242 New Form of Mountain or other Barometer. 
and diameter, the larger the cistern the less will be the vertical 
rise of the fluid in it, in consequence of the displacement by the 
float. If then we call 
W, the weight of the float—for instance in grains ; 
w, the weight of a cubic inch of mercury at any given tempe- 
rature ; 
4, the diameter of the cistern in inches; 
D, the diameter of the float Ki 
d, the external diameter of tube ‘ 
0, the internal diameter “ su 
And z, the circumference of a circle whose diameter is unity ; 
the sum of the correction will be 
Ww ( 1 1 
—([- osayt@rzem)) 5 
reducing to a common denominator, we have 
W . (42-402 —d?) 
ee eran (D2 —d?) ; (42 +5? —D?) 
If we neglect the change of specific gravity by a change of tem- 
perature, as we may in this case safely do, and take w=3426.56 
grs. (which corresponds to a specific gravity for mercury of 
13.5728 at 62° F., and very near 13.6 at the maximum density 
of water,) and give to z its numerical value of 3.14156, etc., the 
correction becomes in round numbers 
Wi. (42-0? =a?) 
—9691. (D?—d*).(4?-40°—D*) 
Such is the quantity to be subtracted from the reading, in order 
to give the height of the barometric column unaffected by the 
weight of the float. 
It only remains to be added, that in comparing the few barom- 
eters of this construction, which have so far been made, with 
some others, among which were those of Mr. Troughton, of Mr. 
Hassler, and of Mr. Green—I mean coming from the hands of 
the persons named—the former, after deducting the amount of 
correction, stood uniformly higher than any of the latter; which 
I attribute not so much to the individual precautions in securing 
a better vacuum as to the greater precision with which the zero 
of the measurement can be ascertained. 
Baltimore, Md., May 25, 1843. 
