24 MEMOIPtS OF THE Js^ATIOXAL ACADEMY OF SCIEXCES. 



fleets or diftuses, part of tbe solar energy must be absorbed and re-radiated as dark heat. We 

 make no doubt, then, that the lunar soil radiates heat tovrard space. The real questions at 

 issue are "At what temperature does it so radiate?" "Can we have any experimental knowl- 

 edge of such dark beat radiation at the earth's surfacel" If we suppose, for instance, the lunar 

 soil to be heated by the sim 50° C. above the temperature of surrounding space, then in the case 

 of this very considerable supposed heating effect, the moon's surface will remain far below zero 

 in the sunshine, and though it may be said in one sense to radiate beat to the earth, yet since it 

 is in this case below the mean temperature of the earth's surface, we should obtain no sensible heat 

 from it, even were our atmosphere altogether absent, while the actual presence of our atmosphere, 

 athermanous, as it is generally believed to be to such radiations, wonld render their determination 

 hopeless. Whether tbe moon be a perfectly diffusive body or the actually imperfectly diffusive 

 one, we get the same amount of heat from it; for it will finally attain a condition of beat equilibriam 

 in which it will send away as much as it receives. In the first hypothesis, what it sends away will 

 be purely reflected or diffused energy, of wave-length corresponding to what it has received from 

 the sun; in the second hypothesis, the radiant energy will be partly reflected, and partly that of 

 much lower wave length emitted by the soil. Tbe second hypothesis, doubtless is tbe true one; 

 but the question before us is, "Is this I'eradiated heat sensible ?" 



From the fact that the lunar energy api)ears less traiismissible by glass than tbe solar it has 

 been assumed that the entire effect is due necessarily to a large beat radiation from the lunar soil, 

 which our atmosi)bere transmits and the glass stops. Before we accept this hypothesis we must 

 repeat that it does not necessarily imi)ly this, for we have only to suppose the selective reflection 

 exercised on the solar rays at the surface of the moon to be such as to send us in the reflected rays 

 an undue proportion of those which glass absorbs, to account, at least in part, for the observed 

 effect. We will pass, therefore, to a series of observations which show more clearly than any jet 

 given that a selective absorption of such a character does actually take place. 



I'BOTOMETIilC OBSERVATIONS IN 18S4. 



Comparative photometric measures of tbe intensities of solar and lunar rays are of im- 

 portance, as we ha\'e seen, to our heat determinations, and especially is this the case when 

 such measures are combined with others (to be shortly given) of tbe comparative amounts of heat 

 received from tbe sun and moon. The complete knowledge desirable would tell us of the special 

 ratio of each separate heat or light ray, but even a knowledge of the ratio of tbe total sun- 

 light to moonlight and tbe total sunheat to moonheat will be valuable. If, for instance, it 

 were found by purely optical means that the intensity of sunlight was m times that of moon- 

 light, and by an instrument like tbe thermoi)ile or bolometer, in which tbe registered effect of 

 the radiation is propoi tional to the amount of energy which resides in it, that the beat received 

 from the sun was only n times that from the moon, even such a result would enable us to draw 

 some inference as to the general character of the lunar energy, and hence of the conditions of tem- 

 perature of the moon's surface. For, in the case above stated (supposing »(>)(), tbe given relation 

 between the ligbt and heat ratios could be explained only on the supposition that the energy was 

 distributed differently in the two spectra, a larger portion of that residing in the lunar rays being 

 unable to produce any physiological effect when received upon the retina or iiicai)able of being in- 

 terpreted as light, and hence that tbe surface of the moon had either selectively reflected tlie solar 

 i-ays or had added to them radiations from its own substance indicative of a considerable individual 

 temperature. We have seen, however, that a difference in the direction of the above supposition 

 is to be expected from the effects of selective reflection at the nioon's surface. 



The chief objection to such a comparison between the light and heat ratios of the sun and 

 moon is tbe difficulty of nniking the necessary measurements witb tlie requisite degree of accu- . 

 racy; so that, unless the difference were extreme, it would be masked by the effects of the errors 

 of observation. The photometric comparisons are generally made with the aid of an artificial 

 source of light of intermediate brightness, which at once introduces a considerable degree of uncer- 

 tainty into the problem on account of its variations in intensity. Differences in altitude ami 

 changes in the state of tbe atmosphere have also great influence upon the result; and it has been 

 shown by the writer bow great is tbe difficulty of making certain allowance for the effect of these 



