703 MECHANICS. 
inch thick going through, and foldered into the hollow 
copper ball, B b. The upper ball of this wire is filed flat 
on one fide, for the Item of the hydrometer, with a mark 
at m, to which it (inks exattly in proof-fpirits, There 
are two other marks, A and B, at top and bottom of the 
Item, to (how whether the liquor be -j~j above proof, as 
when it finks to A, or-j? 3 under proof, as when it emerges 
to B, when a brafs weight, fuch as C, has been fcrew-ed 
on, to the bottom at c. There are a great many fuch 
weights of different fixes, and marked to be fcrewed on, 
inftead of C, for liquors that differ more than' from 
proof, fo as to ferve for the fpecific gravities in all fuch 
proportions as relate to the mixture of fpirituous liquors, 
in all the variety made ufe of in trade. There are alfo 
other balls for (flowing the fpecific gravities quite to com¬ 
mon water, which makes the inftruraent perfect in its 
kind. P/iil. TravJ. vol. xxxvi. p. 277. 
Dr. Defaguliers, who communicated the above inftru- 
vnent to the Royal Society, himfelf contrived an hydro- 
sneter for determining the fpecific gravities of different 
waters to fuch a degree of nicety, that it would (how 
when one kind of water was but (he 40,000th part hea¬ 
vier than another. It confifts of a hollow glafs ball of 
about three inches in diameter, charged with (hot to a 
proper degree; and, having fixed in it a long and very 
(lender wire, of only the 40th part of an inch in diame¬ 
ter, and divided into tenths of inches, each tenth anfwer- 
ing to the 40,000 part, as above. See his Exper. Pkilof. 
voi. ii. p. 234. 
The hydrometer or aerometer, invented by M. de par- 
cieux in 1766, and prefented to the Academy of Sciences, 
is very Ample in its Itrufture, and at the fame time very 
convenient in application, and furniihin.g very accurate 
refults. This inltrument confiffs of a (mall glafs phial, 
about two inches, or at molt two inches and a half, in di¬ 
ameter, and (even or eight inches long. Its bottom muff 
not be bent inwards as bottles and phials commonly are, 
left air fhould be lodged in the cavity when it is immerfed 
in any liquid. The mouth is clofed with a very tight 
cork (iopper, into which is fixed, without palling through 
it, a very ftraight wire (of iron or of brafs) about a line 
in diameter, and twenty-eight or thirty inches long. The 
bottle is then loaded in Inch a manner, by introducing 
into it fmall grains of (hot, that the inltrument when im- 
merfed in the lighted of the liquids to be compared, finks 
fo as to leav.e only the end of the wire above its furface, 
while in the heavieft the wire is immerfed fome inches. 
This may be properly regulated either by augmenting or 
diminifiiing the weight in the bottle ; or by attaching a 
little difh or bafin to the top of the wire, and changing 
the weights in it; or by varying the thicknefs of the wire. 
To perform experiments of this kind it will be neceffary 
to have a veffel for the reception of the fluid to be tried ; 
a convenient fize would be a cylinder of three or four 
inches in diameter, and as many feet in length; it may 
be made of tin plate ; and a fcale divided into inches and 
lines may be carried up above one fide of the veffel, which 
will ferve to meafure the different depths of immerfion in 
the fluid, by noting the divifion on the fcale againft which 
the top of the wire hands. M. de Parcieux recommends 
the ufe of two inftruments ot this kind at once, in order 
that two fluids may be compared at the fame inttant. 
The aerometer, when thus conftru&ed, will exJiibit very 
fenfibly the leaft difference in the fpecific gravities of dif¬ 
ferent liquors, or the changes which the fame liquor may 
experience, in this refpeci, under different circumltances. 
Indeed M. Montucla flays, “ We have fleen an inltrument 
of this kind, the motion of which was fo fenfible, that, 
•when immerfed in water cooled to the ufual temperature, 
it funk feveral inches while the rays of the fun fell upon 
the water,and immediately rofe on the rays of that lumi¬ 
nary being intercepted. A very fmall quantity of fait or 
fu gar thrown into the water made it alfo rife fome inches.” 
Jlutton's Montucla's Recreations, vol. ii. p. 25. 
