93 



the opposite extremity, either very slight indications, or a neutral 

 result, constant vibrations, or the movement of the needle some de- 

 grees in the opposite direction, were always observed. The maxi- 

 mum deflection, at any time attainable by the galvanometer, when 

 the sun was quite unclouded, was about 12°, generally only 10°. It 

 may be observed that in all these experiments the power of the rays 

 was probably somewhat diminished, by passing though the glass pane 

 of the window, and through the glass shade of the instrument itself. 



In order to show that the effect was not thermo-electric action, 

 the extremities of the helix were removed from their mercury cups 

 and wrapped in paper, so as to exclude the mercurialized portion of 

 the copper from the action of the sun's rays ; but no alteration oc- 

 curred in the ordinary results of the experiments. There is, more- 

 over, the author considers, no evidence on record of any thermo- 

 electric action ensuing from the application of heat to copper wire 

 alone, nor without the formation of a complete electrical circuit. 

 But in these experiments hitherto the completion of the circuit 

 had not been attempted. During the course of the experiments 

 the circuit was established by means of a connecting wire between 

 the mercury cups, and the circuit was again and again completed, 

 and as frequently broken, without any deviation occurring in any 

 of the results, either during the progression, stationary condition, or 

 decline of the needle. 



That these phenomena were the result of the action of the sun's 

 rays upon the helix itself, was further shown, from the circumstance 

 that when the sun remained clouded for days together, there was no 

 deflection of the needle ; that when the helix was partly shaded by a 

 pillar, or the window-frame, the instrument indicated an amount of cur- 

 rent corresponding to the number of coils of wire illumined; and that 

 the illumination of the whole bundle of luires at the southern extremity 

 of the helix was necessary to produce the usual results, for when a 

 burning lens of high power was employed to condense the rays and 

 throw them in a focus upon one or two wires only, no deflection of 

 the needle was observed. It was also further shown that the action 

 of the rays upon the helix was attributable to that portion situate 

 chiefly at the southern extremity, for the whole instrument was in a 

 variety of ways and at diff'erent periods shaded from the solar rays ; 

 but its results were unaff'ected, unless the south end was obscured, 

 when the needle immediately declined; or the north end was illu- 

 mined when the deflections were lessened, or the motion of the 

 needle took place in the opposite direction. 



A pile of red-hot burning embers held in the vicinity of one ex- 

 tremity of the helix caused a slight deflection of and when held 

 at the opposite extremity, caused a deflection in the opposite di- 

 rection. 



The author states a remarkable circumstance, viz. that vibrations 

 and neutral action were observed during bright sunshine about the 

 11th of December, and again on the 23rd of January ; that previous 

 to the former period the deflections of the needle were to the left- 

 hand \ between these two periods they were to the right-hand \ and 



