282 * 
v-auM continue to move Gr>r a csnsiderable | 
timo alter. We do not mention the louses ! 
or microscopes that are applied to read off 
t ie divisions at E and at X, or to see the 
coincidence of the plummet-line with a dot 
m irked upon the arc at C, as matters that 
need no particular description. 
In the eye-tubs* of the telescope AD, 
there are certain slender wires, placed in the 
focus of the eye-jen;, and perpendicular to 
the axis of the telescope, which enable the 
observer to distinguish more accurately 
when an object, that is seen through the 
telescope, reaches the axis of the telescope, 
or, as it is more commonly called, the line 
of c d'im .tiou, & c. Now when the stars or 
planets are observed at night, those wires 
in the eye-tube cannot be seem ; therefore, 
to render them visible, an arm or wire is 
fixed occasionally at the end of the telescope, 
which arm holds a small piece of ivory or 
card z, set aslant to the axis of the tele- 
, scope; for when a lighted candle or lantern, 
is situated at a little distance, and is directed 
so as to shine upon the above-mentioned 
ivory or card, the reflection of the light from 
it into the tube of the telescope will enable 
the observer to distinguish the wires at the 
same time that he benolds the celestial ob- 
ject. 
The mural quadrant, fig. 2, is a larger 
instrument like the above, excepting that it 
has no stand ; and its index is prevented 
from bending on account of its great length, 
by means of metallic bars, d,f,b,c. This 
instrument is firmly fixed upon a wall ex- 
actly in the plane of the meridian of the 
observatory, for which purpose it has clamps, 
screws, and other adjustments. It lias like- 
wise a plummet. 
This undoubtedly is the principal instru- 
ment of an observatory ; for by observing 
the times by the clock, of the arrival of any 
celestial object, to the meridian, the right 
ascension of that object is had immediately ; 
and its declination is shewn at the same time 
by the index of the quadrant upon the di- 
vided arch ; deducting the inclination of the 
equator, which is given by the latitude once 
ascertained of the observatory. It is by this 
means that exact catalogues of the places of 
tne fixed stars have been made. 
The transit instrument consists of a tele- 
scope of any convenient length, fixed at 
right angles to a horizontal axis-, which axis 
is supported at its two extremities ; and the 
instrument is generally situated so that the 
line of coiiimation of the telescope may move 
in the plane of the meridian. The use of 
this instrument is to observe the precise 
time of the celestial bodies’ passage across 
the meridian of the observatory. 
Fig. 3. exhibits a transit instrument. N 
M is the t leseope; in the eye-tube of which 
a system' of parallel wires, is situated in the fo- 
cus of the eye-lens. FE is. the horizontal axis, 
in the middle of which the telescope is steadily 
fixed : so that by moving the telescope, the 
axis is forced to turn round its two extremi- 
ties E and F, which rest in the notches o: 
two thick pieces, T, S, of bell-metal, suck 
as arc delineated separately and magnifies 
at X a- d Z. Those pieces are generally fix- 
ed up n two pillars, eituer of cast iron, of 
which is better, of stone, as are shewn in the 
OBSERVATORY. 
figure ; and they are constructed so as to be 
susceptible of a* small motion by means of 
slides -and screws, viz. the piece T backwards 
and forwards, and the piece S upwards and 
! downwards; by which means the axis EF 
[ of the instrument may be set exactly iior;- 
! zontal, and caused to move perpendicular 
i to the plane of the meridian. In order to 
! verify the first of those requisites, viz. to see 
whether the axis is truly horizontal, the long 
! spirit-level P Q is suspended upon it by 
| means of the metallic branches PO and QR ; 
and the situation of the bubble in _ it will 
| immediately shew whether the axis is truly 
| horizontal, or which way it inclines, and of 
| course where it must be raised or depressed, 
j The other requisite, viz. whether the axis 
| is perpendicular to the plane or the meri- 
! dian, or not, may be verified by various 
! means, the best of ‘which is by observations 
i on those circumpolar stars which never go 
i below the horizon of the observatory. ^ l luis, 
I observe the times by the clock, when a 
| circuriipo’ar star, seen through the telescope 
; NM, crosses the meridian both above and 
| below the pole ; and if the times of describ- 
j ing the eastern and western parts ot its cir- 
! cuit are equa 1 , the telescope is then in the 
j plane of the meridian, consequently the axis 
i EF is perpendicular to that plane ; other- 
| wise the notched pieces T and S, which 
support the extremities E, F, of the axis, 
I must be moved accordingly, or until upon 
observation it is found that the above-men- 
tioned times of the stars’ semi-revolutions are 
equal. 
