II YD 
of the case, and commit themselves to the 
water. 
HYDROPHOBIA, in medicine, an 
aversion or dread of water ; a terrible symp- 
tom of the rabies canini. See Medicine. 
IIYDROPHYLAX, in botany, a genus 
of the Tetrandria Monogynia class and or- 
der. Natural order of Rubiaceae, Jussieu. 
Essential character : calyx four-parted ; co- 
rolla funnel-form ; fruit ancipital, one-seed- 
ed. There is only one species, viz. H. ma- 
ritima, found in driving-sand, on the sea- 
shore, near Guduluhr in the East Indies. 
HYDROPHYLLUM, in botany, water- 
leaf, a genus of the Pentandria Monogynia 
class and order. Natural order of Bora- 
gineae, Jussieu. Essential character : co- 
rolla bell-shaped, having five longitudinal 
melliferous streaks on the inside ; stigma bi- 
fid ; capsule globular, two-valved. There 
are two species, viz. H. virginicum, Virgi- 
nian water-leaf ; and H. canadense, Cana- 
dian water-leaf. 
HYDROSCOPE, an instrument an- 
ciently used for the measuring of time. 
The hydroscope was a kind of water-clock, 
consisting of a cylindrical tube, conical at 
bottom: the cylinder was graduated, or 
marked out with divisions, to which the top 
of the water, becoming successively conti- 
guous, as it trickled out at the vertex of the 
cone, pointed out the hour. 
HYD 
HYDROSTATICAL balance, a kind of 
balance contrived for the easy and exact 
finding the specific gravities of bodies, both 
liquid and solid. See Hydrostatics. 
HYDROSTATICS relate to the resting 
equilibrium of non-elastic fluids; and to the 
pressure of solids immersed therein. A 
fluid is a body whose parts are infinitely 
minute, capable of dislocation in conse- 
quence of the smallest force, invariably, 
(when suffered to rest), resuming a perfect 
level surface, and presenting an equal re- 
sistance/throughout every part to the body 
immersed. 
Philosophers consider fluids to be divided 
into two classes, viz. the elastic, such as 
air, vapour, and gas; all which may be 
compressed more or less : and the inelastic, 
viz. water, mercury, spirits, &c. which can- 
not be compressed ; though by being heat- 
ed they distend considerably. It may be 
proper to observe in this place, that Mr. 
Canton in the years 1762 and 1764, pub- 
lished the results of experiments he had 
made, whereby it was endeavoured to be 
proved, that all fluids were compressible 
though in so trifling a degree as not to 
affect their bulks when under examination. 
With the barometer at 29§, and the ther- 
mometer at 50, he declares the following 
compressions were effected. 
Spec. Grav. Compression 
With Spirit of wine 846 66 parts in a million 
Oil of olives 918 48 ditto 
Rain water 1,000 46 ditto 
Sea water 1,028 40 ditto 
Mercury 15,595 3 ditto 
We leave the reader to judge whether 
it be probable by any apparatus of human 
formation, and under human guidance, to as- 
certain that the three millionth parts, said, 
to have been compressed, were really so. 
Indeed, even the sixty-sixth millionth parts, 
suffered to be compressed in the spirits of 
wine, must appear extremely doubtful ; 
though we cannot but conclude that, as 
air exists in every atom of nature, more 
or less, with a sufficient force every fluid 
were subject to compression into a smaller 
space than is occupied by it when perfectly 
at liberty. Speaking generally, the defini- 
tions above given may be considered as 
applicable to all cases with which we are 
acquainted ; and may, perhaps, be com- 
pletely true', 
\ 
We shall commenee the detail, incident 
to this subject, with an account of the 
method of obtaining the specific gravities 
of bodies : that is, by shewing the com- 
parative weights of various solids, and 
fluids, as ascertained by the most careful 
and skilful chemists. The reader must, 
however, consider the weights as taken at 
a medium. See Gravity, specific, where 
is given a table of specific gravities. 
The reader will observe, that the whole 
of the above are compared with rain-water, 
which appear at 1,000 parts ; but it is very 
remarkable that the density of that fluid 
varies greatly according to its temperature ; 
and that it by no means affords a regular 
scale of weight, or of bulk in proportion 
to the degrees of heat. This will be seen 
