_h'_, at once releases the endless screw or the crown-wheel
respectively, so that the telescope can be swept at once through any
desired angle in altitude or azimuth. This method of mounting has other
advantages; the handles are conveniently situated and constant in
position; also, as they do not work directly on the telescope, they can
be turned without setting the tube in vibration.

[Illustration: _Fig. 8._]

I do not recommend the mounting to be exactly as shown in fig. 8. That
method is much too expensive for an alt-azimuth. But a simple
arrangement of belted wheels in place of the toothed wheels _a_ and _c_
might very readily be prepared by the ingenious amateur telescopist; and
I feel certain that the comfort and convenience of the arrangement would
amply repay him for the labour it would cost him. My own
telescope--though the large toothed-wheel and the quadrant were made
inconveniently heavy (through a mistake of the workman who constructed
the instrument)--worked as easily and almost as conveniently as an
equatorial.

Still, it is well for the observer who wishes systematically to survey
the heavens--and who can afford the expense--to obtain a well-mounted
_equatorial_. In this method of mounting, the main axis is directed to
the pole of the heavens; the other axis, at right angles to the first,
carries the telescope-tube. One of the many methods adopted for mounting
equatorials is that exhibited--with the omission of some minor
details--in fig. 9. _a_ is the polar axis, _b_ is the axis (called the
declination axis) which bears the telescope. The circles _c_ and _d_
serve to indicate, by means of verniers revolving with the axes, the
motion of the telescope in right ascension and declination,