496 S. P. Langley—The Solar Atmosphere. 
If it be true then that the sun is surrounded by an atmos- 
phere whose principal action in obscuring the heat radiation is 
due to a thin stratum which cuts off one-half of the heat 
which should reach us, and in whose absence this radiation 
should be doubled,—an atmosphere not independent of the in- 
terior of its globe in such a degree as our own, but one to and — 
from which matter is constantly being added and withdrawn, 
it follows that any change in the ratio of supply and with- 
rawal, or other cause, which should increase its absorption by 
so much as 25 per cent would diminish the mean surface tem- 
perature of our globe by 100° F., whilst a like diminution in 
the envelope would produce a corresponding change in the op- 
posite direction. 
I am unable to see that, if the Allegheny observations are 
correct, whence I have derived the above given approximate 
value of the absorption, any other result would follow, and in 
any case the existence of life in its present forms on this planet 
seems dependent, within certain limits, on the depth and absorp- 
tive power of the solar atmosphere. The reader who may not 
have closely observed how far the validity of this result 1s 
dependent on, and how far independent of, the hypothesis just 
made as to the mean terrestrial temperature, is invited to 
pose were the temperatures of the glacial epochs. : 
Father Secchi seems to think it probable that the difference 