When the instrument has been once so 
adjusted, a mark may be made upon a house, 
or rock, or post, at some distance from the 
observatory, so that when viewed through 
the telescope, this mark may appear to be 
in the direction of the axis of the telescope ; 
by which means the correct situation of the 
instrument may afterwards be readily veri- 
fied. 
The cylindric extremity F is perforated, 
and the perforation passes through the halt 
of the axis, and reaches the inside o! the 
telescope ; that side of the telescope tube 
which is exactly facing F, being also per- 
forated. Within the said tube, and directly 
opposite to the perforation of the end F, 
a plane reflector, or a flat piece of ivory, 
is fixed, making an angle of 45° with the 
axis of the telescope, and having a hole 
through it large enough to admit all the rays 
passing from the object-glass to the eye- 
glass of the telescope. 
When stars or other celestial objects are 
to be observed in the night-time, a small 
lantern Y is set upon a stand just before the 
perforation of the extremity F, so as to throw 
the light within the axis, and upon the slant 
reflector within the tube of the telescope, 
whence it is reflected upon the wires in the 
eye-tube M, and renders them visible. By 
placing the lantern nearer to, or farther from, 
the extremity F, the observer may illumi- 
nate the wires sufficiently lor the purpose, 
and not too much. 
To the other extremity E of the axis, a 
divide^ circle, or sometimes a semicircle, is 
ixed, which turns with the axis ; the index 
wing fixed to the pillar which supports th 
xis. Sometimes the situation of those pari 
is reversed, viz. the circle is fastened to the 
pillar, or to the brass piece which supports 
the axis, and the index is fastened to the 
extremity E of the axis. The use of this 
circle, is to place the telescope in the direc- 
tion of any particular celestial body, when 
that body crosses the meridian ; which incli- 
nation is equal to the collatitude of the 
place, more or less the declination of the 
celestial body, according as that declination 
is north or south. 
To adjust the dock by the sun’s transit 
over the meridian . — N ote the time's by the 
clock when the preceding and following 
edges of the sun’s limb touch the cross 
wires. The difference between the middle 
time and 12 hours, shews how much the 
mean, or time by the clock, is faster or slower 
than the apparent, or solar time, for that 
day ; to which the equation ot time being 
applied, will show the time of mean noon for 
that day, by which the clock may be ad- 
justed. 
Astronomical or equatorial sector, an in- 
strument for finding the difference in right 
ascension and declination between two ob- 
jects, the distance of which is too great to- 
be observed by the micrometer, was invented 
by Graham. Let AB (fig. 4.) represent an 
arc'n of a circle, containing 10 or 12 degrees 
well divided, having a strong plate CD for 
its radius, fixed to the middle of the arch at 
D: let this radius be applied to the side of 
an axis HFI, and be moveable about a joint 
fixed to it at F, so that the plane of the* sec- 
tor may be always parallel to the axis Hi \ 
which being parallel to the axis of the earth, 
the plane of the sector will always be parallel 
to the plane of some hour-circle. Let a 
telescope CE be moveable about the centre 
C of the arch AB, 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 instru- 
ment about the axis HI, till the plane of it 
is successively directed, first to one of the 
stars and then to another, it is easy to move 
the sector about the joint F, into such a po- 
sition, that the arch AB, 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 AB, 
Then, having fixed the plane of the sector a 
little to the westward of both the stars, 
move the telescope CE by the screw G ; 
and observe by a clock the time of each 
transit over the cross hairs, and also the de- 
grees and minutes upon the arch AB cut by 
the index at each transit ; then in the dif- 
ference of the arches, the difference of the 
declinations, and by the difference of the 
times, we have the difference of the right as- 
censions of the stars. 
The dimensions of this instrument are 
these : The length of the telescope, or the 
radius of the sector, is 2\ feet; the breadth 
of the radius, near the end C, is inch; 
and at the end D two inches. The breadth 
of the limb AB is 1| inch ; and its length six 
inches, containing ten degrees divided into- 
quarters, and numbered from each end to. 
the other. The telescope carries a nonius 
or subdividing plate, whose length, being 
equal to sixteen -quarters of a degree, is di- 
vided into fifteen equal parts ; which, in ef- 
. c t , divides the limb into minutes, and, by 
estimation, into smaller parts. The length, 
of the square axis I1EI is eighteen inches. 
