416 Proceedings of the British Association 



m 



From these numbers the difference in time between a right and 

 left oscillation is found to be, from the first trial. 0^*000725 ; from 

 the second, 0^-000710. 



From these observations M. Bravais concludes that a seconds 

 pendulum, revolving conically, would lose three seconds per day 

 when moving from right to left, and gain the same when moving 

 in the contrary direction. A pendulum 33 feet long would be 

 retarded or accelerated 11^-4 per day. From these observations 

 M. Bravais also computed the length of a simple seconds pendu- 

 lum, which he made 39'"- 1255 instead of 39^^-1291 the number 

 commonly adopted, (993"^m-77, instead of 993"^«i-86.) 



Art. XLIII. — Extractsfrom the Proceedings of the Twenty-first 

 Meeting of the British Association^ held at Ipsivich^ Jtily 2.* 



r 



1. On preparing Speculums for Telescope; by the Earl of Rosse. 



The Earl of Rosse said that, having observed by the public prints that 

 the President of the Association had, in his inaugural address, done 

 him the honor of alluding with approbation to the attempts which he 

 had lately made, and with considerable success, to produce plane spec- 

 ulums of silver for reflecting telescopes, — he thought it might be ac- 

 ceptable to the Section to receive a brief account of them, and he had 

 therefore come down from London that morning to give them such an 

 account. In order to help their conception of what he had to say, he 

 drew with chalk the annexed 

 sketch. A, the great concave 

 speculum at the bottom of the 

 telescope tube, collecting the 

 rays of light which came from 

 a radiant point placed at a 

 great distance beyond the 

 mouth of the tube to its con- 

 jugate focus F, These rays were then intercepted by the small 

 mirror B, and by it reflected to the focus F towards the side of the 

 tube where they were received by the eye-piece of the telescope. 

 Now, speaking roughly, it was found that about one-third of all the 

 light which fell upon the great speculum A, was lost in the act of 

 reflection, — and that of the remainder a considerable portion, but not 

 quite one-third, was lost by the second reflexion at B. Of course, 

 since quantity of light is of the utmost importance to the perform- 

 ance of these tubes, this rendered it a matter of the utmost import- 

 ance either to get rid altogether of the second reflexion at B, or to 

 obtain some means of producing it subject to less loss of light ^^^ 

 that caused by speculum metal; and accordingly, Sir William Her- 

 schel, the first who had succeeded in producing reflecting telescopes on 

 a large scale^ had, by inclining the great mirror slightly, brought the 



* From the Athenaeum of July 12 and 19, Nos. 1237, 1238, 





