PROCEEDINGS 



OF 



THE ROYAL SOCIETY. 



1832-1833. No. 12. 



February 21, 1833. 

 FRANCIS BAILY, Esq., Vice-President, in the Chair. 



A paper was read, entitled, " On the Influence of the Sun's Rays 

 on the Oscillations of the Magnetic Needle." By William Snow 

 Harris, Esq. F.R.S. In a letter addressed to Samuel Hunter Chris- 

 tie, Esq. M.A. F.R.S. 



The apparatus employed by the author in the inquiries of which he 

 gives the results in the present paper, is very similar to that he has 

 already described in his former communications to the Royal Society. 

 It allowed of his carrying on a long series of experiments with freely 

 suspended magnets, oscillating in a medium either rare or dense, and 

 either in the sunshine or in the shade. The source of error incident 

 to experiments in sunshine, made under an air-tight receiver, arise 

 from the increased temperature, producing, both in the rare and in 

 the dense medium, an irregular expansion, and a constant circulation 

 of currents of air, which interfere with the equable movements of the 

 bar — a condensation of vapour on the interior of the receiver — an 

 expansion of the bar itself, by which its length, as a pendulum, be- 

 comes changed — and, lastly, a derangement of the original magnetic 

 state of the bar. These disturbing causes he endeavoured to avoid by 

 observing the oscillations, first in the shade, under a close receiver, 

 and next when a beam of sunshine was thrown into the receiver by 

 means of a plane mirror ; in which case the heat was inconsiderable. 

 When the bar had been allowed to return to its former temperature, 

 similar experiments were repeated, after exhausting the receiver. 

 The results of a series of experiments conducted in this manner are 

 given in several tables : and the author concludes from them that the 

 influence of the solar rays on a magnetic bar, oscillating in air, is to 

 increase its apparent rate of oscillation ; while in vacuo, that rate is 

 diminished. 



The author seeks for an explanation of these phenomena in certain 

 changes effected in the surrounding medium. Comparative experi- 

 ments were instituted on a bar of copper of the same dimensions as^ 

 the magnetic bar employed in the former series. The author con- 

 cludes from these inquiries, that the phenomena in question are in- 

 dependent both of the magnetic state of the bar, and also of the in- 

 fluence of solar light. He tried the effect of exposure of the bar to 

 the intense light evolved by lime, acted upon by the influence of the 

 oxy-hydrogen blowpipe ; but with the same negative result. 



