HOW TO WORE WITH THE TELESCOPE. 463 
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 h e and 
Tv’ e respectively work, are so fastened by elastic cords, that an 
upward pressure on the handle h, or a downward pressure on 
the handle 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. As here shown, the 
mounting is too expensive for an alt-azimuth ; but it is clear 
that, in place of the toothed-wheels a and c, a simple arrange- 
ment of belted smooth wheels could be substituted. 
But, for systematic observation of the heavens, the alt- 
azimuth must yield place to the equatorial. In this mode 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 equa- 
torials is that exhibited (with the omission of some minor details) 
in Plate XXII., fig. 2. 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 axis, . the motions of the telescope in right ascension and 
declination respectively. The weight w serves to counterpoise 
the telescope, and the screws s, s, s , s , serve to adjust the in- 
strument so that the polar axis shall be in its proper position. 
The advantage gained by the arrangement above indicated, is 
that only one motion is required to follow a star. Owing to 
the diurnal motion of the earth, the stars appear to move 
uniformly in circles parallel to the celestial equator ; and it is 
clear that a star so moving will be kept in the field of view, if 
the telescope, once directed to the star, be made to revolve 
uniformly and at a proper rate round the polar axis. 
The equatorial can be directed, by means of the circles c and 
d, to any celestial object whose right ascension and declination 
are known. On the other hand, to bring an object into the 
field of view of an alt-azimuth, it is necessary, either that 
the object itself should be visible to the naked eye, or else 
that the position of the object should be pretty accurately 
learned from star-maps, so that it may be picked up by the 
alt-azimuth after a little searching. A small telescope called 
a finder is usually attached to all powerful telescopes in- 
tended for general observation. The finder has a large field 
of view, and is adjusted so as to have its axis parallel to that 
