1895.] on the Radiant Heat from the Moon during an Eclipse. 631 



The ordinary thermopiles, made by Elliott Brothers, were at first 

 used, but after a time thermocouples, consisting of bismuth and 

 bismuth-tin alloy in very thin bars soldered to thin discs of copper 

 to receive the heat, were substituted.* 



Slide 13 represents Professor Langley's " bolometer." It is com- 

 posed of two superimposed gratings of exceedingly thin metal (gene- 

 rally 3L- inch wide and -^oVo inch thick). The planes of the gratings 

 are -^ inch apart. The strips are connected up and down into two 

 systems, which are respectively interposed in the two arms of a 

 Wheatstone's bridge. A slight change of resistance in either system, 

 through a change of temperature under radiant heat, is thus detected 

 and measured by a galvanometer in the cross arm as usual. The 

 strips are indicated by the strong black lines. The course of the 



Fig. 13. 



current is indicated by the dotted lines. The two gratings are shown 

 side by side, and may be imagined hinged and turned so that the 

 correspond iug plain and dotted letters come into electrical contact. 

 There are two circuits : the first in the centre, on to which the radia- 

 tion is alternately directed and then removed ; the second, or screened 

 circuit, on the two sides, which acts simply as a compensator for acci- 

 dental disturbances. 



A pair of single bolometers, made for me by a London artist, did 

 not show more sensitiveness than my thermocouples, but they are not, 

 I think, of the delicacy of the American instruments. It was evident 

 that some experience and practice would be required for their suc- 

 cessful use. 



Langiey has varied the proportions of his bolometers, and the 

 descriptions of them are to be found in his several papers in the 



See Troc. Koy. Soc. No. 112, 1870, p 553. 



•2 u 



