Telescopic Mirrors and Object Lenses. 93 
(q, r), with the double lever (7, s), which moves about (s) in a 
vertical plane, perpendicular to the fixed axis (s, ¢). The double 
lever (7, 5), has between its two sides a tongue of spring steel, 
which in one direction catches the cogs of the cogged wheel (s), 
and in the contrary motion passes over them. To prevent any 
retrograde motion of the cogged wheel, there is another steel tongue 
attached to the iron pillar (A). 
By the oscillation of the lever (7, s), the wheel moves over 
one or more cogs for every stroke of the mirror; and by a worm- 
screw (f), with a series of wheels and axles*, turns a large wooden 
pully (P) very slowly, but with great mechanical advantage. 
A string passes round three-fourths of the circumference of this 
pully, and from the upper side of it passes under the moveable 
pully (Q), over the fixed pully (R), quite round the groove of 
the box containing the mirror, over the fixed pully (8S), quite 
round the groove of the large wheel containing the leaden tool, 
from whence it is gathered directly on to the first pully (P). The 
weight (W’) and moveable pully (Q) are to produce friction on 
the first pully (P); and to gather up the loose string as fast as 
it is delivered off from that pully. The same string passing over 
both wheels, causes the mirror and the leaden tool to move round 
slowly, in opposite directions, with angular velocities inversely 
proportioned to the diameters of the wheels ; that is, in the required 
proportion of two to one. , 
To prevent slipping over any of the pullies, the string should 
be rubbed with resin: also any degree of friction may be produced 
on the first pully by mcreasing the weight attached to the move- 
able pully (Q): or the first pully may be a solid block with several 
grooves of equal radius; the string being made to pass several 
* Nothing can answer better for this part of the machine than a common kitchen jack; 
and as the lever is applied to the worm-screw, and acts at a great mechanical advantage, the 
most defective instrument will serve the purpose as well as the most perfect. 
