HYDROMETER. 
dency be to rise, the fluid will stand round 
the stem in a small protuberance. The 
operator must assist this tendency by apply- 
ing the pincers, with which he takes up his 
weights to the rim of the dish. It is very 
easy to know when the surface of the fluid 
is truly flat, by observing the reflected 
image of the window, or any other fit ob- 
ject seen near the stem in the fluid. In 
this way the adjustment of the weights in 
the dish may, without difficult} 7 , be brought 
to the fiftieth part of a grain. If, there- 
fore, the instrument displace one thousand 
grains of water, the result will be very true 
to four places of figures, or even to five. 
This will be as exact as most scales are ca- 
pable of affording. 
Some writers have spoken of the adjust- 
ment of an hydrometer of this kind, so that 
it shall at some certain temperature dis- 
place one thousand grains of water, as if 
this were a great difficulty. It is true, in- 
deed, that the performance of a piece of 
workmanship of this nature would require 
both skill and judgment on the part of the 
artist ; but it is by no means necessary. 
Nothing more is required on the part of 
the workman, than that the hydrometer 
shall be light enough to float in ether, and 
capable of sustaining at least one-third of 
its own weight in the dish, without over- 
setting in a denser fluid. This last requisite 
is obtained by giving a due length to the 
stem beneath, to which the counterpoise is 
attached. With such an instrument, what- 
ever may be its weight, or the quantity of 
water it displaces, the chemist may pro- 
ceed to make his experiments, and deduce 
his specific gravities by the proportion be- 
fore laid down. Or to save occasional com- 
putation, he may once for all make a table 
of the specific gravities, corresponding to 
every number of the load in the dish, from 
one grain up to the whole number of grains, 
so that by looking for the load in one co- 
lumn, he may always find the specific gra- 
vity in the column opposite. 
This method is very ready and conve- 
nient in practice ; but if it be preferred, 
the weights may be adjusted to the hydro- 
meter, so as to show the specific gravity, 
without computation or reference. For 
this purpose the hydrometer must be pro- 
perly counterpoised in distilled w'ater, at 
the assumed standard temperature ; sup- 
pose 60°, and the whole weight of the 
instrument and its load called 1.000, &c. 
Then the weight of the instrument and 
its load must be separately determined in 
grains and parts, or other weights, by a 
good pair of scales, and as the whole 
weight of the instrument and its load is pro- 
portioned to the weight of the instrument 
alone, so will be the number 1.000, &c. to 
a fourth term expressing the weight of the 
instrument in such parts as make the whole 
1.000, &c. Make an actual set of decimal 
weights of which 1,000, &c. shall be equal to 
the hydrometer and its load ; and it is clear, 
that whatever may be the load in these 
weights, if it be added to the number de- 
noting the weight of the instrument, the 
sum will denote the specific gravity of the 
fluid, wherein the instrument floats with 
that load. 
By following the above easy method, it 
will be found that every hydrometer, where- 
soever made, must give the same results. 
The subject is indeed in itself sufficiently 
simple, and would require scarcely any dis- 
cussion, if it had not happened that many 
philosophers, for want of requisite atten- 
tion, have made their experiments with hy- 
drometers graduated on the stem by no 
certain rule by which operators, at a dis- 
tance from each other, might compare their 
experiments. The hydrometers, or pesd- 
liqueurs of Baurad, though in reality, com- 
parable with each other, are subject, in 
part, to the defect, that their results, hav- 
ing no independent numerical measure, re- 
quire explanation to those who do not know 
the instruments. Thus, for example, when 
a chemist acquaints us that a fluid indicated 
fourteen degrees of the pese-liqueur of 
Baum6, we cannot usefully apply this re- 
sult, unless we have some rule to deduce 
the correspondent specific gravity ; whereas 
we should not have been in any respect at 
a loss, if the author had mentioned the spe- 
cific gravity itself. As a considerable num- 
ber of French philosophers refer to this 
instrument, it will be of use to explain its 
principles. 
M. Bau me appears to have directed his 
attention chiefly to the acquisition of a 
means of making hydrometers with a gra- 
duated stem, which should correspond in 
their results, notwithstanding any differences 
in their balls or stems. There is little 
doubt but he was led into the method he 
adopted, by reflecting-. on that by which 
thermometers are usually graduated. See 
Thermometer. 
As thermometers are graduated, inde- 
pendent of each other, by commencing 
with an interval between two stationary 
points, of temperature, so M. Bauirie adopted 
two determinate densities, for the sake of 
marking an interval on the stem of his hy- 
i 
