QUADRANT. 
535 
the next below the projection lire the qua- 
drant and line of shadows. 
To find the time oj the sun’s rising or set- 
ting, his amplitude, his azimuth, hour of the 
day, fc. by this quadrant. Lay the thread 
on the day of tlie month, and bring the 
bead to the proper ecliptic, either of summer 
or winter, according to the season, which is 
called rectifying; then- by moving the thread 
bring the bead to the horizon, in which ca>e 
the thread will cut the limbin the point of the 
time of the sun’s rising or setting before or 
after 6; and at the same time the bead will 
cut the horizon in the degrees of the sun’s 
amplitude. Again, observing the sun’s alti- 
tude with the quadrant, and supposing it 
found to be 45° on the 5th of May, lay the 
thread over the 5th of May, then bring the 
bead to the summer ecliptic, and carry it to 
the parallel of altitude 45°; in which case the 
thread will cut the limb at 55° 15', and the 
hour will be seen among the hour-lines to be 
either 4 1 in. past 9 in the morning, or 19 m. 
past 2 in the afternoon. Lastly, the bead 
shews among the azimuths the sun’s distance 
from the south 50° 41'. 
But if the sun’s altitude is less than what 
it is at 6 o’clock, the operation must be per- 
formed among those parallels above tiie upper 
horizon ; the bead being rectified to the 
winter ecliptic. 
Quadrant, gunner’s, fig. 5, sometimes 
called the gunner’s square, is used for elevat- 
ing and pointing cannon, mortars, &c. and 
consists of two branches, either of wood or 
brass, between which is a quadrantal arch 
divided into £>0 9 , and furnished with a thread 
and plummet. 
The use of this instrument is very easy ; for 
if the longer branch, or bar, is placed in the 
mouth of the piece, and it is elevated till the 
plummet cuts the degree necessary to hit a 
proposed object, the thing is done. 
Sometimes on the sides of the longer bar, 
are noted the division of diameters and 
weights of iron balls, as also the bores of 
pieces. 
Quadrant, Gunter’s, so called from its 
inventor, Edmund Gunter, (fig. 6) besides 
the apparatus of other quadrants, has a ste- 
reographic projection of the sphere on the 
plane of the equinoctial; and also a calendar 
of the months, next to the divisions of the 
limb ; by which, besides the common pur- 
poses of other quadrants, several useful ques- 
tions in astronomy. Sic. are easily resolved. 
Use of Gunter’s quadrant. 1 . To find the 
sun’s meridian altitude for any given day, or 
conversely the day of the year answering to 
any given meridian altitude. Lay the thread 
to the day of the month in the scale next the 
limb ; then the degree it cuts in the limb is 
the sun’s meridian altitude. And, contrari- 
wise, the thread being set to the meridian al- 
titude, it shews the day of the month. 
2. To find the hour of the day. Having 
put the bead, w'hich slides on the thread, to 
the sun’s place in the ecliptic, observe the 
sun’s altitude by the quadrant ; then if the 
bead is laid over the same in the limb, the 
bead will fall upon the hour required. On 
the contrary, laying the bead on a given hour, 
having first rectified or set it to the sun’s 
place, the degree cut by the thread on the 
limb gives the altitude. 
Note: the bead may be rectified otherwise, 
by bringing the thread to the day of the 
month, and the bead to the hour-line of 12. 
3. To find the sun’s declination from his 
place given, and the contrary. Bring the 
bead to the sun’s place in the ecliptic, and 
move the thread to the line of declination 
ET, so shall the bead cut the degree of de- 
clination required. On the contrary, the 
bead being adjusted to a given declination, 
and the thread moved to the ecliptic, the 
bead will cut the sun’s place. 
4. The sun’s place being given, to find the 
right ascension, or contrariwise. Lay the 
thread on the sun’s place in the ecliptic, and 
the degree it cuts on the limb is the right as- 
cension sought. And the converse. 
5. The sun’saltitude being given, to find his 
azimuth, and contrariwise. Rectify the bead 
for tee time, as in the second article, and ob- 
serve the sun’s altitude; bring the thread to 
the complement of that altitude ; then the 
bead will give the azimuth sought, among the 
azimuth-lines. 
Quadrant, Hadley’s, (fig. 7) so called 
from its inventor, John Hadley, esq. is now 
universally used, as the best of any for nautical 
and other observations. 
Description of Hadley’s quadrant. Fig. 
