OBSERVATORY. 
axis must be so supported, that what is 
called the line of collimation, or line of 
sight of the telescope, may move in the 
plane of the meridian. This instrument is 
made of various sizes, and of large dimen- 
sions in our great observatories ; but the 
following is one of a size sufficiently lai ge 
and accurate for all the useful purposes. 
The axis, A B (fig. 2), to which the middle 
of the telescope is fixed, is about two feet 
and a half long, tapering gradually toward 
its ends, which terminate in cylinders well 
turned and smoothed. The telescope, C D, 
which is about four feet, and an inch and 
a half diameter, is connected with the axis 
by means of a strong cube or die, G, and 
in which tlie two cones, M Q, forming the 
axis, are fixed. This cube, G, serves as 
the principal part of the whole machine. 
It not only keeps together the two cones, 
but holds the two sockets, K H, of fifteen 
inches length, for the two telescopic tubes. 
Each of these sockets has a square base, 
and is fixed to the cube by four screws. 
These sockets are cut down in the sides 
about eight inches, to admit more easily 
the tube of the telescope ; but when the 
tube is inserted, it is kept in firm by screw- 
ing up the tightening screws at the end of 
the sockets at K. and H. These two sock- 
ets are very useful in keeping the telescope 
in its greatest possible degree of steadiness. 
They also afford a better opportunity of 
balancing the telescope, and rectifying its 
vertical thread, than by any other means. 
In order to direct the telescope to the 
given height that a star would be observed 
at, there is fixed a semicircle, A N, on one 
of the supporters, of about eight inches 
and a half diameter, and divided into de- 
grees. The index is fixed on the axis, at 
the end of which is a vernier, which sub- 
divides the degrees into twelve parts of 
five minutes. This index is moveable on 
the axis, and may be closely applied to the 
divisions by means of a tightening screw. 
Two upright posts of wood or stone, Y Y, 
firmly fixed at a proper distance, are to 
sustain the supporters of this instrument. 
These supporters are two thick brass plates, 
R R, having well smoothed angular notches, 
in their upper ends, to receive the cylindri- 
cal arms of the axis. Each of these notched 
plates is contrived to be moveable by a 
screw, which slides them upon the surfaces 
of two other plates immoveably fixed upon 
the two upright pillars ; one plate moving 
in a horizontal, and the other in a vertical 
direction ; or, which is more simple, these 
two modes are sometimes applied only on 
one side, as at V and P, the horizontal 
motion by the screw P, and the vertical 
by the screw V. These two motions set ve 
to adjust the telescope to the plane of the 
horizon and meridian ; to the plane of the 
horizon by the spirit-level, E F (fig. 4) hung 
by D C on the axis M Q, in a parallel direc-' 
tion, and to the plane of the meridian in 
the following manner: Observe by the 
clock when a circumpolar star seen through 
this instrument transits both above and be- 
low the polo ; and if the times of describ- 
ing the eastern and western parts of its 
circuit are equal, the telescope is then in 
the plane of the meridian : otherwise the 
screw, P, must he gently turned, that it 
may move the telescope so much that the 
time of the star’s revolution be bisected by 
both the upper and lower transits, taking 
care at the same time that the axis remains 
perfectly horizontal. AVhen the telescope 
is thus adjusted, a mark must be set at a 
considerable distance (the greater the bet- 
ter) in the horizontal direction of the inter- 
section of the cross wires, and in a place 
where it can be illuminated in the night- 
time by a lanthorn hanging near it ; which 
mark being on a fixed object, will serve at 
all times afterwards to examine the posi- 
tion of the telescope by the axis of the 
instrument being first adjusted by means of 
the level. 
To adjust the Clock by the' Sun’s Transit 
over the Meridian. Note the times 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 twelve hours, shows how much the 
mean time, or time by the clock, is faster or 
slower than the apparent or solar time for 
that day; to which the equation of time 
being applied, will show the time of mean 
noon for that day, by which the clock may 
be adjusted. 
The Equal Altitude Instrument, is an in- 
strument that is used to observe a celestial 
object when it has the same altitude on 
both the east and west sides of the meri- 
dian, or in the morning and afternoon. It 
principally consists of a telescope about 
thirty inches long, fixed to a sextantal or 
semicircular divided arch, the centre of 
which is fixed to a long vertical axis. 
The Equatorial or Portable Observatory, 
an instrument designed to answer a number 
of useful purposes in practical astronomy, 
independent of any particular observatory. 
It may be made use of in any steady room 
