EXAMINATION OF MR. CHRISTIE'S EXPERIMENT. 5 



on a stand by two pillars, B C, a hair, e, was fastened, about the point A ; its other 

 extremity, z, being fastened to a whalebone spring, D, by a thread,/; the extremity, g, 

 of the hair, being bent by the knot on one side, served as an index. On exposing the 

 instrument to the sunbeam, the little index, g, immediately moved, sometimes more than 

 a semicircle. 



8. Thinking to obtain more decisive effects, I concentrated the sunbeam with a lens 



o 



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 uo way affected by exposure to solar light. 



9. It is said expressly 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 was the cause of the derangement in question ; I therefore 

 vibrated a needle, under similar circumstances, with the same result as 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 compass boxes are generally made of sheet brass, with a 

 soldered seam in the side, it was barely possible that the fine line of solder acted with the 

 brass, as a thermo-electric couple, capable of excitation by the warmth of the sunbeam. 

 I therefore made a compound cylinder of copper and zinc, c z (jig. 2, pi. 1), the edges 

 of which, at a and b, were neatly soldered, the junction b being highly polished, and that 

 at a blackened. The needle was suspended in an exhausted receiver, by a silk fibre, g, 

 and a ray of light, c d, coming from an aperture half an inch wide in the shutter, fell 

 upon the junction. The needle used in this experiment was of watch-spring ; its first 

 vibration was performed in an arc of forty degrees, and when the compound cylinder was 

 taken away, it made thirty-two vibrations in sixty seconds in vacuo. On placing it 

 concentrically with the compound cylinder, and suffering the ray to impinge on the 

 polished junction the moment that the arc of vibration had become forty degrees, the 

 number of oscillations in one minute was carefully observed; six experiments gave sev- 

 erally the number thirty-two. On turning the blackened junction to the light, the re- 

 sult was still thirty-two ; and on substituting the solid brass cylinder, three consecutive 

 trials gave thirty-two. The thermometer stood in the sunbeam at 103 ; the barometer 

 at -28.8. 



30. By some this magnetic action of light has been attributed to the violet or more 

 refrangible rays only. A needle made of watch spring, about four inches long, which, 

 in an exhausted receiver, suspended by a filament of silk, exhibited no polarity, had one 

 half of it exposed to the violet ray, cast by an equi-angular prism of flint glass. This ray 



