14 MEMOIliS OF THE NATIONAL ACADEMY OF SCIENCES. 



Tlie amount of lieat received from the moon, and tlie dependent question as to the temi>era- 

 ture of tlie lunar surface, are subjects of greater interest to us than might at first appear. They 

 aie even ones in which we may be said to have a material concern, for until we know the tempera- 

 ture which an airless jdanet* would attain in the sun's rays, we can have no accurate knowledge 

 of the extent to wh ch the atmosphere of our own planet contributes to its heat, nor of some of 

 the most important conditions of our own existence. Those conditions are only lately becoming 

 known, for it has hitiierto been supi)Osed that the temperature of the earth's surface was chietly 

 due directly to the radiation which it receives from the sun. It has l)een admitted, indeed, that the 

 air acts to some extent in increasing the heat by hinderin^the radiation from the soil, bnt the man- 

 ner and extent of this action have scai cely been, as it now appears, even surmised. Thus Sir John 

 Herschel, distinguished as he is as a meteorologist as well as an obseiver, transferring to the moon 

 conceptions drawn from the supposed state of things here, states that the temperatuie of the moon's 

 surface in the lunar day must rise to L'OOo or ;50(to Fahr., and sink nearly as far below zero during 

 the long lunar night, and the idea that the airless snrfaceof the full moon must be intensely hot (in 

 comi)arison witli ordinary terrestrial temperatures) has since been generally accepted, Almost the 

 oidy dissenting voice lias been that of Air. John Ericsson, who asserts that the lunar surface must, 

 on the contrary, be intensely cold. In the ])resent writer's opinion, the temperature supposed h\ 

 Herschel, if it exist on the moon, will imply the presence of an atmosphei'e there : and if we can set 

 aside the weight of traditional belief and preconceived imiucssion the snpi)osition that the surface 

 of the moon (if absolutely airless) must be cold, even in lull sunshine, is one which, however ])ara- 

 doxical it may a])pcar, we are led to entertain by evidence at the C(Uiimand of everybody who can 

 ascend high in our atmosphere. As we go up a mountain, we do not find the soil growing hotter, 

 but colder, in the sunshine; and at great elevations, where the barometer is low, and we are partly 

 approaching the conditions which must pievail on the moon (if airless), we find the surface covered 

 with peri)etnal snow, even under the intenser solar blaze. The direct rays, indeed, are hotter, but 

 the radiation from the soil is so far greater than below that, on the whole, with every upward 

 step that diminishes the protection of our atmosi)heric envelope, the surface tencis to grow 

 colder. This is a matter of the most frequent observation. It is confirmed by the analogous 

 experience of aeronauts, and it bears to my mind but one interpretation, that if we ascended 

 still higher, until the air had been left altogether behind, we should find there regions of still 

 intenser cold than any which we have experienced at the highest altitudes attainable by man. It 

 has indeed been urged that the cold of high altitudes is largely due to the expansion of ascending 

 air currents and to analogous causes, but our conclusions also rest on means whicli are indepeiulent 

 of this hypothesis. In ISSl the expedition under the writer's charge to Mount Whitney, in the 

 Sierra Nevadas, made many hundred actiiiometric observations at altitudes of from 3,000 to l.">,000 

 feet upon the direct solar radiation, and its power to heat a thermometei- bulb virtually renioveil 

 from every disturbing influence, so that it was possible to estimate the result which would follow 

 if the air between it and the sun were wholly withdrawn : the conclusion being that the tempera- 

 ture of the surface of the earth in full per|)etual sunshine would, in the entire absence of its atmos- 

 phere, not rise much more than 4-S^ C. over that of surrouiuling space. Mow. the "temperature of 

 space" must be conceded to be a vague and unsatisfactory term. To gi\e a meaning to the 

 expression we must ask what final temperature the earth's surface would attain were the sun's 

 radiation and its own internal heat wholly cut oft', and it were warmed oidy by radiations from 

 other heavenly bodies, visible or invisible, or by the dynamic effects of meteorites, &c. I'ouillet's 

 conclusions are well known. The writer has reached much lower values, which he will not under- 

 take to here state or exi>lain. l''or the present [mrpose it is sufHc-ient to say that, in his belief, the 

 surface temiierature of our planet, so far as it is due to direct solar radiation, would jirobably be 

 such, that every licpiid we know, and perhaps every gas, would exist only as a solid, though beneath 

 the vertical rays of the sun. 



It is hence almost wholly to our atnir»pliere and its capacity (by selective absoiiition) of stor- 

 ing the solar heat that, in the writer's view, we owe the high temperatuie which makes our exist- 



" CVrtaiii ili.S(ie]>iiiicifs lietweeii oliseiviilimis on tcnesli.Tl :iinl lunar radiiitiiin snggest to ns, liowever, tin- )iossi- 

 bility of the fxistencf of a iiiinnte Innar atirjosplieie, too small lor ift'ognition liy tin- t<-lesro])f. 



