226 WRITINGS OF JOSEPH HENRY. [1845 



was nearly square, of about three-fourths of an inch on the 

 side, and a little larger than the end of the thermo-pile. 



The method of observation consisted in first placing, for 

 example, a portion of the picture of the luminous surface of 

 the sun in connection with the face of the pile, and after 

 noting the indication of the needle of the galvanometer the 

 telescope was then slightly moved so as to place the dark 

 part of the spot directly on the face of the pile, the indication 

 of the needle being again noted. In the next set of experi- 

 ments the order was reversed, the picture of the spot at the 

 beginning was placed in connection with the pile and after- 

 wards a new part of the luminous portion of the disc was 

 made to occupy the same place. 



The thermo-electrical apparatus used in these experiments, 

 was made by Ruhmkorff, of Paris ; and in order to render 

 the galvanometer more sensitive, two bar magnets, arranged 

 in the form of the legs of a pair of dividers, were placed with 

 the opening downward, in a vertical plane, above the needle, 

 so that by increasing or diminishing the angle the directive 

 power of the needle could be increased or diminished, and 

 consequently the sensibility of the instrument could be 

 varied, and the zero point changed at pleasure. 



In the present experiments, in order to mark more defi- 

 nitely the difference in temperature, after the needle had 

 been deflected by the heat of the sun, the magnetic bars 

 above mentioned were so arranged as to repel it back to near 

 zero point, so that it might in this position receive the max- 

 imum effect of any variation in the electrical current. 



Twelve sets of observations were made on the first day, 

 all of which, except one, gave the same indication, namely, 

 that the spot emitted less heat than the surrounding parts of the 

 luminous disc. The following is a copy of the record made 

 at the time of observation. The degrees are those marked 

 on the card of the galvanometer and are of course arbitrary : 



Spot, 3° i. Sun, 5° i 



Sun, 4° 1. Spot, 4°. 



Sun, 3°. Spot, 4° i 



Spot, 1° |. Sun, 5°. 



