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



that it has not been given to a question of merely abstract in- 

 terest, but that the whole subjects of terrestrial radiation and 

 the conditions of organic life upon our planet are intimately 

 related to the present research. 



Spectroscopy has hitherto dealt almost exclusively with light, 

 but in this new held we consider chiefly that great invisible 

 spectral region in which the entire radiation of the soil of our 

 own planet is to be found, a region of which we have until 

 quite recently known nothing. To see how the question of 

 the lunar heat affects our knowledge on the whole subject of 

 our planet's temperature, we must remember that until a few 

 years past, it had been assumed by all writers of repute that 

 the earth's atmosphere acted exactly like the glass cover of a 

 hot-bed, and kept the planet warm in exactly the same way 

 that the hot-bed is warmed, by admitting the light-heat of the 

 sun, which was returned by the soil in the invisible radiation 

 of greater wave-length to which the atmosphere was supposed 

 to be impervious, and that thus the heat was stored, glass being 

 till lately supposed to be practically athermanous to all infra- 

 red heat. It was a necessary part of this assumption that all 

 or very nearly all the infra-red was absorbed by our atmos- 

 phere, but in 1881, the observations of the Mount Whitney 

 expedition, supplementing previous ones made at this observa- 

 tory, showed that through the infra-red, as far as it had then 

 been explored, the atmosphere transmitted the invisible rays 

 with greater facility even than the luminous heat, so that the 

 ordinarily received idea must be essentially modified, and, if 

 the absorption of the telluric radiation did indeed take place as 

 supposed, it must be in spectral regions then entirely unknown. 

 It is in an examination of these, till now, quite unknown 

 regions beyond the extreme boundaries of former researches 

 on the infra-red, and in the study of the radiations of corre- 

 sponding wave-length emitted from the lunar soil, that we find 

 the principal subject matter of the present memoir.* 



It is, in introduction, again pointed out that the absorption 

 of the earth's atmosphere for these radiations, as for all others, 

 is not simple, but eminently complex, and that the old formulae 

 lead to gross errors in practice. Further, as the amount of 

 radiation of a planet is like that of any other body, dependent 

 on that of its surroundings, reasons are repeated for believing 



* The reader is reminded that the words "infra-red" have obtained an exten- 

 sion of meaning since we have been able to show in previous memoirs, not only 

 the vast amount of the energy in this region (which, in the case of the sun, is 

 over 100 times that in the ultra-violet,) but that in this invisible infra-red there 

 is every variety of condition, greater differences than there are e. g. between 

 violet aud orange light, and that Melloni's anticipatory comparison of varieties of 

 radiant heat to varieties of color actually understates the truth. 



