aluable measurements obtained in our modern
observatories are yielded by that instrument of precision known as the
_meridian circle_. It is impossible, in any adequate account of the
Story of the Heavens, to avoid some reference to this indispensable aid
to astronomical research, and therefore we shall give a brief account of
one of its simpler forms, choosing for this purpose a great instrument
in the Paris Observatory, which is represented in Fig. 8.

The telescope is attached at its centre to an axis at right angles to
its length. Pivots at each extremity of this axis rotate upon fixed
bearings, so that the movements of the telescope are completely
restricted to the plane of the meridian. Inside the eye-piece of the
telescope extremely fine vertical fibres are stretched. The observer
watches the moon, or star, or planet enter the field of view; and he
notes by the clock the exact time, to the fraction of a second, at which
the object passes over each of the lines. A silver band on the circle
attached to the axis is divided into degrees and subdivisions of a
degree, and as this circle moves with the telescope, the elevation at
which the instrument is pointed will be indicated. For reading the
delicately engraved marks and figures on the silver, microscopes are
necessary. These are shown in the sketch, each one being fixed into an
aperture in the wall which supports one end of the instrument. At the
opposite side is a lamp, the light from which passes through the
perforated axis of the pivot, and is thence ingeniously deflected by
mirrors so as to provide the requisite illumination for the lines at the
focus.

The fibres which the observer sees stretched over the field of view of
the telescope demand a few words of explanation. We require for this
purpose a material which shall be very fine and fairly durable, as well
as somewhat elastic, and of no appreciable weight. These conditions
cannot be completely fulfilled by any metallic wire, but they are
exquisitely realised in the beautiful thread which is spun by the
spider. The delicate fibres are stretched with nice skill across the
field of view of the telescope, and cemented in their proper places.
With instruments so beautifully appointed we can understand the
precision attained in modern observations. The telescope is directed
towards a star, and the image of the star is a minute point of light.
When that point coincides with the intersection of the two central
sp