7, represents a quadrant, or octant, of 
the common construction. The following 
parts are those which require the particular 
attention of the observer: 
I. BC the arc. 
II. AD the index, ah the nonius scale. 
III. E the index-glass. 
IV. F the fore horizon-glass. 
V. G the back horizon-glass. 
VI. K the dark glasses or screens. 
VII. Ill the vanes or sights. 
\ III. d he arc BC is called the limb or qua- 
drantal arc ; the arc cd, lying from o 
towards the right, is called the arc of 
excess. 
The quadrant consists of an arc BC, firmly 
attached to two radii, or bars, AB, AC, 
which are strengthened and bound together 
by the two braces LM. 
The index D is a flat bar of brass, that 
turns on the centre of the octant. At the 
lower end of the index there is an oblong 
opening: to one side of this opening a vernier 
is fixed, to subdivide the divisions of the arc ; 
at the bottom or end of the index there is a 
piece of brass, which bends under the arc, 
carrying a spring to make the vernier lie close 
to the divisions : it is also furnished with a 
screw to fix the index in any desired position. 
See Vernier. 
The circular arcs on the arc of the qua- 
drant are drawn from the centre on which the 
index turns : the smallest excentricity in the 
axis of the index would be productive of con- 
siderable errors. 
The position of the index on the arc after 
an observation, points out the number of de- 
grees and minutes contained in the observed 
angle. 
Upon the index E, and near its axis, is 
fixed a plain speculum, or mirror of glass, 
quicksilvered. It is set in a brass frame, and 
is placed so that the face of it is perpendicular 
to the plane of the instrument; this mirror be- 
ing fixed to the index, moves along with it, 
and lias its direction changed by the motion 
thereof. 
This glass is designed to receive the image 
of the sun, or any other object, S, and reflect 
i it upon either of the two horiaon-glasses F 
and G, according to the nature of the ob- 
servation. 
The brass frame with the glass is fixed to 
the index by the screw C ; the other screw 
serves to replace it in a perpendicular position, 
if by any accident it has been deranged. 
The index-glass is often divided into two 
parts, the one silvered, the other black with a 
small screen in front. A single bhu k surface 
has indeed some advantages ; but if the glasses 
are well selected, there is little danger to be 
apprehended of error from a want oS parallel- 
ism ; more is to be feared from the surfaces 
not being flat. 
On the radius AB of the octant, are two- 
small speculums F and G. The surface of 
the upper one is parallel to the index-glass, 
when the counting division cf the index is at 
o on the arc ; but the surface of the lower 
one is perpendicular to the index-glass, when- 
the index is at 0 degrees on the arc ; these 
mirrors receive the reflected rays from the 
object, and transmit them to the observer. 
t he horizon-glasses are not entirely quick- 
silvered; the upper one F, is only silvered 
on its lower part, or that half next the qua- 
drant, the other half being transparent ; and 
the back part of the frame is cut away, that 
nothing may impede the sight through the 
unsilvered part of the glass. The edge of 
the foil of this glass is nearly parallel to the 
plane of the instrument, and ought to be very 
sharp, and without a flaw. 
The other -horizon-glass G, is silvered at 
both ends; in the middle there is a trans- 
parent slit, through which the horizon, or 
other object, may be seen. 
Each of these glasses is set in a brass frame, 
to which there is an axis ; this axis passes 
through the wood work, and is fitted to a. 
lever on the underside of the quadrant; by 
this lever the glass may be turned a few de- 
grees on its axis, in order to set it parallel to 
the index-glass. The lever has a contrivance 
to turn it slowly, and a button to fix it. To 
set the glasses perpendicular to the plane of 
the quadrant, there are two sunk screws, one 
before and one behind each glass; these 
screws pass through the plate on which the 
frame is fixed, into another plate ; so that by 
loosening one and tightening the other of 
these screws, the direction of the frame with 
its mirror may be altered, and thus be set 
perpendicular to the plane of the instrument. 
There are two red or dark glasses, and one 
green one K; they are used to prevent the 
bright rays of the sun, or the glare of the 
moon, from hurting the eye at the time of ob- 
servation. They are each of them set in a 
brass frame, which turns on a centre; so tha 
they may be used separately, or together, as 
the brightness of the sun may require. The 
green glass may be used also alone, if the sun 
is very faint ; it is also used for taking the 
altitude of the moon, and in ascertaining her 
distance from a fixed star. 
When these glasses are used for the fore 
observation, they are fixed as at K ; when 
used for the back observation, they are re- 
moved to N. 
Each of the vanes H and I, is a perforated 
piece of brass, designed to direct the sight 
parallel to the plane of the quadrant. That 
which is fixed at I is used for the fore, the 
other for the back, observation. 
