1869.] 



Heat from the Moon. 



437 



Professor Piazzi Smyth had conducted a series of experiments on the 

 Peak of Teneriffe with a thermopile, but apparently without any means 

 of concentrating the moon's heat beyond the ordinary polished metal cone. 



Melloni had employed a glass lens of considerable diameter (I believe 

 about three feet) ; but as glass absorbs rays of low refrangibility, it was 

 not so well adapted to concentrate heat as a metallic mirror. 



In the following experiments the point sought to be determined was, in 

 what proportions the moon's heat consists of 



(1) That coming from the interior of the moon, which will not vary 

 with the phase. 



(2) That which falls from the sun on the moon's surface, and is at 

 once reflected regularly and irregularly. 



(3) That which, falling from the sun on the moon's surface, is absorbed, 

 raises the temperature of the moon's surface, and is afterwards radiated as 

 heat of low refrangibility. 



The apparatus consisted of a thermopile of four elements, the faces half 

 an inch square, on which all the moon's heat which falls on the large 

 speculum of the 3 -foot telescope is concentrated, by means of a concave 

 mirror of 3£ inches aperture, 2*8 inches focal length. 



As it was found difficult to compensate the effects of unequal radiation 

 on the anterior face of the pile, by exposing the posterior face also of the 

 same pile to radiation from the sky, during the later experiments (be- 

 ginning with March 23rd) two piles were used, and the following was the 

 form of apparatus adopted. 



D E is the large mirror of the telescope ; F G the two small concave 

 mirrors of 3£ inches aperture, and 2*8 inches focal length, fixed in the 

 plane of the image formed by the large mirror D E. The two thermo- 

 piles are placed respectively in the foci of F and G, their anterior faces 

 shielded from wind and other disturbing causes by polished brass cones, 

 and their posterior faces kept at a nearly uniform temperature by means 

 of brass caps filled with water. The thermopiles and accompanying 

 mirrors are supported by a bar screwed temporarily on the mouth of the 

 tube. Two wires are connected with the two poles of each pile ; and the 

 ends of the wires are connected, two and two, close to the galvanometer, 

 in such a manner that a given amount of heat on the anterior face of one 

 pile will produce a deviation equal in amount, and opposite in direction, to 

 that produced by an equal amount of heat on the anterior face of the 

 other pile. Thomson's Reflecting Galvanometer was the one used. 



2 k 2 



