530 
The vane 1 b: s two holes : one exactly at 
■the height of tiie quicksilvered edge of the 
horizon-glass ; the. other somewhat higher, to 
direct the sight to the middle of the trans- 
parent-part of the mirror, for those objects 
v. Inch are bright enough to be reflected from 
the unsilvered part of the mirror. 
Directions to held the instrument. It is 
recommended to support the weight of the 
instrument by the right hand, and reserve the 
left to govern the index. Place the thumb 
of the right hand against the edge of the 
quadrant, under the swelling part of which 
-tiie tore sight 1 stands, extending the lingers 
across the back of the quadrant, so as to lay 
-hold on the opposite edge, placing the fore 
linger above, and the other lingers below the 
.swelling part, or near the fore horizon-glass; 
thus you may support the instrument con- 
veniently, in a vertical position, by the right 
-hand only; by resting the thumb of the left 
hand against the side, or the lingers against 
•the middle bar, you may move the index gra- 
dually either way. 
In the back observation, the instrument 
should be supported by the left hand, and the 
index be governed by the right. 
Of the two objects which are made to coin- 
cide by this instrument, the one is seen 
-directly by a ray passing through, the other 
hy a ray reflected from, the same point of 
the horizon-glass to the eye. This ray is 
called tiie visual ray; but when it is con- 
sidered merely as a line drawn from the mid- 
dle of the horizon-glass to the eye-hole of the 
sight vane, it is called the axis of vision. 
The axis of a tube, or telescope, used to 
direct the sigh:, is also called the axis of 
"vision. 
The quadrant, if it is held as before direct- 
ed, may be easily turned round between the 
lingers and thumb, and thus nearly on a line 
parallel to the axis of vision ; thus the plane 
of the quadrant will pass through the two ob- 
jects when an observation is made, a circum- 
stance absolutely necessary, and which is 
more readily .effected when the instrument is 
furnished with a telescope. Within the tele- 
scope are two parallel wires, which by turning 
the eye-glass tube may be brought parallel to 
the plane of the quadrant, so that by bring- 
ing the object to the middle between them, 
you are certain of having the axis of vision 
parallel to the plane of the quadrant. 
Of the observations. There are two sorts 
of observations to be made with this instru- 
ment: the one is when the back of the ob- 
server is turned towards the object, and there- 
fore called the back observation ; the other 
when the face of the observer is turned to 
wards the object, which is called the fore ob- 
•servation. 
To rectify the instrument for the fore ob- 
servation. Slacken the screw in the middle 
of the handle behind the glass F; bring the 
index close to the button h ; hold the •instru- 
ment in a vertical position, with the arch 
downwards-; look through the right-hand hole 
in the vane I, and through the transparent 
part of the glass F, for the horizon ; and if it 
dies in the same right line with the image of the 
horizon seen on the silvered part, the glass F 
is rightly adjusted ; but if the two horizontal 
lines disagree, turn the screw which is at the 
end of the handle backward or forward, till 
-those lines coincide ; then fasten the middle 
QUADRANT. 
screw of the handle, and the glass is rightly 
adjusted. 
Intake the sun's altitude by the fore ob- 
servation, Having fixed the screens above 
the horizon-glass F, and suited them pro- 
portionally to the strength -of the sun’s rays, 
turn your face towards the sun, holding the 
instrument _ with your right hand, by the 
braces L and M, in a vertical position, with 
the arch downward ; put your eye close to 
the right-hand hole in the vane I, and view the 
horizon through the transparent part of the 
horizon-glass F, at the same time moving the 
index D with the left hand, till the reflex so- 
lar spot coincides with the line of the horizon ; 
then the degrees counted from that end 
next your body, will give the sun’s altitude 
at that time, observing to add or subtract 16 
minutes according as the upper or lower edge 
of the sun’s reflex image is made use of. 
But to get the sun’s meridian altitude, 
which is the thing wanted for finding the lati- 
tude, the observations must-be continued; 
and as the sun approaches the meridian, the 
index D must be Continually moved towards 
B, to maintain the coincidence between the 
reflex solar spot and the horizon ; and con- 
sequently as long as this motion can maintain 
the same coincidence, the observation must 
be continued tiil the sun lias reached the 
meridian, and begins to descend, when the 
coincidence will require a retrograde motion 
of the index, or towards C ; and then the ob- 
servation is finished, and the degrees counted 
as before will give the sun’s meridian altitude, 
or those from B will give the zenith distance; 
observing to add the semidiameter, or 16.', 
when his lower edge is brought to the hori- 
zon, or to subtract 16 when the horizon and 
upper edge coincide. 
