258 Description of a Compensation Barometer, [May, 
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very great proper motion in 
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VIII.—Description of a Compensation Barometer, and Observations on 
Wet Barometers. By J. Prinsep, Sec. &c. 
Where a daily register of the Barometer is kept, it becomes a seri- 
ous labour to apply the correction for temperature to every observa- 
tion : this inconvenience has led to the suppression of the correction alto- 
gether in the tables published at the Surveyor General’s office; but who- 
ever may have occasion to use these valuable meteorological records must 
himself reduce the indications of the Barometric columns to the freezing 
point, and therefore little is gained by omitting the correction in the 
first instance. 
With a stationary barometer, in a climate liable to but small and 
regular alternations of atmospherical pressure, it is very easy to avoid 
all this labour, by attaching a compensation tube for the adjustment 
of the index point. I have been in the habit of using one with the 
instrument of which a register is kept at the Assay Office, and as it is 
very simple and easily made, I shall beg leave to describe it, referring 
to the drawing of it in fig. 3, Plate VIII. 
The height of the mercurial column in a barometer depends directly 
upon the weight of the atmosphere, and inversely upon the density, or 
specific gravity, of the quicksilver, which is liable to alteration by 
