lated walls, which form
an imposing feature on the lawn at Birr Castle. This instrument cannot
be turned about towards every part of the sky, like the equatorials we
have recently been considering. The great tube is only capable of
elevation in altitude along the meridian, and of a small lateral
movement east and west of the meridian. Every star or nebula visible in
the latitude of Parsonstown (except those very near the pole) can,
however, be observed in the great telescope, if looked for at the right
time.

[Illustration: Fig. 8.--Meridian Circle.]

Before the object reaches the meridian, the telescope must be adjusted
at the right elevation. The necessary power is transmitted by a chain
from a winch at the northern end of the walls to a point near the upper
end of the tube. By this contrivance the telescope can be raised or
lowered, and an ingenious system of counterpoises renders the movement
equally easy at all altitudes. The observer then takes his station in
one of the galleries which give access to the eye-piece; and when the
right moment has arrived, the star enters the field of view. Powerful
mechanism drives the great instrument, so as to counteract the diurnal
movement, and thus the observer can retain the object in view until he
has made his measurements or finished his drawing.

Of late years reflecting telescopes have been generally made with
mirrors of glass covered with a thin film of silver, which is capable of
reflecting much more light than the surface of a metallic mirror. Among
great reflectors of this kind we may mention two, of three and five feet
aperture respectively, with which Dr. Common has done valuable work.

We must not, however, assume that for the general work in an observatory
a colossal instrument is the most suitable. The mighty reflector, or
refractor, is chiefly of use where unusually faint objects are being
examined. For work in which accurate measurements are made of objects
not particularly difficult to see, telescopes of smaller dimensions are
more suitable. The fundamental facts about the heavenly bodies have been
chiefly learned from observations obtained with instruments of moderate
optical power, specially furnished so as to enable precise measures of
position to be secured. Indeed, in the early stages of astronomy,
important determinations of position were effected by contrivances
which showed the direction of the object without any telescopic aid.

Perhaps the most v