To take the altitude of a star by the fore 
observation. Through the vane H, and the 
transparent slit in the glass G, look directly 
to the star; and at the same time move the 
index, till the image of the horizon behind 
you, being reflected by the great speculum, 
' is seen in the silvered part of G, and meets 
the star; then will the index shew the de- 
grees of the star’s altitude. 
To rectify the instrument for the back ob- 
servation. Slacken the screw in the middle 
of the handle, behind the glass G ; turn the 
button h on one side, and bring the index as 
many degrees before 0 as are equal to double 
the dip of the horizon at your height above 
the water; hold the instrument vertical, with 
the arch downward; look through the hole of 
the vane FI ; and if the horizon seen through 
the transparent slit in the glass G, coincides 
with the image of the horizon seen in the sil- 
vered part of the same glass, then the glass 
G is in ils proper position ; but if not, set it 
by the handle, and fasten the screw as be- 
fore. 
To take the sun’s altitude by the baric ob- 
servation. Put the screens as at K; and m 
proportion to the strength or faintness of the 
sun’s rays, let either one or both or neither of 
the frames of those glasses be turned close to 
the iace of the limb ; hold the instrument in 
a vertical position, with the arch downward, 
by the braces L and M, with the left hand ; 
turn your back to the sun, and put one eye 
close to the hole in the vane H, observing 
the horizon through the transparent slit in the 
horizon-glass G ; with the right hand move 
the index D„till the reflected image of the sun 
is seen in the silvered part o. the glass G, 
and in a right line with the horizon ; swing 
your body to and fro, and if the observation 
is well made, the sun’s image will be observ- 
ed to brush the horizon, and the degrees 
reckoned from C, or that part of the arch 
farthest from your body, will give the sun’s 
altitude at the time of observation ; observing 
to add 16', or the sun’s semidiameter, it the 
sun’s upper edge is used, and subtract tire 
same for the lower edge. 
The direction just given, for taking alti- 
tudes at sea, would be sufficient, but for two 
corrections that are necessary to be made 
before the altitude can be accurately deter- 
mined, viz. one on account of the observer’s 
eye being raised above the level of the sea, 
and the other on account of the refraction of 
the atmosphere, especially in small altitudes. 
The following tables, therefore, shew the 
corrections to be made on both these ac- 
counts : 
TABLE I. 
TABLE II. 
zon of the 
sea. 
m 
altitude. • 
Height 
of the 
eye. 
Dip of 
the ho- 
rizon. 
Appar. 
altit. in 
deg. 
Refrac- 
tion. 
App. 
alt. in 
deg. 
refrac- 
tion. 
Feet. 
/ 
// 
O 
/ 
// . 
0 
/ 
// 
1 
0 
57-1 
0 
33 
0 
11 
4 
47 
O 
1 
21 ' 
X 
30 
35 
12 
4 
23 
3 
1 
39 i 
x 
28 
22 ; 
15 
3 
30 
5 
2 
8 i 
1 
24 
29 
20 
2 
35 
10 
3 
1 
2 
18 
35 : 
25 
2 
2 
15 
3 
42 ‘ 
3 
14 
36 
30 
1 
38 
20 
4 
1C 
4 
11 
51 
35 
1 
21 
25 
4 
46 
5 
9 
54 
40 
1 
8 
30 
5 
14 
6 
8 
29 
45 
0 
57 
35 
5 
39 
7 
7 
20 
50 
0 
48 
40 
6 
2 
8 
6 
29 
60 
0 
33 
45 
6 
24 
9 
5 
48 
70 
0 
2! 
50 
6 
44 j 
10 
5 
15 
80 
0 
10 
Refractions of the stars, &c. 
General rules for these corrections. 1 . In 
the fore observations, add the sum of' both 
corrections to the observed zenith distance, 
for the true zenith distance; or subtract the 
said sum from the observed altitude, for the 
true one. 2. In the back observation, add 
the dip and subtract the retraction for alti- 
tudes; and for zenith distances do the con- 
trary, viz. subtract the dip, and add the re- 
traction. 
Example. By a back observation, the alti- 
tude of the. sun’s lower edge was found by 
Hadley’s quadrant to be 25° \2>, the eye 
being 30 feet above the horizon. By the 
tables, the dip on 30 feet is 5' 14", and the 
refraction on 25° 12' is 2' 1". Hence 
Appar. alt. lower limb 25° 12' 0" 
Sun’s semidiameter, sub. 0 16 0 
Appar. alt. of centre 
24 
56 
3 
Dip of horizon,- add 
0 
5 
14 
25 
1 
i4 
Refraction, subtract 
0 
2 
1 
True alt. of centre 24 59 13 
In the case of the moon, besides the two 
corrections above, another is to be made for 
her parallaxes. But for all these particulars, 
see the requisite tables for the Nautical Al- 
