on the Temperature of the Sun, 327 



metal, 0. The case was in the form of a straight prism with 

 a rectangular base. Each base, back and front, was mo- 

 vable, and had in its centre a window, which admitted the rays 

 as far as the junctures in the pile. These windows could, how- 

 ever, be closed by double shutters working in horizontal 

 grooves set in the outer wall of the case. The interior walls 

 of the case were covered with lampblack ; they could not, 

 however, radiate towards the faces of the pile, because the 

 latter were protected by two tubes E E, E E (rectangular in 

 section), which were slightly divergent towards the exterior 

 and reached nearly as far as the windows of the case. The 

 case with the pile could be inclined more or less to the horizon 

 by means of a toothed pinion, Gr, worked by a rack, F, fixed 

 on the case. This rack was supported by a pillar, H, which 

 was movable along a divided scale, T, and could be fixed in 

 any convenient position by means of a clamp -screw. The pile 

 could thus be moved in two directions — one along a horizontal 

 line, the other about a horizontal axis. A third movement, of 

 rotation about a vertical axis, could be obtained by turning the 

 foot L by hand. For the purpose of accurately arranging the 

 pile so that its two faces should be perpendicular to the rays, 

 two sights (e and/) were fixed to the top of the case. The 

 line passing through two holes made in these sights was parallel 

 to the longitudinal axis of the pile. In employing solar ra- 

 diation, it was certain that the faces of the pile were perpendi- 

 cular to the rays of the sun when the pencil of light which 

 passed through the hole in the front sight fell on the hole 

 in the hinder one. 



The thermoelectric pile made by M. Gourjeon (No. 2) was 

 only used in a few special researches. It was composed of a 

 great number of bismuth-antimony elements arranged in the 

 form of a straight cylinder with a circular base. It was more 

 sensitive than pile So. 1. 



2nd, Galvanometer. — An excellent Wiedemann's galvano- 

 meter measured the electric current generated in the pile 

 when exposed to radiation. In this galvanometer the devia- 

 tions of the magnet were read by means of a telescope on a 

 divided scale situated below the telescope and some distance 

 from the needle. The current which deflected the magnet 

 traversed a wire which was wrapped on two horizontal bob- 

 bins, which were placed on the two sides of the magnetic disk; 

 they were insulated from one another, and could be moved 

 either backwards or forwards. On each of the bobbins there 

 were wound two wires, in order that the instrument might be 

 used as a differential galvanometer. I arranged the con- 

 nexions so as to make the electric current travel over the two 



Z2 



