ELECTBICITY, MAQNETISMj AND GRAVITATION. 



167 



6° to left. Covering the lower half of the helix now appears 

 to diminish the amount of deflection. 



February 26th, lOh. 55m.— The needle marks 3° to 4° to 

 the left hand, and by interposing the screen the indicator 

 returns to zero. 



March 3rd, 1 . 40 p.m. — Sun rather clouded ; the galvano- 

 meter indicates 6° to the left hand. Clouds intervene, and in 

 a few moments the needle declines to 2i°. 



Ih. 45m. The sun again emerged, and the galvanometer 

 deflected to 6° to the left, and continued to move in one 

 direction or the other just in proportion to the brightness or 

 obscured condition of the solar rays. 



March 4th, lOh. 54m. — The sun being very bright, the 

 galvanometer was observed to deviate 10° to the left hand. 



In order to see if any similar eflfect could be produced by 

 ordinary heat, I employed a spirit lamp, held near the 

 extremity of the helix, but no deflection ensued. 



Afterwards I held a pile of red hot burning embers at 

 the north extremit}'^ of the helix, resting upon a piece of 

 sheet copper, and a deviation of the needle to the left 

 hand ensued, equal to about i°. The embers were then 

 removed to the opposite extremity, when, as the north end 

 became cooler, the needle passed zero and deflected about 

 i° to the right. This was performed several times with 

 similar results. Afterwards the mercury cups were removed, 

 but the needle deflected under the influence of the embers 

 just as before. 



The shade of the apparatus was heated very considerably 

 by this proceeding, and much more than by the solar rays for 

 the production of a deflection of 10°. 



June 27th, 1851. — I have since discovered that not only 

 is the deflection of the needle produced by the solar rays 

 falling upon the southern extremity of the helix, and ob- 

 structed by the illumination at the same time of the northern 

 extremity, but that their projection upon the upper surface 



