160 



ON THE IDENTITY OF LIGHT, HEAT, 



to the sun's rays when unobstructed, and at all times to the 

 same distance. (See later experiments.) 



3h. p.m. — The needle has now deflected 4^° to 5° in the 

 sun's rays. On interposing the screen, it returns 2°, and on 

 removing it, vibrates again to 5^ ; and this action is main- 

 tained as long as I please, by the mere interposition or 

 removal of the screen. 



Is THIS Thermo-Electric Action ? 



In order to see whether the effect was due to any thermo- 

 electric influence, I retained the south end of the helix in 

 the mercury cup, exposed to the solar rays, whilst the screen 

 was interposed ; but this in nowise altered the results. I 

 then removed the mercury cups altogether ; but the action 

 of the needle was just as before. The sun is now beginning 

 to illuminate the north end of the helix as much as the oppo- 

 site; the action becomes weaker, and in a short time I*' is 

 the whole effect produced. 



December 12th. — The galvanometer has now been sta- 

 tionary two nights and one day. No sunshine yesterday, 

 and no deflection. 



On the emerging of the sun from behind a cloud this 

 morning at 11, the needle deflected gradually to 3'^ towards 

 the right hand. On interposing a screen to obstruct the 

 rays, it declined speedily to 2^. 



When the screen was removed, the sun shining brightly, 

 the galvanometer indicated 4°. Interposed the screen, gal- 

 vanometer, 0°. Removed the obstruction, and in one 

 minute galvanometer indicated 2l°. The power of the solar 

 rays is indicated with as great precision as with a thermo- 

 meter ; a cloud intervening, 2° is indicated. 



December 13th. — A cloudy day. llh. 55m., the sun has 

 just emerged, aAd the galvanometer moves 2i° to the right. 

 The sun is again obstructed, and the indicator stands at zero. 

 Ih. 55m., some cloudy films intervening, the galvanometer 



