536 Royal Astrotiomical Society. 



that superior degree of regularity which is given by machinery, the 

 operation of polishing by hand. Proceeding from the quick motions 

 to the slow ones, the order of movements is as follows : — (I.) By a 

 steam-engine, a rotatory motion round a vertical spindle is given to 

 a crank, which by a connecting rod acts upon a sliding rod that 

 moves the grinder or polisher backwards and forwards. This sHding 

 rod passes through a fixed guide at the end next the connecting rod, 

 and through a slowly moving guide [described under (2)] at the other 

 end ; and thus every stroke of the grinder is very nearly a straight 

 stroke. The sliding rod is interrupted in its middle, and there its 

 place is supplied by a hoop which loosely surrounds the grinder : 

 this construction is necessary, because, on account of the great size 

 and weight of the grinder, it is necessary that it be supported by 

 counterpoises in various parts, at the same time that it is necessary 

 that (in order to prevent the formation of striae in a definite direction 

 on the grinder) the grinder be left free to revolve : that motion of 

 revolution is given merely by the friction upon the mirror, whose 

 rotation will be mentioned under (3), and depends upon the weight 

 with which the grinder presses the mirror. (2.) A band from the 

 crank-spindle (1) passes round a wheel which carries a crank, in 

 which is the forked guide (turning by a spindle in the crank end) 

 that guides the distant end of the sliding rod ; and thus the strokes 

 of the grinder do not pass uniformly over the centre of the mirror, 

 but pass in their direction a certain distance to the right and left. 

 But as, in the ordinary crank motion, the duration of the strokes «t 

 the extreme right and left would be too great, the wheel on the 

 spindle of this grinding- crank is elliptical, the proportion of its axes 

 being about three to one : its angular motion is therefore unequal ; 

 and the strokes are thus made to dwell a shorter time near the ex- 

 treme right and left, and a longer time near the centre. (3.) Anothev 

 band from the crank spindle (1) passes round a large wheel on the 

 vertical axis, that passes through a stuffing-box in the bottom of the 

 grinding-trough, and there carries a broad frame, on which is placed 

 the fixed frame of the mirror, supporting the mirror itself. Thus the 

 mirror turns slowly round to receive the strokes of the grinder in 

 every direction. It is only necessary to add to this, that a large 

 part of the grinder's weight is equally supported at twelve points by 

 a lever counterpoise above it, which supports the centre of a triangle, 

 each point of which (by a roller over which a cord passes) supports 

 two points, each of which i;)oints is the middle of a straight lever 

 whose ends are attached to the grinder. 



In Mr. Lassell's apparatus (omitting some details, which are not 

 now necessary, as a detailed account of this apparatus has lately 

 been given by Mr. Lassell himself to the Society), the steam-engine 

 puts in motion (1) a vertical spindle below, which turns the mirror 

 slowly, and (2) a vertical spindle above, which has a horizontal arm, 

 in which is a planet- wheel turned by its tooth-connexion with a 

 fixed sun -wheel. The axis of this planet- wheel carries another 

 wheel that works in a third wheel, whose spindle is carried by the 

 horizontal arm, but whose place can be fixed at pleasure near to or 

 far from the centre of the vertical spindle (2). An arm from the 



