required _altitude_ (or elevation), the other giving it the required
_azimuth_ (or direction as respects the compass points).

For small alt-azimuths the ordinary pillar-and-claw stand is
sufficiently steady. For larger instruments other arrangements are
needed, both to give the telescope steadiness, and to supply slow
movements in altitude and azimuth. The student will find no difficulty
in understanding the arrangement of sliding-tubes and rack-work commonly
adopted. This arrangement seems to me to be in many respects defective,
however. The slow movement in altitude is not uniform, but varies in
effect according to the elevation of the object observed. It is also
limited in range; and quite a little series of operations has to be gone
through when it is required to direct the telescope towards a new
quarter of the heavens. However expert the observer may become by
practice in effecting these operations, they necessarily take up some
time (performed as they must be in the dark, or by the light of a small
lantern), and during this time it often happens that a favourable
opportunity for observation is lost.

These disadvantages are obviated when the telescope is mounted in such a
manner as is exhibited in fig. 8, which represents a telescope of my own
construction. The slow movement in altitude is given by rotating the rod
_he_, the endless screw in which turns the small wheel at _b_, whose
axle in turn bears a pinion-wheel working in the teeth of the quadrant
_a_. The slow movement in azimuth is given in like manner by rotating
the rod _h'e'_, the lantern-wheel at the end of which turns a
crown-wheel on whose axle is a pinion-wheel working in the teeth of the
circle _c_. The casings at _e_ and _e'_, in which the rods _he_ and
_h'e'_ respectively work, are so fastened by elastic cords that an
upward pressure on the handle _h_, or a downward pressure on the handle