jj and its thickness is about a quarter of an inch : 
the diameters of tiie circles are each 5 inches ; 
i the thickness of the plates, and the other 
: measures, may be taken at the direction ot 
a workman. 
This instrument may be rectified, for mak- 
i iug observations, in this manner : by placing 
the intersection oT 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 mo- 
| tion of the telescope upon the limb will 
1 be sufficiently true, or free from conical cur- 
vity : which may be examined hy suspend- 
ing a long plumb-line at a convenient dis- 
j tance from the instrument ; and by fixing 
the plane of the sector in a vertical position, 
j and then by observing, while the telescope 
I is moved by the screw along the limb, whe- 
' titer the cross hairs appear to move along 
: the plumb-line. 
The axis h f o (see figure below) may 
I be elevated nearly parelkl to the axis ot 
I the earth, by means of a small common 
1 quadrant ; and its error may be correct- 
ed by making the line ot sight follow 
■ the circular motion of any of the circum- 
polar stars, while the whole instrument 
i is moved about its axis hf o, the telescope 
being fixed to the limb : for this purpose, 
j let the telescope k l be directed to the star 
| a, when it passes over the highest point of 
[ its diurnal circle, and lot the division cut by 
the nonius be then noted : then alter twelve 
hours, when the star comes to the lowest 
I point of its circle, having turned the instru- 
ment half-round its axis to bring the tele- 
scope into the position in n ; 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 is necessary to move the telescope into 
the position uv, in order to point to this star 
at c, the arch m u, which measures the angle 
inf u or bfc, 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 thus the trial must be repeat- 
ed till the true elevation of the axis is ob- 
tained. By making the like observations 
upon the same star on each side the pole, 
in the six o’clock hour-circle, the error ot 
the axis, towards the east or west, may also 
be found and corrected, till the cross-hairs 
follow the star quite round the pole: for 
supposing a o p b c to Up an arch ot 
the meridian, make the angle af p equal 
to half the angle a f c, and the line 
fp will point to "the pole; and the angle 
ofp, which is the error of the axis, will be 
equal to half the angle bfc, or mju , found 
by the observation ; because the difference 
of the two angles afb, af c, is double the 
difference of their halves af o and afp. 
Unless the star is very near the pole, al- 
lowance must be made for refractions. 
Equatorial or portable observatory : an 
instrument designed to answer a number of 
useful purposes in practical astronomy, inde- 
pendantly of any particular observatory ; it 
mav be made use of in any steady room or 
and performs most of "the useful pro- 
blems in the science. 
The principal parts of this instrument (fig. 
5.) are, 1. The azimuth or horizontal circle 
OBSERVATORY. 
A, which represents the horizon of the place, 
and moves on an axis B, called the ver- 
tical axis. 2. The equatorial or hour circle 
C, representing the equator, placed at right 
angles to the polar axis D, or the axis ot 
the earth, upon which it moves. 3. 1 he 
semicircle ot declination E, on which the 
telescope is placed, and moving on the axis 
of declination, or the axis of motion of the 
line of collimation F. These circles are 
measured and divided as in the following 
table : 
Divided by 
nonius into 
parts of inc. 
1350th 
1350th 
1260th 
1 
Divided on 1 
limb into | 
parts of inc. i 
45th 
45th 
45th 
Nonius of ■ 
SO gives J 
seconds, j 
o o s o 
Limb divided 
to 
15' 
$ 15 '. 
£1' in time 
15' 
Radius 
in dec. 
