110 On a new Hydrostratic Balance. 
at one-third, 3, at one-fourth, .4, &e. Also at two-thirds the 
Jength is rriarked 14, at two-fifths, 24, &c. And so of all the 
fractions, sufficiently minutely. These numbers extend as high 
as the specific gravity of platina ;—the pivot of the instrument 
represents unity, and a notch is made at the further end. 
In using this instrument, any convenient weight is suspended 
bya hook from the notch at the end of the scale. The body un- 
der examination is to be suspended to the other end by a horse= 
hair, and slid along till an equilibrium is produced. It is then, 
without altering its situation on the beam, to be immersed in 
water, and balanced a second time by sliding the weight. The 
hook of the latter then marks the specifie gravity on the scale. 
The demonstration of this is very simple. The instrument 
being supposed in equilibrium, and BD (see figure) and the 
weight of the counterpoise being constant, the weight of the 
body varies as the distance of the counterpoise from "BS hy the 
common principle of the lever. Hence, if C be the place of the 
weight at the conclusion of the operation, 
Weight in water : weight in air:: BC: BA. And, by snb- 
traction, the loss of weight in water : weight in air:: AC: AB; 
and hence 
wt. in air AB 
a = AE = the spec. grav. ; which is the rule. @. E.D. 
Substances lighter than water may have, if necessary, their. 
specific gravity ascertained by the usual method; a scale of equal 
parts being cut on the opposite side of the beam, and the article 
to be weighed placed in a notch for the purpose. For mineralogy, 
however, this will seldom be necessary, The bottom of the notch 
A (at the smaller end) should be in a line with the edge of the 
scale, its sides being a little raised. The top of the shorter end 
should be rather the thickest part of it, to allow the horse-hair, 
by which the mineral is suspended, to swing clear. This mode 
will be found very delicate and accurate, and a hook must not 
be used, as it cannot be balanced. 
The instrument, in this form, is exceedingly compact, and may 
be reduced to a simple rod. 
The principle is capable of being applied {as in an instrument 
I have made) to an are of a circle, with a rod resembling in its 
application a common bent lever, 
oS == 
XXV. True apparent Right Ascension of ‘Dr. MaskELYNE’s 
36 Stars for every Day in the Year 1821. By the Rev. 
J. Groosy. 
[Continued from p. 52.] 
Argu- 
