S. -P. Langley — Temperature of the Moon. 439 



The result of the comparison of the (of course unabsorbed) 

 electric arc with the radiation of the sun after absorption, as 

 shown in the sixth column, is that this solar radiation in the 

 orange and red is nearly ten times that of the arc, while to- 

 wards the violet end of the spectrum, the relative superiority 

 of the absorbed solar heat diminishes, evidently because of the 

 progressive increase of the atmospheric absorption in that direc- 

 tion, which lessens the solar intensity without sensibly affect- 

 ing that of the arc. The solar efficiency continues greater 

 through all the infrared spectrum known until very lately, 

 while in the extreme portions recently investigated, it falls be- 

 low that of the arc. This is partly due to the fact that radia- 

 tion from a source at a lower temperature (the arc in this case) 

 is relatively more powerful in the longer than in the shorter 

 waves ; yet it can hardly be doubted that here also (that is, in 

 the extreme infra red) a very large atmospheric absorption has 

 taken place. 



There is reason to believe that a considerable part of this ab- 

 sorption takes place in the first few meters of air, while we con- 

 clude, from all the evidence in our possession, that the real 

 telluric absorption, being a locally selective one, is much greater 

 than the comparison of high and low altitude observations 

 alone would indicate. 



The import of this comparison will be still more evident 

 from a consideration of the seventh column, where by means 

 of the Allegheny tables of the solar absorption, we have 

 calculated the ratio of the arc heat to that of the sun before ab- 

 sorption by the earth's atmosphere. Although a large absorp- 

 tion by the solar atmosphere has already taken place, we see 

 that in the ultra-violet the solar radiation is from 20 to 30 

 times that of the arc, while that of the absorbed sun is only 

 about 6 or 7 times. When we reach the region of the red and 

 upper infra-red, we see that these ratios are nearly the same in 

 the absorbed and unabsorbed solar radiation, showing that the 

 terrestrial absorption in this region (which was once supposed 

 to be its principal seat), is in fact very small, while in the 

 regions of the extreme infra-red corresponding to temperatures 

 not greatly exceeding that of the terrestrial soil (regions only 

 revealed by quite recent investigation) the telluric absorption 

 again becomes considerable. 



The general result of this comparison is to enhance our ideas 

 as to the rate of solar radiation, and as to the solar temperature. 

 Comparisons of the total solar radiation with the total arc 

 radiation have (it may be observed) been made before, but so 

 far as I am aware, comparisons of the heat in different portions 

 of their spectra are here presented for the first time. 