-1 rH 10 
US V} *0 
Measures of the several 
circles, and divisions | 
on them. ; 
Azimuth, or horizon- £ 
tal circle - S 
Equatorial, or hour I 
circle - 5 
Vertical semi-circle, 1 
for declination or > 
latitude - J 
4. The telescope in this equatorial may be 
brought parallel to the polar axis, as in the 
figure, so as to point to the pole-star in any 
part of its diurnal revolution; and thus it 
lias been observed near noon, when the sun 
has shone very bright. 5. r I he apparatus 
for correcting the error in altitude occasi- 
oned bv refraction, which is applied to the 
eye-end of the telescope, and consists of a 
slide G moving in a groove or dovetail, and 
carrying the several eye-tubes of the tele- 
scope, on which slide there is an index cor- 
responding to 5 small divisions engraved on 
the dovetail ; a small circle called the refrac- 
tion circle, H, moveable by a finger-screw at 
the extremity of the eve-end of the telescope; 
which circle is divided into half-minutes, one 
entire revolution of it being equal to 3' 18'', 
and by its -, motion raises the centre of the 
cross hairs on a circle of altitude; and like- 
wise a quadrant I of an inch and a halt ra- 
dius, with divisions on each side, one ex- 
pressing the degree ot altitude ot the object 
viewed, and the other expressing the mi- 
nutes and seconds of error occasioned by 
refraction, corresponding to that degree of 
altitude : to this quadrant is joined a small 
round level K, which is adjusted partly by 
the pinion that turns 1 he whole of this appa- 
283 
rat us, and partly by the index of the quad- 
rant ; for winch purpose the refraction-circle 
is set to the same minute, &c. -which the in- 
dex points toon the limb ot the quaarant; 
and if the minute, & c. given by the quad- 
rant exceeds the 3' IS' 7 contained in one en- 
tire revolution of the ret: action-circle, this 
must be set to the excess above one or more 
of its entiie revolutions; then the centre 
of the cross hairs will appear to be. raided on 
a circle of altitude to t he additional height 
which the error of refraction will occasion at 
that altitude. 
The principal adjustment in this instru- 
ment is that of making the line ot collimation 
to describe a portion of an hour-circle in the 
heavens ; in order to which, the azimuth- 
circle must be truly level ; the i’fne ot colli- 
matiou, or. some corresponding line represent- 
ed by the small brass rod M parallel to it, 
must be perpendicular to the axis ot its 
own proper motion ; and this last axis must 
be perpendicular to the polar axis. On the 
brass rod M there is occasionally placed a 
hanging level N, the use of which will appear 
in the following adjustments. 
The azimuth-circle may be made level by 
turning the instrument till one ot the levels 
is parallel to an imaginary line joining two 
of the feet-screws ; then adjust that level 
with these two feet-screws ; turn the circle 
half round, that is, 180"; and if the bubble 
is not then right, correct half the error by 
the screw belonging to the level, and the 
other half error by the two feet-screws ; re- 
peat this till the bubble comes right ; then 
turn the circle 90° from the two former po- 
sitions, and set the bubble right, if it is 
wrong, by the foot-screw at the end of the 
level; when this is done, adjust the other 
level by its own screw, and the azimuth-circle 
will be" truly level. The hanging level must 
then be fixed to the brass rod by two hooks 
of equal length, and made truly parallel to it : 
for this purpose make the polar axis per- 
pendicular or nearly perpendicular to the 
horizon ; then adjust the level by the pinion 
of the declination-semicircle, reverse the 
level, and if it is wrong, correct half the 
error by a small steel screw that lies under 
one end of the level, and the other half 
error by the pinion of the declination-semi- 
circle ;" repeat this till the bubble is right 
in both positions. In order to make tiie 
brass rod on which the level is suspended, at 
right angles to the axis of motion of the 
telescope or line of collimation, make the 
polar axis horizontal, or nearly so ; set the 
declination-semicircle to 0", turn the hour - 
circle till the bubble comes right ; then turn 
the declination-circle to 90" ; adjust the 
bubble by raising or depressing the polar 
axis (first’ by hand till it is nearly right, af- 
terwards tighten with an ivory -key the socket 
which runs on the arch with the polar axis, 
and then apply the same ivory key to the 
adjusting screw at the end of the sail! arch 
till the bubble comes quite right) ; then turn 
the declination-circle to the opposite 90" ; 
if the level is not then right, correct half the 
error bv the aforesaid adjusting screw at the 
end of the arch, and the other half error by the 
2 screws which raise or depress the end of the 
brass rod. The po ar axis remaining nearly 
horizontal as before, and the declination- 
semicircle at 0°, adjust the bubble by the 
