MEMOIR XII.] THE MAGNETIC EFFECTS OF LIGHT. 193 



the incident light, what results would be obtained. If 

 the needle was suspended on a point, or by a thread 

 without torsion, the time and the number of vibrations 

 were the same whether the needle was exposed to the 

 sunbeam or not. But if the needle was suspended by a 

 hair or other organic substance having torsion, the sun- 

 beam would occasion a twist in the hair on its first 

 exposure to light ; and if the direction of that twist hap- 

 pened to coincide with the direction of the needle's 

 motion, the momentum of the needle was increased and 

 the vibrations continued longer. A needle which vi- 

 brated forty-four times in one minute would occasional- 

 ly, owing to this cause, vibrate forty-six when suspended 

 by a hair; but if by a silk fibre, its vibrations were al- 

 ways forty-four, the first arc of vibration being in every 

 instance 40. 



Thinking to obtain more decisive effects, I concen- 

 trated the sunbeam with a lens on the south pole of the 

 suspended needle, and found that the needle was thrown 

 into a rapid, tremulous motion. But here the hot air 

 ascending from the needle acts upon it as upon the sail 

 of a windmill ; and the same effect ought to take place 

 to a certain extent on simple exposure of the half of a 

 vibrating needle to direct light. But I found that a 

 needle suspended in the vacuum of an air-pump by a 

 thread without torsion is in no way affected by exposure 

 to solar light. 



It is said in the account of Christie's experiment that 

 the needle was contained in a brass compass -box. It 

 might have been that electrical currents were excited in 

 that box which were the cause of the derangement in 

 question. I therefore vibrated a needle under similar 

 circumstances, with the result above stated. I should 

 mention that this was done in a solid cylinder or ring 

 of brass without any seam or soldered junction ; but as 



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