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$70 Description of an Universal Hydrometer 
have at different periods been resorted to for ascertain- 
ing it. 
In point of accuracy, perhaps, the best mode of 
taking the specific gravity of a body is by a very good 
hydrostatic balance. This instrument, however, we 
may venture to affirm, can scarcely ever be obtained 
sufficiently perfect to be depended on for so nice a pur- 
pose. 
Persons who are in the habit of adjusting balances, and 
those who use them with considerable care, well know 
the various sources of error to which they are liable. 
The circumstance of the arms of a beam being in equili- 
brio, is no proof of its correctness, unless it will remain so 
when either loaded or unloaded, and with exchange of 
scale-pans. The necessity of having a piece of steel for 
the beam which shall be perfectly homogeneous ; the un- 
certainty with regard to the exact equality of the arms, 
in both weight and length ; and, even when very nicely 
adjusted, its liability to acquire polarity, and consequent 
derangement by magnetism ; the expansion of either arm 
by the heat of the hand, or its contraction by a current of 
air, renders those instruments extremely liable to give 
anomalous results. 
But supposing the balance not liable to error, it is too 
complicated in its use for any other than the man of sci- 
ence, in his closet, where time and close attention may 
be afforded ; and since the application of science to the 
arts has become so general, chemists, manufacturers of 
acids, brewers, dyers, distillers, and all others whose 
manufacture consists of any cheminal process, require a 
more simple and expeditious mode of ascertaining the 
specific gravity, and consequently the value of their arti- 
cles, than by the hydrostatic balance. Indeed, in many 
concerns its use would be impracticable, it being necessa- 
ry to intrust the business of examining the qualities of the 
