Gr R A 
G It A 
G R A 8^9 
a moveable ruler or index, ED, with two [ 
sights P, P ; which turns round the centre, • 
and retains any situation given it. See Plate ' 
Misct 1. fig. 108. i 
To measure by this instrument any angle 
ACB (fig. 109), in any plane, and compre- 
hended between the right lines AC and BC, 
drawn from two points A and B, to the place 
of station C. Let the graphoineter be placed 
at C, supported by its fulcrum ; and let the 
immoveable sights on the diameter of the in- 
strument DE, be directed towards the point 
A ; and likewise while the instrument remains 
immoveable, let the sights of the ruler FG, 
which is moveable about the centre C, be \ 
directed to the point B, Now it is evident I 
that the moveable ruler cuts off an arch DG, 
which is the measure of the angle ACB, 
sought. Moreover, by the same method, the i 
inclination of DE, or FG, may be observed j 
with the meridian line, which is pointed out j 
by the magnetic needle inclosed in the box, | 
and moveable about the centre of the instru- i 
ment. ! 
GRAPNELS, a sort of anchors with four ! 
Hooks, serving for boats to ride by. 
There is also a kind called fire and chain- 
grapnels, made with four-barbed claws in- j 
stead of flooks, and used to catch hold of the 1 
enemies’ rigging, or any other part., in order 
for boarding them. See Plate Miscel. fig. 
110 . 
GRASS. See Botany. For the culture 
of the different sorts of grass, see Husband- 
ry. 
GRASSHOPPER, in zoology. See 
Gryllus. 
GRATINGS, in a ship, a kind of lattice- 
work formed of ledges and battens, the square 
holes of which baing three or four inches 
wide, are for the current footing of men over 
hatchways, to give air below, and vent for the 
smoke in an engagement. 
G RATIO LA, hedge-hyssop, a genus of the 
monogynia order, in the diandria class of 
plants. The corolla is irregular ; there are 
two barren stamina ; the capsule is bilocular ; 
the calyx has seven leaves, with the two ex- 
terior ones patulous. There are 12 species ; 
the most remarkable of which is the officina- 
lis, or common hedge-hyssop. This grows 
naturally in the Alps and other mountainous 
parts of Europe. It has a thick, fieshy, 
fibrous, creeping root, which propagates very 
much, when planted in a proper soil and 
situation. From this arise several upright 
square stalks, with narrow spear-shaped 
leaves, placed opposite. The flowers are 
produced on the side of the stalks at each 
foint ; they are shaped like those of the fox- 
glove, but are small, and of a pale-yellowish 
colour. This herb has an emetic and purga- 
tive virtue ; to answer which intentions it 
was formerly used by the common people in 
England, but was never much prescribed by 
the physicians, and at last fell totally into dis- 
use. Of late, however, it has been the sub- 
ject of a dissertation by Dr. James Kostr- 
zewski, of Warsaw, in Poland, who gives 
some remarkable accounts of its effects in 
mania and obstinate venereal cases. It was 
given in powder, or in extract, to the quan- 
tity of half a drachm of the first, and a whole 
drachm of the second, at each dose. 
GRAVE, in music, is applied to a sound 
which ig of alow os deep tone. 
GRAVEL, in natural history and garden- 
ing, a congeries of pebbles, which, mixed 
with a stiff loam, makes lasting and elegant 
walks ; an ornament peculiar to our gardens, 
and which gives them the advantage over 
those of all other nations. 
Gravel. See Medicine. 
GRAVIMETER, the name given by M. 
Guyton to an instrument for measuring spe- 
cific gravities : he adopts this name rather 
than either areometer or hydrometer, be- 
cause these latter terms are grounded upon 
the supposition that a fluid is always the 
thing weighed ; whereas, with regard to so- 
lids, the liquid is the known term of compa- 
rison to which the unknown weight is re- 
ferred. 
Guyton’s gravimeter is executed in glass, 
and is of a cylindric form, being that which 
requires the smallest quantity of fluid, and is 
on that account preferable, except so far as 
it is necessary to deviate for the security of a 
vertical position. It carries two basins ; one 
of them superior, at the extremity of a thin 
stem, towards the middle of which the fixed 
point of immersion is marked. The other, 
or lower basin, terminates in a point ; it con- 
tains the balls, and is attached to the cylin- 
der by two branches. The moveable sus- 
pension by means of a hook has the inconve- 
nience of shortening the lever which is to se- 
cure the vertical position. 
