S. P. Langley — Temperature of the Moon. 435 



the glass, as rays of this wave-length from a source at a tempera- 

 ture below that of boiling water are known to be. This test of 

 the glass, employed by Lord Rosse in the direct lunar beam, is 

 here, it will be observed, applied at different parts of the lunar 

 heat-spectrum and its result in the latter case, corroborating that 

 already obtained by the respective wave-lengths of the maxima, 

 brings evidence of a radiation of heat from the lunar soil at a 

 temperature at any rate below that of boiling water. 



Other observations furnish the means of computing the 

 relative absorption of the earth's atmosphere as exhibited in 

 extended cold bands in the region of the lower maximum. 

 Then, from a combination of the two, we are enabled to reach 

 a certain approximation to the position and magnitude of this 

 maximum as it would appear if the atmosphere had not inter- 

 vened. The existence of some of the principal atmospheric 

 cold bands in this region, due to the absorption exercised by 

 an atmospheric column 100 meters in length, has been quite 

 independently determined by means of the great radiator 

 already referred to. During moist summer weather two prin- 

 cipal maxima have been found in its spectrum, the larger at 

 deviation 37° 15', nearly agreeing with the lunar curve in 

 summer, a second smaller maximum at deviation 38° 45 r , and 

 between them a cold band with its minimum at deviation 

 38° 20'. Remembering that the unabsorbed spectrum from a 

 radiating surface of lamp-black, at the temperature of boiling 

 water, has its maximum at 38° 25', or very near the deepest 

 depression of the cold band, it will be recognized that we have 

 evidence of a considerable absorption at this point. 



To this must be added the fact, shown by our observations, 

 that in the case of solids the greater part of the whole heat is 

 always found oelow the maximum of the (unabsorbed) pris- 

 matic curve. If this law hold in the case of the sun, since 

 little heat is found below its actual prismatic maximum (near 

 deviation 39° 40'), the inference is that absorption in that 

 region (i. e. the extreme infra-red), must have been great. 



Arguments on these different lines, combined with another 

 derived from a direct comparison of sun and electric arc radia- 

 tion, which will be described farther on, enable us to present a 

 curve (Plate X) showing with the degree of approximation 

 compatible to the first attempt in such a field, the atmospheric 

 absorption in all parts of the spectrum. 



The final result of the measures, extending over the three 

 years from 1884 to 1887, is given in Plate XI, in which ab- 

 scissae correspond to deviations of a rock salt prism of 60°, 

 vertical ordinates to directly observed heat from radiations, 

 while the dotted curve indicates what seems to be the most 

 probable position and amount of the lower maximum, as it 

 would be observed were there no intervening atmosphere. 



