Mechanical Science. 165 



repulsion at a distance. Nevertheless, I dare not affirm that they 

 establish this mode of action, though I have observed its existence 

 in another manner, because the calorific repulsions for intervals of 

 some millimetres are so feeble that I can hardly believe them ca- 

 pable of overcoming the friction of the drop of liquid on the wire. 



I had uselessly endeavoured, for a long time, for the verifica- 

 tion of certain hypotheses, to move a small disc of foil attached to 

 the extremity of a very light horizontal stem, supported by a 

 thread of silk in vacuo, by the action of the solar rays collected 

 together by a lens. Since then I proposed to try whether this 

 mobile disc would not be repelled by a heated body brought near 

 to it ; but I should no doubt have delayed the execution of this 

 project, if M. Libri had not communicated to me his interesting 

 observations. They, by inducing me to consider the success as 

 probable, caused me the sooner to make the experiment. 



For its convenient performance a very fine steel wire, mag- 

 netized, and suspended by a silk fibre, had attached to its extre- 

 mities a disc of foil, and a disc cut from a plate of mica, for the 

 purpose of trying an opaque and a transparent body in the same 

 apparatus : the fixed body, intended to repel the balance of torsion 

 was also a disc of foil. A vacuum was carefully made within the 

 glass jar which enclosed the apparatus ; the elasticity of the re- 

 maining air indicated by the mercurial gauge was not more than 

 one or two millimetres. The jar was then placed in the sun's 

 rays, and so turned that the magnetized steel wire was but little 

 out of the magnetic meridian, yet sufficient to cause one of the 

 discs fixed at its extremity to exert a very slight pressure against 

 the fixed disc, so that it should remain in contact with it. The 

 apparatus thus arranged, I threw the sun's rays by a lens, some- 

 times on the fixed disc, sometimes on the moveable one, and im- 

 mediately the latter separated quickly from the former. I re- 

 tained it separate, and sometimes at the distance of a centimetre, 

 (0.39371 of an inch,) by continuing to heat the discs. When I 

 removed the lens, the balance of torsion did not return imme- 

 diately to the fixed body, but gradually approached it, performing 

 small oscillations. It is very probable that if I had employed 

 thicker bodies, and such as would cool more slowly, the return to 

 the original position would have been more gradual. 



It seemed to me as if the transparent disc was not so strongly 

 repelled as the disc of foil. I observed also that the most ad- 

 vantageous manner of heating the bodies, so as to retain them at 

 the maximum distance, was to send the focus of the lens on to one 

 of the opposing surfaces. I do not suppose that this effect is due 

 to reflection, but merely to the facility obtained in this way of 

 more highly heating the surface which is to exert the repulsive 

 action. 



That I might be assured these phenomena were not occasioned 