The cylinder is 22 millimetres (0.71 inch) 
in diameter; and 21 centimetres (6.85 
inches) in length. It carries in the upper basin 
an additional, constant weight of five grammes 
(115 grains). These dimensions might 
be increased so as to render it capable of re- 
ceiving a much more considerable weight ; 
1 but this is unnecessary. M. Guyton has 
added a piece which he calls the plongeur, 
because in fact it is placed in the lower basin 
when used, and is consequently entirely im- 
mersed in the fluid. It is a bulb of glass 
leaded with a sufficient quantity of mercury, 
in order that its total weight may be equal to 
the constant additional weight, added to the 
weight of the volume of water displaced by 
j this piece. It will he readily understood that 
the weight being determined at the same tem- 
perature at which the instrument was origi- 
nally adjusted, it will sink to the same mark 
on the stem, whether it is loaded with a con- 
stant additional weight in the upper basin, or 
whether the effect of this weight be produced 
by the additional piece in the lower dish. 
From this explanation there will be no diffi- 
culty in seeing how this instrument may be 
adapted to every case in practice. 
It may be used, 1 . for solids. The only 
condition will be, that the absolute weight of 
the body to be examined shall be rather less 
than the constant additional weight, which in 
this instrument is five grammes, or 115 
grains. 
2. For liquids of less specific gravity than 
water the instrument, withoqt the additional 
weight above-mentioned, weighs about two 
decagrammes (459 grains) in the dimensions 
before laid down. It would be easy to limit 
its weight to the utmost accuracy. We have 
therefore the range of one-fifth of buoyancy, 
and consequently the means of ascertaining 
all the intermediate densities from water to 
the most highly rectified -spirit of 'wins, which 
10 
is known to bear in this respect the ratio of 
eight to ten with regard to water. 
3. When liquids of greater specific gravity 
than water are to be tried, the constant 
weight being applied below, by means ot the 
additional piece, which weighs about six 
grammes (138 grains), the instrument can re- 
ceive in the upper basin more than four tinier 
the usual additional weight, without losing' 
the equilibrium of its vertical position. In 
this state it is capable of shewing the specific 
gravity of the most concentrated acids. 
4. It possesses another property, namely, 
that it may be used as a balance to determine 
the absolute weight of such bodies as do no^ 
exceed its additional load. 
5. Lastly, the purity of the water being 
known, it will indicate the degrees of rare- 
faction and condensation in proportion to its 
own bulk. 
This instrument may be readily construct- 
ed by any workman in glass. The additional 
piece for the lower basin will require some 
attention to make it perfectly agree with the 
constant upper weight, as to the immersion 
of the instrument. But this object may, by 
careful adjustment, be ascertained with the 
utmost certainty and accuracy. The bulb- 
of glass is for this purpose drawn out to a tine 
point ; a sufficient quantity of mercury is 
then introduced to sink it, and the aperture 
closed with a little piece of wax. The bulb 
being then placed in the lower basin of the. 
instrument, the upper basin is to be loaded 
until the mark on the stem becomes accurate- 
ly coincident with the surface oi the water. 
The sum of the weights added above is pre- 
cisely equal to that of the quantity of mer- 
cury necessary to be added to that in the 
glass bulb ; which done, nothing more is 
needed than to seal the point by fusion, taking 
care not to change its bulk. 
The whole is rendered portable by means 
of a case in which all the delicate parts are 
secured from pressure, and the heavier parts 
supported in such a manner as to resist the 
excess of motion they are capable of acquir- 
ing by virtue of their mass. this last cii- 
cumstance is frequently overlooked by such 
workmen as are employed in the package ot 
instruments; whence it necessarily follows, 
that some strain or fracture must be produced 
when matters of very unequal density are ex- 
posed to receive a common impulse. 
To find the specific gravity of any solid by 
the gravimeter, observe this rule: “from 
the weight in the upper dish, when the in- 
strument is properly immersed in the un- 
known fluid, take the weight which is placed 
with the body in the same scale at the like- 
adjustment. The remainder is the absolute 
weight of the solid. Multiply this by the 
specific gravity of the fluid, and reseive the 
product. From the additional weight when 
the body is placed in the lower basin, take 
the weight when it was placed. in the uppei . 
The remainder will be the loss of weight by 
immersion. Diride the reserved product b> 
the loss by immersion, and the quotient will 
be the specific gravity of the solid with regard 
to distilled water at the standard temperature 
a,nd pressure.” . . . 
To find the specific gravity of a fluid pro- 
ceed thus: “To the weight of the gravime- 
ter add the weight required in the upper ba- 
sin to sink it in the unknown liuid. Again, 
rto the weight of the gravimeter add. th» 
