PROCEEDINGS 



OF 



THE ROYAL SOCIETY. 



1837. 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 various periods, and an account of that which is now 

 adopted, for correcting the observed height of the mercury in the 

 Society'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 view to ascertain whether, at the end of 

 any given period, the one may have had any greater chemical effect 

 on the mercury than the other, and thus affected 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 be 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 vernier 

 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 be viewed 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 barometers 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 effectually 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. B 



