244 Royal Astronomical Society. 



The author states that the Admiralty having left to his decision 

 the form and construction of a new pendulum, which they had re- 

 solved on sending out to the Cape, for the purpose of being swung 

 by Mr. Maclear at the several stations of the trigonometrical survey 

 now in progress in that colony, he had not hesitated in adopting 

 the Jar-pendulum, as by far the best and most convenient for a 

 travelling instrument. The pendulum which has been accordingly 

 constructed is a brass bar, sixty inches long, two inches wide, and 

 about half an inch thick. It was formed of several thin plates which 

 were pressed together by a rolling machine, and is, consequently, 

 very compact and hard. Its specific gravity was 8" 60, and its rate 

 of expansion for one degree of Fahrenheit's thermometer, •00001034. 

 It is furnished with four knife-edges, thereby affording the advan- 

 tages of four distinct pendulums on one and the same bar, and which 

 thus serve as a check on each other. As the construction of the 

 pendulum did not allow of much filing away at the ends without 

 cutting into the knee-pieces, the vibrations on the several knife- 

 edges were rendered nearly isochronous (for absolute isochronism 

 can hardly be obtained), by fastening a circular piece of brass 

 weighing 3000 grains, about an inch and a half from the centre of 

 the bar ; the weight and position having been determined by re- 

 peated preliminary experiments. After every thing was finished, 

 seven sets of experiments were made on each knife-edge, the 

 mean results of which were respectively as follows;. — knife-edge A, 

 85906-322 vibrations ; B, 85905-725 ; C, 85904-107 ; D, 85903-427; 

 in a mean solar day. The computations and corrections were made 

 in the usual manner, with the exception of the correction for the 

 height of the barometer, which can only be determined accurately 

 by swinging the pendulum in vacuo. For this there was not time 

 before the pendulum was sent off, and the correction was assumed 

 to be the double of that which is given by the formula which was 

 usually employed prior to the experiments of M. Bessel. The agate 

 planes, which were made expressly for this pendulum, are attached 

 to a solid frame of brass, three-quarters of an inch thick, and having 

 three foot-screws for the purpose of levelling the planes. 



Observations of the Second Comet of 1840, made at the Obser- 

 vatory at Hamburgh. By Mr. Rumker. 



The observations give the apparent right ascension and declina- 

 tion from January 29 to March 24, 1840. 



Dec. 11. — The following communications were read : — 



On a large Achromatic Object- Glass of a Telescope worked by 

 Mr. Dollond, the flint glass of which was prepared by the late Dr. 

 Ritchie. By the Rev. Samuel King, M.A., F.R.A.S/ 



In a paper by Mr. Simms, " On the Optical- Glass prepared by 

 the late Dr. Ritchie," which was read to the Society on the 14th of 

 June, 1839, and is now printed in vol. xi. of the Memoirs, refer- 

 ence was made to an object-glass of 7f inches aperture, the flint 

 glass of which was worked by Mr. Dollond out of a disc prepared 

 by Dr. Ritchie ; and it was intimated that Mr. King, who had the 

 object-glass at that time under trial, would probably report to the 



