144 Proceedings of Philosophical Societies. [Feb. 



to the other (south) side of the circle, and a corresponding 

 observation made, without reversing the circle, but merely by the 

 motion of the telescope on the limb. The difference of the two 

 zenith distances so read oif is double the error of the zenith or 

 horizontal point of the graduation, and their semi-sum is the 

 true zenith distance of the collimating point, or the co-inclina- 

 tion of the axis, of the collimating telescope to the horizon. 



By the experiments detailed in Capt. Kater's paper, it appears 

 that the error to be feared in the determination of the horizontal 

 point by this instrument can rai'ely amount to half a second if a 

 mean of four or five observations be taken. In a hundred and 

 fifty-one single trials, two only gave an error of two seconds, and 

 one of these was made with a wooden float. In upwards of a 

 hundred and twenty of these observations, the error was not one 

 second. 



For further details we must refer to the original communication. 



Jan. 20. — Capt. F. W. Beechy, RN. was admitted a Fellow of 

 the Society, and the following paper was read : — 



On the Construction of the Barometer ; by J. F. Daniell, 

 Esq. FRS. 



In a former communication to the Royal Society on the Con- 

 struction of the Barometer, the author had inferred from some 

 experiments therein detailed, that the capillary depression of the 

 mercury in barometer-tubes was decreased one-half by boiling; 

 and the first object of the pi'esent paper was to describe some 

 new experiments that he had made on this subject, the results 

 of which confirmed his former deductions. In these the depres- 

 sion of the mercury in tubes of from -^ to -j-^^ of an inch internal 

 diameter was measured to the -y-^ts P^rt of an inch, by a par- 

 ticular apparatus constructed for the purpose, and described in 

 the paper ; and their results very nearly agreed with those given 

 in Dr. Young's tables, calculated from the experiments of Lord 

 Charles Cavendish : on repeating the experiments after boiling- 

 mercury in the tubes, Mr. Daniell found the amount of the 

 depression to be one-half of what it was before ; as he had for- 

 merly concluded. 



Mr. Daniell proceeded to detail some facts relating to the 

 gradual deterioration of Ijarometers by the insinuation of air 

 between the mercury and the tube, and to describe the means 

 he had devised for obviating this defect in the instrument. 

 He had been informed that the m.ercury in the barometer con- 

 structed under his superintendence, and set up in the apart- 

 ments of the Royal Society, by the direction of the Meteorolo- 

 gical Committee, exhibited a peculiar speckled appearance ; 

 and on examination he found a number of minute bubbles of 

 air between the glass and the m-ercury, increasing in size 

 towards the top. 



