OBSERVATORY. 
H I, which being parallel to the axis of the 
eartli, tlie plane of the sector will always 
be parallel to the plane of some hour cir- 
cle. Let a telescope, C E, be moveable 
about tlie centre, C, of the arch, A B, from 
one end of it to the other, by turning a 
screw at G, and let the line of sight be 
parallel to the plane, of the sector. Now, 
by turning the whole instrument about the 
axis, H I, till the plane of it be successively 
directed, first to one of tlie stars, and then 
to another, it is easy to move the sector 
about the joint, F, into such a position, 
that the arch, A B, when fixed, shall take 
in both the stars in their passage, by the 
plane of it, provided the difference of their 
declinations does not exceed the arch, A B. 
Then, having fixed the plane of the sector 
a little to the westward of both the stars, 
move the telescope, C E, by the screw, G, 
and observe by a clock the time of each 
transit over the cross hairs, and also the 
degrees and minutes upon the arch, A B, 
cut by the index at each transit ; then in 
the difference of the arches, the difference 
of the declinations, and by the difference 
of the times, we have the difference of the 
right ascensions of the stars. The dimen- 
sions of this instrument are these ; the 
length of the telescope, or the radius of the 
sector, is two feet and a half: the breadth 
of the radius, near the end, C, is an inch 
and a half, and at the end, D, two inches : 
the breadth of the limb, A B, is one inch 
and a half, and its length six inches, con- 
taining ten degrees, divided into quar- 
ters, and numbered from either end to the 
other. 
The telescope carries a nonius, or subdi- 
viding plate, whose length being equal to 
sixteen quarters of a degree, is divided 
into fifteen equal parts, which, in effect, 
divides the limb into minutes, and, by esti- 
mation, into smaller parts. The length of 
the square axis, H I F, is eighteen inches, 
and of the part, HI, twelve inches; and 
its thickness is about a quarter of an inch. 
The diameters of the circles are each five 
inches ; the thickness of the plates, and 
the other measures, may be taken at the 
direction of a workman. This instrument 
may be rectified for making observations in 
this manner: By placing the intersection of 
the cross hairs at the same distance from 
the plane of the sector as the centre of the 
object-glass ; the plane described by the 
line of sight, during the circular motion of 
the telescope upon the limb, will be sufii- 
ciently true, or free from conical curvity, 
which may be examined by suspending a 
long plumb-line at a convenient distance 
from the instrument, and by fixing the 
plane of the sector in a vertical position ; 
and then by observing, while the telescope 
is moved by the screw along the limb, whe- 
ther the cross hairs appear to move along 
the plumb-line. The axis, hf o, may be ele- 
vated nearly parallel to the axis of the earth, 
by means of a small common quadrant, 
and its error may be corrected by making 
the line of sight follow the circular motion 
of any of the circumpolar stars, while the 
whole instrument is moved about its axis, 
k f 0, the telescope being fixed to the 
limb; for this purpose let the telescope, 
k I a, be directed to the star a, when it 
passes over the highest point of its diurnal 
circle, and let the division, cut by the no- 
nius, be noted; then, after twelve hours, 
when the star comes to the lowest point of 
its circle, having turned the instrument 
half round its axis, to bring the telescope 
into the position, mu; if the cross hairs 
cover the same star supposed at b, the ele- 
vation of the axis, h f o, is exactly right ; 
but if it be necessary to move the tele- 
scope into the position, ufc,in order to 
point to this star at c, the arch, m u, which 
measures the angle m f u, or bf c, will 
be known; and then the axis, hfo, must 
be depressed half the quantity of this 
given angle if the star passed below b, or 
must be raised so much higher if above it ; 
and then the trial must be repeated till 
the true elevation of the axis be obtained. 
By making the like observations upon the 
same star on each side the pole in the six 
o’clock hour circle, the error of the axis, 
toward the east or west, may also be found 
and corrected, till the cross hairs follow the 
star quite round the pole; for supposing 
a 0 p b c, to be an arch of the meridian, 
make the angle, af p, equal to half the 
angle, afc, and the line,/p, will point to 
the pole ; and if the angle, o f p, which is 
the error of the axis, will be equal to half 
the angle, b f c, or m f u, found by the 
observation ; because the difference of the 
two angles, afh, afc, is double the dif- 
ference of their halves, afo, and afp. 
Unless the star be very near the pole, 
allowance must be made for refractions. 
See Quadrant. 
VI. Transit and Equal Altitude Instru- 
ments. The transit instrument is used for 
observing objects as they pass over the 
mei'idian. It consists of a telescope fixed 
.at right angles to a horizontal axis ; which 
