306 
MR. W. LASSELL ON POLISHING THE 
long shaft or lever C, as shown in fig. 7. On the spindle E is keyed a pulley 8*3 inches 
diameter, connected by a crossed band with another pulley of 9 inches diameter, keyed 
on to the main spindle or shaft B. These pulleys, differing but little in diameter, are 
intended to be prime to each other, to avoid a repetition of the same strokes in the 
crank-arms. 
Attached to the long shaft C is an apparatus for securing a regular slow motion of 
the polisher on a vertical axis. On the hack of the polisher is a circular rack of 128 
teeth, driven by a pinion of 15 teeth, the shaft of which works in a little frame attached 
to the long shaft, as shown at F. On this small shaft are two pulleys, either of which, 
by means of two direction-pulleys (G) and a round hand, may be driven by any of the 
pulleys which are keyed on the upper portion of the driving-pin of the crank-arm H. 
The axis of the direction-pulleys G is secured to a separate piece of wood, which can be 
fastened to the long shaft, or raised somewhat from it by means of two wooden screws, 
as shown in the drawing ; thus the band can be kept at a proper degree of tension. If 
the direction-pulleys were to be brought into immediate connexion with any of those 
on the shaft H, the speed would be too great, and therefore two supplementary systems 
of speed- or cone-pulleys are introduced between G and H. These afford abundant scope 
for alteration of speed ; and by crossing any one of the bands the motion of the polisher 
on its axis may be either in the direction of that of the sun or the reverse. At J is a 
hook, attached by a cord going over a large pulley in the ceiling to a counterpoise- 
weight, by which the whole or any portion of the weight of the long shaft C may be 
supported. The teeth of the circular rack and of the pinion are made as long as can 
be, consistently with their working well together ; and the counterpoise is so regulated 
that they remain engaged without the apices of any of the teeth coming into contact 
with the opposite bases. Therefore the weight of the polisher, which is of course a 
constant quantity, or very nearly so, is the only weight pressing on the surface of the 
speculum. The axial motion of the polisher is usually in the opposite direction to that 
of the speculum, and its speed is slower. These constitute the ordinary motions of both 
the speculum and polisher. 
The polisher, equal in diameter to the speculum, is made of two strata of white deal, 
such as is generally used for the inside carpentry of dwelling-houses, the grain of the 
wood being placed at right angles in the two disks, which are about 1^ inch thick, cut 
out of adjacent parts of the same well-seasoned board. One surface of each board is 
planed as flat as possible, and then they are united together with the best glue under 
strong and equal pressure. While the pressure is still applied and the glue warm, the 
disks are further secured in contact by about two dozen countersunk screws, equally 
distributed. Calling the disks A and B, half the screws are entered on the side A, and 
half on the side B, each disk having been previously bored and countersunk for its own 
screws (for expedition’s sake), so that only what boring may be necessary in the other 
disk is done after gluing. The two external surfaces of the polisher are now to be 
wrought or planed, for symmetry’s sake, to fit approximately the concave gauge of cur- ' 
