106 



Mr. Samuel Hunter, 



For a 2-feet speculum H K revolves about 132 times per 

 minute ; the eccentric S 17 times; V 347. The speculum A, 

 0*46, T being 1*7 inches from its centre to centre of spindles, Mr, 

 and Y being 1*4 inches from centre of T. The curve described 

 by the centre of the polisher in these conditions is shown in 

 figure 7. An exactly similar curve would be obtained if S did 

 not revolve at all (the revolutions of T being unaltered), and the 

 speculum at a speed of 1*7+ 0*46, the sum of their velocities, 

 equivalent to 1 turn of speculum to about 16 of Y. The dotted 

 line in figure 7 shows the path of the centre of T, the centre 

 of the speculum being at C. It was found by Mr. Lassell that 

 the polishing was apt to proceed in rings to the detriment of the 

 figure, he caused the speculum to rest on a sliding bed instead of 

 directly on the wheel C, which being acted on by a roller fastened 

 to the wall plate, received a thrust of about one and a-half inches 

 on two opposite sides. Delarue made a further improvement by 

 causing the speculum slowly to revolve on this, so that the thrust 

 did not always occur across the same diameter. 



Mr. Grubb's First Machine. 



Fig. 8. 



This machine gives Lassell's curves, and is capable also of 

 approximating closely to Lord Rosse's. 

 /is a worm-shaft driving the wheel b of 90 teeth ; c also of 90 



