A SILVERED GLASS TELESCOPE. 21 



machine, except those parts liable to wear liy friction, is made of wood. The ends 

 oo' of the horizontal bar r, arc defended by brass tnlws working in mahogany, and 

 have even now but little shake, though many liundred thousands of reciprocations 

 ha\e l)eeu made. 



The foot power consists of an endless band with wooden treads a', passing at 

 one end of the apparatus over iron wheels /,//', which carry the band-wheel c upon 

 their axle. At the other end it goes over the rollers d d'. Two pairs of inter- 

 mediate wheels ee', serve to sustain the weight of tin 1 man or animal working in it. 

 The treads are so arranged that they interlock, and form a platform, which will 

 not yield downwards. ( hving to its inclination when a weight is put on the plat- 

 form n', it immediately moves from b toward (/ and the band-wheel turns. By a 

 moderate exertion, equivalent to walking up a slight incline at a slow rate, a power 

 more than sufficient to polish a 15J inch mirror is obtained. This machine, 

 in which very little force is lost in overcoming friction, is frequently employed 

 for dain u-e. and is moved commonly in the State of New York by a sheep. I 

 ha\e generalh myself walked in the one used by me, and have travelled some days, 

 during five hours, more than ten miles. 



In order to give an idea of the method of using a grinding and polishing machine, 

 the following extract from the workshop note-book is introduced: 



A disk of plate glass 15^ inches in diameter, and 1^ inch thick was pro- 

 cured. It had been polished flat on both sides, so that its internal constitution 

 might be seen. 1 It was fastened upon the table b of the machine, by four blocks 

 of wood as at c, Fig. 21. Underneath the glass were three thick folds of blanket, 

 15 inches in diameter, to prevent scratching of the lower face, and avoid risk of 

 fracture. A convex disk of lead weighing 40 pounds having been cast, was laid 

 upon the upper surface of the glass, and then the screw I was depressed so as to 

 catch in a perforated iron plate n, at the back of the lead m, and press downward 

 strongly. 



" Emery as coarse as the head of a pin having been introduced, through a hole 

 in the lead, motion was commenced and continued for half an hour, an occasional 

 supply of emery being given. The machine made 150 eight-inch cross strokes, and 

 the mirror 50 revolutions per minute. The grinder m was occasionally restrained 

 from turning by the hand. At the end of the time the detritus was washed away, 

 and an examination with the gauge made. A spot 11 inches in diameter, and ^\ 

 of an inch deep, was found to have been ground out. The same process was con- 

 tinued at intervals for ten hours, measurements with the gauge being frequently 

 made. The concave was then sufficiently deep. The leaden grinder was kept of 

 the right convexity by beating it on the back when necessary. A finer variety of 

 coarse emery, and after that flour emery were next put on, each for an hour. These 

 left the surface moderately smooth, and nearly of the right focal length. The 

 leaden grinder was then dismissed, and the iron one, Fig. 6, put in its stead. The 



1 The glass that I have used has generally been such as was intended for dead-lights and sky- 

 lights in ships. 



