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QUADRANT. 
index is open, so as to show the gradations 
on the arc : the lower edge is chamfered, that 
it may come close down to them, and is there 
divided into smaller portions : this scale is 
called the nonius, and shows the smaller di- 
visions in a more correct and obvious man- 
ner than could be done by the quadrantal 
arc, on which each degree is subdivided 
into no more than three equal parts, of 20' 
each. Now the nonius, being divided into 
21 equal parts, shows at what portions of 
the arc the index cuts the division of 20 mi- 
nutes ; therefore it shows every minute. 
THE USE OF HADLEY’S QUADRANT. 
For the Fore-Observation. Bring the index 
close to the bottom, so that the middle of 
the Vernier’s scale, or nonius, stand against 
0 degrees. Hold the plane of the instru- 
ment vertical, with the arch downwards ; 
look through the right-hand hole in the 
vane, and direct the sight through the trans- 
parent part of the horizon-glass, to observe 
the horizon. If the horizon-line, seen both 
in the quick-silvered part, and through the 
transparent part, should coincide, or make 
one straight line, then is the glass adjusted ; 
but if one of the horizon-lines should stand 
above the other, slacken the screw in the 
middle of the lever, backwards or forwai'ds, 
as there may be occasion, until the lines 
coincide : fasten the screw in the middle of 
the lever, and all is ready for use. 
To take the Sun’s Altitude. Fix the 
screens above the horizon-glass, using ei- 
ther or both of them, according to the 
strength of the sun’s rays, by turning one 
or both the frames of those glasses close 
against the plane or face of the instrument ; 
then your face being turned towards the 
sun, hold the quadrant by the braces, or by 
either radius, as is found most convenient, 
so as to be in a vertical position, with the 
arch downwards. Put the eye close to the 
right-hand-hole in the vane, look at the ho- 
rizon through the transparent part of the 
horizon-glass, at the same time sliding the 
index with the left hand, until the image 
of the sun, seen in the quicksilvered part, 
falls in with the edge of the horizon, taking 
either the upper or the under edge of the 
solar image. Swing your body gently from 
side to side ; and when the edge of the sun 
is observed not to cut, bnt to touch the ho- 
rizon-line, like a tangent, the observation is 
made, 'riien will the degrees on the arch, 
reckoning from tlie end next your body, 
give the altitude of that edge of the sun 
which was brought to the horizon. If the 
lower edge was observed, then sixteen mi- 
nutes, added to the said degrees, gives the 
altitude of the sun’s centre ; but if the up- 
per edge was used, the sixteen minutes 
must be subtracted. 
To take the Altitude of a Star. Look di- 
rectly up at the star, through the vane, and 
transparent part of the glass ; the index be- 
ing close, to the button : then will the image 
of the star, by refraction, be seen in the sil- 
vered part, right against the star seen 
through the other part. Move the index 
forward, and, as the image descends, let 
the quadrant descend also, to keep it in the 
silvered part, till it comes down in a line 
with the horizon, seen through the transpa- 
rent part, and the observation is made. 
To make an Artificial Horizon. Often 
when the atmosphere is clear above, the ho- 
rizon is so laden with vapours, as to prevent 
an observation being taken. In such case, 
an artificial horizon is to be made thus : fill 
into any vessel, having a diameter of about 
three inches, and about half an inch deep, 
from one to two pounds of quicksilver, on 
which lay a metal speculum, or a piece of 
plain glass, whose diameter may be about 
one-third of an inch less than that of the 
surface of quicksilver ; in this the image of 
the sun may be seen distinctly.. Sling the 
vessel, so that it may remain level, and take 
an observation with a stained glass, which , 
w'ill subdue the great brilliancy of the re- 
flection. The observation thus taken, will 
be as correct as if taken by means of the 
natural horizon. 
As refraction causes each ray of light to 
assume a curved direction, all objects, when 
observed, especially by means of instru- 
ments, appear with an excess of altitude 
beyond their actual height. The refrac- 
tions, to be deducted, follow' : 
