Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 19, Scripture-SUG

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ecific dividing the weight by the weight of the ftandard. The
: avity. quotient is the ipeeific gravity of the fluid. But in all
r'Y 1 ■■J other cales this is a very difficult problem : it requires
very nice hands, and an accurate eye, to make two
bodies of the fame bulk. An error of one hundredth
part in the linear dimenfions of a folid body makes an
error of a 30th part in its bulk ; and bodies of irregular
fliapes and friable fubifanee, fueh as the ores of metals,
cannot be brought into convenient and exact dimenfions
for meafure merit.
From all thefe inconveniences and difficulties we are
freed by the celebrated Archimedes, who, from the prin¬
ciples of hydroltatics difcovered or eftabliffied by him,
deduced the accurate and eafy method which is now
univerfally pradlifed for difcovering the fpecific gravity
and denfity of bodies. (See Archimedes and Hy¬
drodynamics). Inftead of meafuring the bulk of
the body by that of the dilplaced fluid (which would
have been impoffible for Archimedes to do with any
thing like the neceffary preeifion), we have only to ob-
ferve the lofs of weight fuftained by the folid. This can
be done with great eale and exactnef-. Whatever may
be the bulk of the body, this lofs of weight is the
weight of an equal bulk of the fluid •, and we obtain the
fpeeifie gravity of the body by Amply dividing its whole
weight by the weight loit : the quotient is the fpecific
gravity when this fluid is taken for the ftandard, even
though we Ihould not know the abfolute weight of any
given bulk of this ftandard. It alfo gives us an eafy
and accurate method of afcertaining even this fundamen¬
tal point. We have only to form any folid body into
an e.xadf cube, fphere, or prifm, of known dimenfions,
and obferve w hat weight it lofes when immerfed in this'
ftandard fluid. This is the weight of the fame bulk of
the ftandard to be kept in remembrance *, and thus we
obtain, by the bye, a moft eafy and accurate method
for meafuring the bulk or folid contents of any body,
however irregular its fhape may be. We have only to
fee how much weight it lufes in the ftandard fluid ; we
can compute what quantity of the ftandard fluid will
have this weight. Thus ffiould we find that a quantity
of fand, or a furze bufh, lofes 250 ounces when im¬
merfed in pure water, we learn by this that the folid
meafure of every grain of the fand, or of every twig
and prickle of the furze, when added into one fum,
amounts to the fourth part of a cubic foot, or to 432
cubic inches.
To all thefe advantages of the Archimedean method
of afcertaining the fpecific gravity of bodies, derived
Irom his hydroftatical doftrines and difcoveries, rve may
add, that the immediate ftandard of comparifon, namely,
water, is, of all the fubftances that we know, the fitteft
for the purpofe of an univerfal ftandard of refere nce.
In its ordinary natural ftate it is fufficiently eonftant and
uniform in its weight for every examination where the
utmoft mathematical accuracy is not wanted ; all its
variations arife from impurities, from which it may at
all times be feparated by the Ample procefs of diiiilla-
tion : and we have every reafon to think that when
pure, its denfity, when of the fame temperature, is in¬
variable.
Water is therefore univerfally' taken for the unit of
that fcale on which we meafure the fpecinc gravity of
bodies, and its weight is called 1. The fpecific gravity
of any other body is the real weight in pounds and Specific
ounces, when of the bulk of one pound or one ounce of gravity.
water. It is therefore of the firft importance, in all " v
difeuflions refpetiing the fpecific gravity of bodies, to
have the precife weight of fome known bulk of pure
■water. We have taken fome pains to examine and com¬
pare the experiments on this fubjeft, and ffiall endea¬
vour to afeertain this point with the precifion which it
deferves. W e ffiall reduce all to the Engliih cubic foot
and avoirdupois ounce of the Exchequer ftandard, on
account of a very convenient circumftance peculiar to
this unit, viz. that a cubic foot contains almoft precifely
a thoufand ounces of pure water, fo that the fpecific
gravity of bodies exprefles the number of fuch ounces
contained in a cubic foot.
We begin with a trial made before the houfe of com¬
mons in 1696 by Mr Everard. He weighed 2145.6
cubic inches of water by a balance, which turned fen-
fibly with 6 grains, when there were 3© pounds in each
fcale. The weights employed were the troy weights,
in the depofit of the Court of Exchequer, which are ftill
preferved, and have been moft fcrupuloufly examined
and compared with each other. The weight was 1131
ounces 14 pennyweights. This wants juft 11 grains of
a thoufand avoirdupois ounces for 1728 cubic inches, or
a cubic foot 5 and it would have amounted to that
weight had it been a degree or two colder. The tem¬
perature indeed is not mentioned 5 but as the trial was
made in a comfortable room, we may prefume the tem¬
perature to have been about 550 of Fahrenheit’s ther¬
mometer. The dimenfions of the veflel were as accu¬
rate as the nice hand of Mr Abraham Sharp, Mr Flam-
ftead’s afliftant at Greenwich, could execute, and it was
made by the Exchequer ftandard of length.
1 his is confided in by the naturalifts of Europe as a
very accurate fiandard experiment, and it is confirmed
by many others both private and public. The ftandards
of weight and capacity employed in the experiment are
ftill in exifter.ee, and publicly known, by the report of
the Royal Society to parliament in 1742, and by the
report of a committee of the houfe of commons in 17 58.
This gives it a fuperiojity over all the meafures which
have come to our knowledge-.
The firft experiment, made with proper attention,
that we meet with, is by the celebrated Snellius. about
the year 1615, and related in his Eratq/lhenes Batuvus.
He weighed a Rhinland cubic foot of diftilled water,
and found it 62.79 Amfterdam pounds. If this was the
ordinary weight of the {hops, containing 7626 Engliflr
troy grains, the Engliffi cubic foot muft be 62 pounds
9 ounces, only one ounce more than by Everard’s expe¬
riment. If it was the Mint pound, the weight was 62
pounds 6 ounces. The only other trials which can come
into competition with Mr Everard’s are fame made by
the Academy of Sciences at Paris. Picart, in 1691,
found the Paris cubic foot of the water of the fountain
d’Arcueil to weigh 69.588 pounds, poids de Paris. Hu
Hamel obtained the very fame refult ; but Mr Monge,
in 1783, fays that filtered rain-water of the temperature
I 20 (Reaumur)' weighs 69.3792. Both thefe meafures
are confiderably below Mr Everard’s, which is 62.^,
the former giving 62.053, and ^6 latter 61.868. M.
Lavoifier ftates the Paris cubic foot at 70 pounds, which
makes the Englifti foot 62.47. ^ut there is an incon-
fiftency