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XXIV. Description of a new Barometer, recently fixed up in the Apartments 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. 
Received October 25, — Read November 16, 1837. 
The Barometer here alluded to may in some measure be considered as two sepa- 
rate and independent barometers, inasmuch as it is formed of two distinct tubes dip- 
ping 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 at- 
tached, 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 discern- 
ible* ; and the other end of which bears the scale of inches, on which I have set off 
with great accuracy, from the standard scale of the Royal Astronomical Society, the 
distance of 30 inches from the above-mentioned agate point. Above and below this 
mark of 30 inches, the usual scale of inches, tenths, &c. is engraved ; and there is a 
separate vernier for each tube. A piece of thin brass projects from the zero point of 
each vernier, across its contiguous tube, which, when the height of the mercury is 
read off, is brought down so that the lower edge of it forms a tangent to the column 
of mercury, in the usual manner. 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 per- 
pendicularity 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 usually formed for general use in this 
country. The absolute heights are more correctly and more satisfactorily determined ; 
and the permanency 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 
* The motion of this rack-work is much too slow, and might be greatly improved if made more rapid. 
