. Mechanical Science. 167 



4his observation be supposed correct, and the temperatures equal 

 in the two cases, it may be concluded that the force which causes 

 deviation of the magnetic needle depends upon the extent of sur- 

 face only, and does not emanate from all the particles comprised 

 in the thickness of the heated disc. By trying bodies of different 

 kinds, and especially those which are transparent, and varying 

 also in thickness, it will, perhaps, be possible to determine to 

 what degree they intercept the repulsive action, arising from ele- 

 vation of temperature. 



" When the mobile disc is of some thickness, and its external 

 surface is heated, it often happens that it remains for a long time 

 in contact with the fixed disc, and separates from it on withdraw- 

 ing the lens. This is probably occasioned by the great difference 

 of temperature between the two surfaces of the mobile disc, from 

 which it may result that the surface receiving the solar rays, is as 

 much repelled by the surface of the bell-glass, as the other sur- 

 face is by the fixed disc. This I offer only as a doubtful expla- 

 nation, not having had time to verify it by new experiments. 



M. Fresnel afterwards says, " new experiments have shewn me 

 that the explanation at the end of the note, of the particular phe- 

 nomena of thick discs cannot be admitted ; for in that case the 

 face of the mobile disc heated by the sun's rays would suffer a 

 sensible repulsion from the neighbouring surface of the bell-glass, 

 and by throwing the focus of the lens on to the moveable disc 

 away from the fixed one, the needle should be deviated ; this, how- 

 ever, does not happen. 



With pieces of copper of a hundredth part suspended from the 

 extremities of the magnetic steel wire, I have obtained very ap- 

 parent effects of attraction. When the solar rays were thrown on 

 the exterior face of the mobile disc near to the fixed disc, it ap- 

 proached and adhered to it as if attracted. This attraction was 

 not occasioned by a developement of electricity, for the solar rays, 

 reunited on the other mobile disc, produced no sensible effects, 

 though the two suspended discs were connected by the steel wire. 



I have observed actions similar to these in many other circum- 

 stances, but I have as yet too slightly studied these singular phe- 

 nomena to give a general and exact description ; I can only say 

 that the experiments I have thus far made confirm me in the 

 opinion that both the attractions and repulsions, produced by heat, 

 do not arise from the developement of electric tension ; and if they 

 belong to a momentary state of emanation of the heated discs, it 

 appears to me at least that the distribution of magnetism here 

 follows particular laws. — Ann. dc Chimie, xxix. 57, 107. 



6. Polarized State of Halo Light. — M. Arago announced to the 

 Academy, that upon examining a halo round the sun towards 

 eleven o'clock in the morning, with an instrument of his inven- 



