160 Proceedings of the British Association. 



as those obtained by Mr. Talbot's process. Sir D. Brewster said, 

 this was the germ of one of the most extraordinary discoveries 

 of modern days ; by it there seemed to be some thermal effect 

 which became fixed in the black substance ; and not only so, 

 but M. Bessel informed him, that different lights seemed to affect 

 different vapors variously, so that there seemed to be something 

 like a power of rendering light latent ; a circumstance, which if 

 it turned out so, would open up very new and curious concep- 

 tions of the physical nature of light : on the emission theory, it 

 would be easy to account for this ; — on the undulatory theory, 

 he could not conceive how it could be possible. 



Prof. Bessel, of Konigsberg, made a communication on the 

 Astronomical Clock. Having ever been of opinion, that this 

 indispensable instrument to the astronomer, the transit clock, 

 could only acquire perfection, if the pendulum, (separated from 

 the works,) were made to vibrate in equal time, whatever the 

 temperature and the arc might be : he would submit, whether 

 the expeditious method of coincidences might not be employed 

 for checking the pendulum in both respects. The pendulum, 

 apart from the clock, being suspended from the wall ; a clock, 

 taken out of its case, might be placed before it, at a distance of 

 six or eight feet ; an object-glass of three or four feet focal length 

 might be placed between both, so as to produce, exactly at the 

 lower end of the pendulum of the clock, an image of the lower 

 end of the other pendulum. Then the coincidences of both 

 might be accurately observed by a telescope placed at a conven- 

 ient distance. Similar contrivances had been described in an 

 account of some pendulum experiments made at Konigsberg : 

 and the accuracy of the method was such, that the relative rate 

 of both pendulums might be ascertained with sufficient accuracy 

 in a very short time, — in from ten to twenty minutes. The rate 

 of the pendulum was to be tried at different temperatures, being 

 placed in a box, having an opening at the lower end, covered 

 with glass, and so fastened to the wall, that the pendulum could 

 swing within it. In the construction of the pendulum, attention 

 should be paid to one thing, which seemed to have been much 

 overlooked. It often happened that thermometers affixed to the top 

 and bottom of a clock case did not agree, whence it was evident 

 that the compensation, acting only below, would not compensate 

 for the variation of the whole rod. He should prefer, on this 



