 

 The eclipse observations confirm those made by the 

 method devised in 1868 for studying the prominences 

 when the sun is not eclipsed. We thus learn definitely 

 that this envelope, consisting of a mixture of hydrogen, 

 helium (the unknown solar element), sodium, and 

 occasionally magnesium, calcium, and other elements, 

 shrinks during the time when the sun is free from 

 spots to about a third of the height which it attains 

 when the sun shows many spots and is, we may infer, 

 most actively disturbed. As the height is in this case 

 a fair measure of the volume, I may say that the 

 sierra has lost two-thirds of the volume it possessed in 

 1871 ; and again, as that volume exceeded that of the 

 earth more than 13,000 times, it follows that the loss 

 of volume experienced by the sierra since 1871 exceeds 

 some 10,000 times the entire volume of this earth on 

 which we live. It is clear that this implies a very 

 remarkable change in the condition of the sun himself, 

 though fortunately for us the change is not one which 

 affects in any striking or obvious manner his emission 

 of light and heat. 



With regard to the prominences, the evidence 

 given by the eclipse accords (as I expected it would) 

 with that obtained by spectroscopists. They have 

 observed that the prominences had become less and 



