, of different sizes, made by Mr
Fontin with the utmost nicety, and, excepting those made by Mr Ramsden
of London, I do not think any can compare with them for precision and
sensibility. The largest of these is about three feet long in the beam
for large weights, up to fifteen or twenty pounds; the second, for
weights of eighteen or twenty ounces, is exact to a tenth part of a
grain; and the smallest, calculated only for weighing about one gros, is
sensibly affected by the five hundredth part of a grain.

Besides these nicer balances, which are only used for experiments of
research, we must have others of less value for the ordinary purposes
of the laboratory. A large iron balance, capable of weighing forty or
fifty pounds within half a dram, one of a middle size, which may
ascertain eight or ten pounds, within ten or twelve grains, and a small
one, by which about a pound may be determined, within one grain.

We must likewise be provided with weights divided into their several
fractions, both vulgar and decimal, with the utmost nicety, and verified
by means of repeated and accurate trials in the nicest scales; and it
requires some experience, and to be accurately acquainted with the
different weights, to be able to use them properly. The best way of
precisely ascertaining the weight of any particular substance is to
weigh it twice, once with the decimal divisions of the pound, and
another time with the common subdivisions or vulgar fractions, and, by
comparing these, we attain the utmost accuracy.

By the specific gravity of any substance is understood the quotient of
its absolute weight divided by its magnitude, or, what is the same, the
weight of a determinate bulk of any body. The weight of a determinate
magnitude of water has been generally assumed as unity for this purpose;
and we express the specific gravity of gold, sulphuric acid, &c. by
saying, that gold is nineteen times, and sulphuric acid twice the weight
of water, and so of other bodies.

It is the more convenient to assume water as unity in specific
gravities, that those substances whose specific gravity we wish to
determine, are most commonly weighed in water for that purpose. Thus, if
we wish to determine the specific gravity of gold flattened under the
hammer, and supposing the piece of gold to weigh 8 oz. 4 gros 2-1/2
grs. in the air[55], it is suspended by means of a fine metallic wire
under the scale of a hydrostatic balance, so as to be entirely