260 Description of a Compensation Barometer, [May, 
fer 29-inches. 5). .20.4/. ser 33.8 
QS dittoer 1.2405. arcieto dee $2.5 
Dk OUEOn sto dk tdci Lawl 31.3 &e. 
But, should such length be inconvenient, advantage may be taken of 
tubes that are not quite cylindrical, by placing the tapering end upper- 
most, and calculating the effect of the excess of mercury below upon 
the range of the narrower part of the column: thus, in the instrument 
of which the drawing is given, 32 inches was the compensating 
length required. 
To prove that the indications of this instrument were equally trust- 
worthy with the equated results of a common barometer, a series of 
comparisons was made both with the Surveyor General’s standard 
instrument (through the medium of the printed register) and with an 
excellent Dottonp’s barometer placed close to mine: the results were 
as follows : 
My barometer lower than Surveyor General’s in June, 1832. —.043 
' in March, 1833. — 044 
Ditto lower than a Dollond’s barometer, ....in Nov. 1832. —.101 
in April 1833. —.102 
12 comparisons at 10 a. m. in May gave.... ...... —.093 Therm. 86.3 
10 Ditto at 4Pp.Mm._ ditto ditto.... ...... —.093 88.7 
These accordances at opposite seasons of the year are sufficient to 
establish confidence in the compensation barometer. I should add, that 
am indebted to Mr. Barrow, H. C. Instrument maker, for carrying 
my views into practice, and adapting the slow-motion screws and clamps 
in a neat and efficient manner. 
While on the subject of barometers, I would take occasion to can- 
tion all observers who are in the habit of using ENGLEFrELp’s open tubes, 
that they should only be filled with mercury when the air is in a very 
dry state. This remark was elicited by a series of experiments made 
by Lieutenant Waueu of the Engineers, when comparing his stock of 
ENGLEFIELD barometers, with my standard, before his departure on sur- 
vey to the hills of Amerkantak. 
One tube having been filled with every precaution was found to 
stand 0.211 lower than the standard Barometer. It was emptied and 
Tenbed sy Mtien S6QOUs, . che crt, os Sine 6 a6 sce a Sodas eee —.499 
A tiie Cie TERMCC is cielo wks Lise 5 cde ve u's vie wlclene —.609 
BP AOULGM oir see taistits NORM Eee sic ie) a, oa tet ee ce ee —.652 
CRS pee ep Tee ea oats -clvaie, pis (oc Uta each cae rata ee meee —.659 
Be SUREHES OSs TCO tS Seen Wah etr tee ete ceione ee Niete sisters —.653 
a seventh, eo eoeereore Peo ere oeHT oo POP OPE He OH HO EO —./00 
