175 



readily daguerreotyped, had the telescope been furnished with an 

 equatorial movement. The form of the penumbra of the spot, as it 

 appeared on the screen, was that of an irregular oblong, about two 

 inches in one direction, and an inch and a half in the other. The 

 dark central spot within the penumbra 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 con- 

 nexion 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 

 experiments the order was reversed; the picture of the spot at the be- 

 ginning of the experiment was placed in connexion with the pile, and 

 afterward 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 galvano- 

 meter more sensitive, two bar magnets, arranged in the form of the 

 legs of a pair of dividers, were placed with the opening downwards, 

 in a vertical plane, above the needle, so that, by increasing or dimi- 

 nishing 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 definitely 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 the zero point, so that it might, in this po- 

 sition, receive the maximum 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 the obser- 

 vations. The degrees are those marked on the card of the galvano- 

 meter, and are of course arbitrary. 



Spot, 3°i. Sun, 5^i. 



Sun, 4°i. Spot, 4°. 



