20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



together with bolometric measurements of the total hinar radiation and 

 its transmission, which we here briefly summarize. 



" Experiment showed that the moon sends us a Httle more than 

 I /i 00000 part of the heat which we receive from the sun. Of this 

 lunar radiation we found at the beginning of December only 14 per- 

 cent transmitted by a specimen of glass which allowed over 75 percent 

 of the solar rays to pass. An ebonite disk, which was almost com- 

 pletely opaque to light, transmitted 32 i^ercent of the solar and only 

 7 percent of the lunar radiation. Very little difference was found in 

 the apparent transmission of the solar and of the lunar beam by the 

 earth's atmosphere as inferred from comparisons at high and low 

 altitudes above the horizon. 



" Photometric spectral comparisons showed that sunlight is much 

 richer in the violet rays than moonlight, indicating a selective reflec- 

 tion by the lunar surface, which, however, becomes less marked as 

 the red end of the spectrum is approached. 



" Comparisons, made in the month of December, 1884, between the 

 total radiation of the moon and that from a blackened vessel of hot 

 water, subtending the same angle, showed that the heating effect of 

 the moon (as received through our absorbing atmosphere) could be 

 replaced by the (unabsorbed) heat of a lamp-blacked surface at about 

 -|-8o°C., or 353°C. above absolute zero. A part of the lunar radia- 

 tion is reflected from the sun and a part never reaches us, being 

 absorbed by the atmosphere. Due allowance for the former would 

 diminish, and for the latter would increase, the indicated lunar tem- 

 perature ; but owing to the selective character of the reflection to 

 which we have already alluded, to our ignorance of the moon's emis- 

 sive power, and to the fact that the radiations of our atmosphere itself 

 are of a wave-length similar to a considerable part of those we now 

 study, no precise deduction can be made 



" We have in the last three years pursued these researches with 

 constantly improving instrumental means, and the following pages are 

 a description of them and of the results. It will be seen that the great 

 labor bestowed on them has been given, not to determine a point of 

 abstract or merely theoretical interest, but that it is justified by the 

 fact that the whole subject of terrestrial radiation, the temperature of 

 the surface of our planet, and the conditions of organic life upon it 

 are intimately related to that of our present research. The entire 

 radiation of the soil of our earth towards space goes on in a spectral 

 region of which we have hitherto known nothing. These observations, 

 in connection with those recently published on invisible spectra and 



