TRANSACTIONS OF THE SECTIONS. 21 



as the centre portion of the moon's crust at the last quarter has been exposed to 

 some 180 additional hours of uninterrupted solar radiation, it is probable that the 

 total heat attained must be very gi-eat indeed. Whatever temperature is acquired, 

 the maximum -will, it is believed, occur as stated, at or near last quarter. 



As regards the date of the gTeatest cold in the moon, the German physicist already 

 cited arrives at the conclusion that it occm's about half a day after first quarter ; 

 at the period, in fact, when, as was said before, the region of the moon opposite to 

 us has been the longest time unexposed to the sun's rays. If a temperature of 

 — 92° Fahr. occurs at the time fixed by Althaus, it would suppose a tail of 940^ 

 Fahr. (522° Cent.) in about fifteen days. The author believes this interval would 

 be required ; for the conduction of heat through the moon's strata would be very 

 gradual : and though it is true that bodies at veiy high temperatm'es cool, both in 

 air and in vacuo, with great rapidity, yet it has been proved that the rate of 

 cooling is greatest in air, by reason of its convection of heat. This is one of the 

 laws laid down by Dulong and Petit, and admitted by those whose judgment in the 

 matter is most to be relied on. The author has submitted the point to direct ex- 

 periment in the receiver of the large air-pump at Kew, when the velocity of cooling, 

 shown by a thermometer with a half-inch bulb coated with lampblack, for tempe- 

 ratm-es a little above the boiling-point, was found, for the first 100°, to be 25 per cent, 

 greater in the glass filled with air, than in the exhausted receiver. Thus it would 

 seem that the absence of an atmosphere would, in the case of the moon, favour an 

 aecumidation of heat, though in a difterent manner from that in which the presence 

 of air and vapour aifects the earth, where the slight heat stored up in its crust would 

 be speedily lost if it were not for the counter-radiation from cloud and vapom*. 



As regards the theory that the solar rays would have no power to heat matter if 

 surroimded by aether, there would seem no reason to believe that this is the case. 

 It would be necessary that the observations which are supposed to point to that 

 conclusion should be verified before the possibility of a result so unlooked for is 

 admitted. Sir H. Davy foimd by experiment that absorption of heat fi-om the coal- 

 points of the electric light took place in vacuo ; and the author's own experiment 

 with the solar rays upon the blackened bulb of a mercmial thermometer, heated by 

 means of a lens, in the l{3-incli receiver already referred to, though undecisive as 

 I'egards the relative speed of heating in air and vanio 'with the sun as the source of 

 heat, showed a gain of 160° Fahr. in two minutes (71° 11' Cent.) in a vacuum of 

 about one-eighth of an inch. Also in several experiments with thermometers with 

 black and blackened glass bulbs enclosed in exhausted 2-inch globes, by Mr. Casella, 

 and one with a lamp-blackened bidb in a globe tilled with air made for the pm-pose, 

 the thermometers in the exhausted globes (and more especially the one with the 

 blackened bulb) were found to rise quicker and reach a higher maximum, in equal 

 intervals of time, than the one in the globe filled with air. On a view of the whole 

 case at the present time, there would seem to be reason to believe that the sun's 

 rays must penetrate the moon's crust to a depth that would present the possibility 

 of her acquired heat being easily or speedily dissipated, and consequently that the 

 accumulation of heat rmder her vertical sun would reach a higher point than is 

 generally- supposed. 



On the Dmrnal Period of Tem'perature in relation to other Physical and 

 Meteorological Phenomena. By Prof. Hennisst, F.R.S. 



On Meteoric Bhoivers considered ivifh reference to the Motion of the Solar 

 System. By Prof. Hemjessy, F.B.S. 



On a Table of Pairs of Stars for approximately finding the Meridian. 

 By W. J. Macqtjoen Eaxeine, C.E., LL.D., F.B.SS. L. 6f E. 



The author stated that the object of the Table referred to was to give increased 

 accuracy and utility to the well-known process of finding a meridian line approxi- 

 mately^ by choosing a pair of stars having nearly the same right ascension, and 

 observing their direction at the instant of their being in the same vertical plane. 