Mr. hlicholfon has made an improvement by which the 
hydrometer is adapted to the general purpofe of finding 
the fpecific gravity both of folids and fluids. A, fig. 14. 
is a hollow hall of copper ; B, a difh affixed to the ball by 
a (hort (lender ftein D ; C is another difh affixed to the 
oppofite fide of the ball by a kind of ftirrup. In the in¬ 
ltrument actually made, the (lem D is of hardened (feel, 
40 of an inch in diameter, and the difh C is fo heavy as in 
all cafes to keep the Item vertical when the inftrument is 
made to float in any liquid. The parts are fo adjulted, 
that the addition of 1000 grains in the upper difli B, will 
juft fink it in diflilled water (at the temperature of 6o° of 
Fahrenheit’s thermometer) fo far that thefurface (hall in- 
.terfeft the middle of the ftem D. Let it now be required 
to find the fpecific gravity of any fluid : Immerfe the in¬ 
ltrument in it, and by placing weights in the difli B caufe 
it to float, fo. that the middle of its ftem D (hall be cut by 
the furface of the fluid. Then, as the known weight of 
the inftrument, a'dded to 1000 grains, is to the fame known 
weight added to the weights uied in producing the lalfc 
equilibrium, fo is the weight of a quantity of diftilled wa¬ 
ter difplaced by the floating inltrument, to the weight of 
an equal bulk of the fluid under examination. And thefe 
weights are in the direct ratio of the fpecific gravities. 
Again, let it be required to find the fpecific gravity of 
a folid body whofe weight is lefs than 1000 grains. Place 
the inltrument in diftilled water, and put the body in the 
dilh B. Make the adjuftment of finking the inltrument 
to the middle of the (lem, by adding weights in the fame 
dilh. Subtract thofe weights from 1000 grains, and the 
remainder will be the weight of the body. Place now 
the body in the lower dilh C, and add more weight in the 
upper dilh B, till the adjuftment is again obtained. The 
weight lalt added will be the lofs the folid fultains by im¬ 
merfion, and is the weight of an equal bulk of water. 
Confequently the fpecific gravity of the folid is to that 
of water, as the weight of the body to the lofs occafioned 
by the immerfion. Mr. Nicholfon lays, “This inftru¬ 
ment was found to be fufficiently accurate to give weights 
true to lefs than one-twentieth of a grain.” Nicholfon’s 
Philofophy, vol. ii. p. 16. 
Gravimeter is the name given by M. Guyton to an in¬ 
ftrument for meafuring fpecific gravities. He adopts this 
name rather than either aerometer or hydrometer, becaufe 
thefe latter terms are grounded upon the fuppolition that 
the liquid is always the thing weighed ; whereas, with 
regard to folids, the liquid is the known term of compa¬ 
nion to which the unknown weight is referred. Guy¬ 
ton’s gravimeter is executed in glafs, and is of a cylindri¬ 
cal form, being that which requires the fmalleft quantity 
of fluid, and is on that account preferable, except fo far 
as it is neceffary to deviate for the fecurity of a vertical 
pofition. Like Nicholfon’s hydrometer, it carries two ba¬ 
lms ; one of them fuperior, at the extremity of a thin 
ftem, towards the middle of which the fixed point of im¬ 
merfion is marked. The other, or lower bafin, termi¬ 
nates in a point; it contains the ballaft, and is attached 
to the cylinder by two branches. The moveable fufpen- 
lion by means of a hook has the inconvenience of (hort- 
ening the lever which is to fecure the vertical pofition. 
The cylinder is 22 millimetres (0^71 inch) in diameter 
and 21 centimetres (6 85 inches) in length. It carries in 
the upper bafin an additional conftant weight of 5 gram¬ 
mes (115 grains). Thefe dimenfions might be increafed 
fo as to render it capable of receiving a much more con- 
liderable weight; but this is unneceffary. M. Guyton 
has added a piece which he calls the plongeur , becaufe in 
fndt it is placed in the lower bafin when uled, and is con¬ 
fequently entirely immerfed in the fluid. It is a bulb of 
glafs loaded with a fufflcient quantity of mercury, in or¬ 
der that its total weight may be equal to the conftant ad¬ 
ditional weight, added to the weight of the volume of wa¬ 
ter difplaced by this piece. It will be readily underftood 
that, the weight being determined at the fame temperature 
at which the inftrument was originally adjulted, it will 
dink to the fame mark on the Item, whether it be loaded 
with 
