TEMPEKATUKE OF THE SURFACE OP THE MOON. 17 



INatnro, XVI, \i. 43S (1877). Lofter by Lord Rosso, ropl.viii^ to M. Railliiid, who aitiibntcs the reddish tiiifie of the 

 totally oilipsed moon to selt'-liiniiiiosity due to the high teiiiperatiirc^ acciuired iiikUu' the snu's rays, and cites 

 olisi'i'valions of Lord Rosse in support of his views.] 



M. Rnillard is mistaken in supposing- that Lord Rosse estimated the temperature of the lunar .surface at 500° 

 Fahr. This was the range of temperature which a lanip-blaclicned vessel must have in order to exhiliit etfecis similar 

 to those of the moon in its dill'erent phases, deduced from early ob.servatious. More accurate ob.servutions (descrilied 

 in previous papers) show that this range is much more nearly 100'^ C, a large error having crept into the previous 

 work. The observations made during the total Innar eclipse show that the diminution of heat ke|)l pace with that of 

 light. Probably not nu)re than 5 per cent, of the heat, a squired since new moon is retained till the midiUe of a total 

 eclipse, although it lias beeu shown that this heat has been absorbed by the lunar surface and reradiated. We must 

 therefore fall back upon the usual cxplauatiou for the reddish oolor of the moon's surface during a total eclipse. 



Urania I (IHHl). 



On comparing the heat curves between new and full moon with that between full and new moon there appeared 

 no conclusive evidence that the lunar surface required liuie to acquire the temperature dno to the radiation falling on 

 it. Accordingly, observations were made at the eclip.ses of November 1-1, 1872, and August 23, 1879, but the result of 

 these was to show that the decline and subsequent increase of the heat took place as rapidly as that of the light. 



[Nature, XXX, p. 589 (October 16, lS-'4). Description of observations made during the total luuar eclipse of October 

 4, l884^by Otto Bocddicker, at the Earl of Rosse's observatory.] 



The apparatus used was the same as that already described. Clouds prevented observations until 30 minutes 

 before the beginning of the total phase, when the sky became exceptionally clear. 



Two hundred and eleven readings of the galvanometer were taken, the time of exposure being 1 minute for each. 

 (A curve representing these observations is given.) No observations were made during the total phase on account of 

 the difficulty of judgiug wheu the image fell on the thermopile, but near the beginning and end of totality the effect 

 was masked by the irregularities of the galvanometer, and was smaller than the probable error of observation. The 

 minimum of heat seems to be later than that of light. As the moon emerged from the earth's shadow, so slowly did 

 the reailings of the galvanometer increase again that, about twenty minntes after the total phase was over, the almost 

 entire absence of any effect led the observer to think that the small condensing mirrors must be covered with dew, 

 which however was not the case. * 



PRELIMINARY OBSERVATIONS AT ALLEGHENY. 



OBSERVATIONS ON LUNAR HEAT. 



The first irieii.sares of liinar heat at Allegheny were made ou the evening of November 12, 

 ]8S0,t more with a view to testing the sensitiveness of the then recently invented bolometer than 

 for the sake of the measures themselves. The lunar rays were concentrated upon the face of the 

 bolometer by means of the 13-inch equatorial of the observatory and a smaller convex lens near 

 its focus, and an average deflection of 42 divisions of the galvanometer scale was obtained. The 

 exposures were made by directing the tel< scope upon the moon after the galvanometer needle had 

 come to rest, while the telescope was pointed at the neighboring sky. 



On June 21, 1SS3, the bolometer and its adjuncts having been much improved in the interval, 

 measurements of the luuar heat were resumed with apparatus better adapted to the purpose. The 

 thick glass lenses, used in the previous experiments, absorb and reflect a large proportion of the 

 already snfliciently minute amount of heat at disposal, and the rays absorbed are those whose 

 presence or absence chiefly affects the conclusions to be drawn from the results of the observa- 

 tions. In the new experiments, therefore, the dioptric system for condensing the lunar rays was 

 rejtlaced by two silvered-glass mirrors, the absorption of silver for every iieat ray of thes])ectriim 

 having beeu determined by previous exijeriments here. This absorption, as is well known, afi'ects 

 chiefly the blue and violet rays, in which but a small proportion of the total energy resides. Our 

 own observations show this absorpti(m (by silver) to be very small and tiearly constant throughout 

 the infra red. The sensitiveness of the bolometer, and its accuracy, enable us, as will be seen, to 



*Althovigh over one fourth of the moon's surface must have emerged from shadow at this time, it must be remem- 

 bered that it was still covered by the earth's penumbra, so that the small heating etl'ect is less surprising. If the 

 moon parts with its ac(|nircd beat so soon as these eclipse observations seem to indicate, it is difficult to see how the 

 maximum of heat could occur at an ajipreciable time after full moon. It is in any case hard to admit that this beat 

 from the lunar surface, which the moon has been absorbing during many days of continuous sunshine, is parted with 

 at once, the whole earthward surface of the planet cooling almost instantaueou.sly. 

 t American Journal of Science, CXXI (March, 1881). 

 S. Mis. 69 3 



