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a study, with those who court an intimate 
acquaintance with hydraulics. 
Fig. 18, shows the section' of three forcing 
pumps, o p q, with their pistons, as acted 
upon by three cranks, a b c, each equally 
radiated from the branch d, e, and moved 
by a water wheel, of which / 1 is the axis : it 
is plain that the several cranks stand at an 
angle of 120 degrees respectively. By this 
means there is a counterbalance among 
them mutually, and each gives one stroke 
or plunge during each revolution of the 
wheel. If the wheel is large, it will of 
course move slowly ; and, unless the pumps 
be very large, but little water will be raised : 
therefore it is usual to accelerate the motion 
of the branch bearing the cranks, by means 
of a spur, or of a trundle, turned by the 
water-wheel, and bearing such proportion 
thereto as the required increase of velocity 
may demand. For the manner of applying 
such a spur, &c. see the article Mill- 
Work. 
HYDRAULICQN, water organ, in mu- 
sic, an instrument acted upon by water, 
the invention of which is said to be of higher 
antiquity than that of the wind organ. 
HYDROCELE, in surgery, denotes any 
herhia arising from water, but is particu- 
larly used for such a one of the scrotum, 
which sometimes grows to the size of one’s 
head, without pain, but exceeding t rouble- 
some to the patient. See Surgery. 
HYDROCEPHALUS, in surgery, a pre- 
ternatural distention of the head to an un- 
common size, by a stagnation and extrava- 
sation of the lymph, which, when collected 
within side of the bones of the cranium, the 
hydrocephalus is then termed internal ; as 
it is external, when retained between the 
common integuments and the cranium. See 
Medicine. 
HYDROCHARIS, in botany, a genus of 
the Dioecia Enneandria class and order. 
Natural order of Palm®. Hydrocharides, 
Jussieu. Essential character : male, spathe 
two-leaved ; calyx, trifid ; corolla, three- 
petalled ; filaments the three inner style 
bearing : female, calyx trifid ; corolla 
three petal led ; styles six ; capsule six-cel- 
led, many seeded, inferior. There is but 
one species, with many varieties, viz. H. 
morsus ran®, frog bit. 
HYDROCOTYLE, in botany, marsh 
pennywort, a genus of the Pentandria Digy- 
nia class and order. Natural order of Um- 
bellate. Essential character : umbel sim- 
ple, with a four-leaved involucre ; petals 
HYD 
entire; seeds semi-orbiculate, compressed. 
There are fifteen species. 
HYDRODYNAMICS treat of the pow- 
ers, forces, and velocities, of fluids in mo- 
tion. Having entered fully into the de- 
tail of all relating thereto while treating of 
Fluid, Hydraulics, Hydrostatics, 
Mills, and Water wheels, we forbear 
from that repetition which would trespass 
on the space allotted to other articles, re- 
ferring the reader to those heads for what 
appertains thereto. 
HYDROGEN. It had been long known 
to the chemists, that a vapour or air is dis- 
engaged in some processes, which kindled 
on the approach of an ignited body. Van 
Helmont gave this the name of gas igneum, 
and it seems to have attracted the attention 
of Boyle, Mayow, and Hales. The che- 
mists knew, that such a vapour or air was 
commonly disengaged during the solution 
of certain metals, in muriatic or dilute sul- 
phuric acid, that it burnt at the mouth of 
the phial, and if mixed with atmospheric 
air, exploded when kindled by a match. 
Mr. Cavendish, however, first examined 
its properties fully, shewed that it is per- 
manently elastic, not absorbed by water, 
and that it is much lighter than atmosphe- 
ric air. (Philos. Trans, vol. Ivi. p. 141 ). 
This substance forming water when com- 
bined with oxygen, and being therefore the 
radical of that compound, the name hydro- 
gen was given to it, at the formation of the 
new nomenclature. 
It is always obtained from the decompo- 
sition of water, as it cannot, from other sub- 
stances in which it exists, be easily disen- 
gaged in perfect purity. Some substance 
is made to act on water, which exerts an 
attraction to the oxygen, without combin- 
ing with the hydrogen, when, of course, the 
hydrogen is disengaged, and passes into the 
elastic form. 
At the common temperature of the globe, 
this decomposition cannot be effected with 
rapidity by any single affinity. Iron, moist- 
ened with water, decomposes it very slowly, 
and evolves hydrogen ; but at the tempera- 
ture of ignition, the decomposition is more 
rapid. If a coil of iron wire, or a quantity 
of iron filings be put into an iron or coated 
glass, or earthern tube, which is placed 
across a small furnace, and surrounded with 
burning fuel, so as to be brought to a red 
heat, on distilling water from a retort con- 
nected with it, the vapour, in passing over 
the surface of the ignited iron, is decom- 
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