Temperature of the Moon. 53 



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 absorption takes place in the first few metres of air ; 

 while we conclude 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 

 absorption by the earth's atmosphere. Although a large ab- 

 sorption 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 por- 

 tions of their spectra are here presented for the first time. 



Principal Conclusion. 



Of the numerous conclusions to be drawn from this research, 

 we here only direct the reader's attention to what we consider 

 the most important one, namely : — That the mean temperature 

 of the sunlit lunar soil is much lower than has been supposed, 

 and is most probably not greatly above zero Centigrade. 



Post Scriptum. 



I would ask to be allowed here to state that the very con- 

 siderable expense for the special means and reduction of the 



