PHYSICAL GEOGRAPHY AND CLIMATE OF THE COAL PERIOD. 381 



i. e., the internal heat for every 50 feet of depth increases twenty times 

 the surface-temperature, so far as this is due to internal causes. Now, 

 it has been shown by Fourier and Hopkins that the same would be 

 true whatever be the surface-temperature from internal causes. For 

 example, if the surface-temperature from internal causes be 1°, then 

 for every 50 feet of depth the' interior heat would increase 20°. If the 

 surface-temperature from internal causes be 10°, then for every 50 feet 

 of depth the interior heat would increase 200° — a condition of things 

 entirely inconsistent with the growth of plants, since all the springs 

 would be boiling. We can not, therefore, attribute, as many have 

 done, even a few degrees' increase of mean temperature to causes in- 

 terior to the earth. In fact, it seems almost certain that during the 

 whole recorded history of the earth, i. e., during the time it has been 

 inhabited by organisms, the surface-temperature of the earth has 

 been almost wholly due to external causes. Now, the composition 

 of the atmosphere is an external cause, which greatly affects the sur- 

 face-temperature, but which has hitherto been almost wholly neg- 

 lected. The thorough explanation of this point will require some dis- 

 cussion of the properties of transparent media in relation to light and 

 heat. 



Many bodies which are transparent to light are opaque to heat. 

 Such bodies, however, will freely transmit heat, if the heat be accom- 

 panied with intense light. It is as if the light carried the heat through 

 with it. Heat thus associated with light is sometimes called light-heat, 

 while that which is not thus associated is called dark heat. Now, the 

 bodies spoken of are transparent to light-heat, but opaque to dark heat. 

 Glass is such a body. If a pane of glass be held between the face and 

 the sun, the heat passes freely and burns the face, but the same pane 

 would act as a partial screen before a fire, and as a perfect screen be- 

 fore a hot, but not incandescent, cannon-hall. 



It is in this way we explain the fact that a glass greenhouse, even 

 in the coldest sunshiny winter's day, becomes insupportably warm if 

 shut up. The sun-light and heat pass freely through the glass, and 

 heat the ground, the benches, the flower-pots; but the light-heat there- 

 by becomes converted into dark heat, and thus is imprisoned within.* 

 Now, the earth and its atmosphere are such a greenhouse. The light- 

 heat passes readily through, warms the ground, changes into dark heat, 

 and is in a measure imprisoned by the partial opacity of the atmosphere 

 to this kind of heat. The atmosphere is a kind of blanket put about 

 the earth to keep it warm. So much has long been recognized. But 



* On Mount Whitney, in the sunshine, Langley got, in a box covered with glass, a 

 temperature af 236° Fahr. or 113'3° C, while in the shade of the open air the temperature 

 was only 58*6° F. or 14-8° C. 



