HYDROMETER. 
base; capsule two-celled, two-valved. There 
are four species. 
HYDRO MAN CY, a method of divina- 
tion by water, practised by the ancients in 
this manner : they filled a cup or bowl of 
water; then fastening a ring to a piece of 
thread tied to their finger, held it over the 
water, and repeated a certain form of 
words, desiring to be satisfied with regard 
to their enquiry; and if the question was 
answered in the affirmative, the ring would 
strike the sides of the bowl of its own ac- 
cord. 
HYDROMETER. The best method of 
weighing equal quantities of corrosive vola- 
tile fluids, to determine their specific gra- 
vities, appears to consist in inclosing them 
in a bottle with a conical stopper, in the 
side of which stopper a fine mark is cut with 
a file. , The fluid being poured into the 
bottle, it is easy to put in the stopper, be- 
cause the redundant fluid escapes through 
the notch or mark, and may be carefully 
wiped off. Equal bulks of water and other 
fluids, are by this means weighed to a great 
degree of accuracy, care being taken to 
keep the temperature as equal as possible, 
by avoiding any contact of the bottle with 
the hand or otherwise. The bottle itself 
shows, with much precision, by a rise or 
fall of the liquid in the notch of the stop- 
per, whether any such change has taken 
place. See Gravity, specific. 
But as the operation of weighing requires 
considerable attention and steadiness, and 
also a good balance, the floating instrument 
called the hydrometer, has always been 
esteemed by philosophers, as well as men 
of business. It consists of a hollow ball, 
either of metal or glass, capable of floating 
in any known liquid ; from the one side of 
the ball proceeds a stem, which terminates 
in a weight, and from the side diametrically 
opposite proceeds another stem, most com- 
monly of an equal thickness throughout. 
The weight is so proportioned that the in- 
strument may float with the last mentioned 
stem upright. In the less accurate hydro- 
meter this stem is graduated, and serves to 
sho w the density of the fluid, by the depth 
to which it sinks ; as the heavier fluids will 
buoy up the instrument more than such as 
are lighter. In this way, however, it is 
clear, that the stem must be comparatively 
thick, in order to possess any extensive 
range ; for the weight of vitriolic ether is 
not equal to three-fourths of the same bulk 
of water, and therefore such an hydrome- 
ter, intended to exhibit the comparative 
densities of these fluids, must have its stem 
equal in bulk to more than one fourth of the 
whole instrument. If this bulk be given 
chiefly in thickness, the smaller differences 
of density will not be perceptible, and it 
cannot, with any convenience, be given in 
length. 
To remedy this imperfection, yarious con- 
trivances have been proposed, for the most 
part grounded on the consideration, that a 
change in the ballast, or weight employed 
to sink the ball, would so far change the 
instrument, that the same, short range of 
gradations on a slender stem, which were 
employed to exhibit the densities of ardent 
spirit, might be employed in experiments 
upon water. Some have adjusted weights 
to be screwed upon the lower stem, and 
others, with more neatness and accuracy, 
have adjusted them to be slipped upon the 
extremity of the upper stem. But the me- 
thod of Fahrenheit appears to be on all 
accounts the simplest and most accurate. 
The hydrometer of Fahrenheit consists of 
a hollow ball, with a counterpoise below, 
and a very slender stem above, terminating 
in a small dish. The middle, or half length 
of the stem, is distinguished by a fine line 
across. In this instrument every division 
of the stem is rejected, and it is immersed 
in all experiments to the middle of the 
stem, by placing proper weights in the little 
dish above. Then as the part immersed is 
constantly of the same magnitude, and the 
whole weight of the hydrometer is known ; 
this last weight, added to the weights in 
the dish, will be equal to the weight of fluid 
displaced by the instrument, as all writers 
on hydrostatics prove. And accordingly 
the specific gravities for the common form 
of the tables will be had by the proportion. 
As the whole weight of the hydrometer and 
its load, when adjusted in distilled water, is 
to the number 1,000, &c. so is the whole 
weight, when adjusted in any other fluid, 
to the number expressing its specific gra- 
vity. 
In order to show the degree of accuracy 
an instrument of this kind is capable of, it 
may in the first place be observed, that the 
greatest impediment to its sensibility arises 
from the attraction or repulsion between 
the surface of the fluid and that of the stem, 
If the instrument be carefully wiped with 
a clean soft linen cloth, the metallic surface 
will be equally disposed to attract or repel 
the fluid. So that if it possess a tendency 
to descend, there will be a cavity surround- 
ing the stem ; or if, on the contrary, its ten- 
