Intelligence and Miscellaneous Articles. 231 



of the pile. The spot was very clearly defined, and might have been 

 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 

 beginning 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. 



j ; 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 

 position, receive the maximum effect of any variation in the electri- 

 cal 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 part* of the luminous disc. The follow- 

 ing is a copy of the record made at the time of the observations. 

 The degrees are those marked on the card of the galvanometer, and 

 are of course arbitrary. 



Spot, 3°^ Sun, 5°|. 



Sun, 4°f. Spot, 4°. 



Sun, 3°. Spot, 4%. 



Spot, l°f . Sun, 5°. 



Spot, 2°. Sun, 4%. 



Sun, 3°. Spot, 3°£. 



Sun, 2°±. Sun, 2°. 



Spot, 2°. Spot, 3°£*. 



Spot, 2°. Spot, 0°f. 



Sun, 2°£. Sun, 2°l. 



* At this observation a slight cloud probably passed over the sun's disc. 



