Page:Proceedings of the Royal Society of London Vol 4.djvu/23

Rh PROCEEDINGS

OF

THE ROYAL SOCIETY.

183’". No. 30_

November 16, 1837. FRANCIS BAILY, Esq., V.P. and Treasurer, in the Chair,

" Description of a new Barometer, recently ﬁxed up in the Apart- ments of the Royal Society; with remarks on the mode hitherto pursued at variom: periods, and an account of that which is now adopted, for correcting the observed height of the mercuryin the Sooiety’s Barometers.” By Francis Baily, Esq., Vice-President and Treasurer, R.S.

The barometer, here alluded to, may in some measure be consi- dered as two separate and independent barometers, inasmuch as it is formed of two distinct tubes dipping into one and the same cistern of mercury. One of these tubes is made of ﬂint glass, and the other of crown glass, with a ViEVV to ascertain whether, at the end of any given period, the one may have had any greater chemical eﬂ‘cct on the mercury than the other, and thus aﬁected the results. A brass rod, to which the scale is attached, passes through the framework, between the two tubes, and is thus common to both : one end of which is furnished with a ﬁne agate point, which, by means of a rack and pinion moving the whole rod. may he brought just to touch the surface of the mercury in the cistern, the slightest contact with which is immediately discernible; and the other end of which bears the usual scale of inches, tenths, &c.; and there is a separate vemier for each tube. A small thermometer, the bulb of which dips into the mercury in the cistern, is inserted at the bottom : and an eye- piece is also there fixed, so that the agate point can he w'ewed with more distinctness and accuracy. The whole instrument is made to turn round in azimuth, in order to verify the perpendicularity of the tubes and the scale.

It is evident that there are many advantages attending this mode of construction, which are not to be found in the burometers as usu- ally formed for general use in this country. The absolute heights are more correctly and more satisfactorily determined; and the per- manency of true action is more elfectmlly noticed and secured. For, every part is under the inspection and control of the observer ; and any derangement or imperfection in either of the tubes is imme- diately detected on comparison with the other. And, considering the care that has been taken in ﬁlling the tubes. and setting off the

voL. IV. 3