A SILVERED GLASS TELESCOPE. 21 



machine, except tliose parts liable to wear by friction, is made of wood. Th(> ends 

 ov of the horizontal bar r, are defended liy brass tubes working- in mahoj^-any, and 

 have even now l)ut little shake, though many hundred tliousands of reciprocations 

 have been made. 



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

 one end of the apparatus over iron wheels J> //, whicli carry the band-wh(>el r uixm 

 their axle. At tlie otlur end it goes over tlie rollers r/ J'. Two pairs of inter- 

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

 The treads are so arranged that they interlock, and form a i)lafform. wliicli will 

 not yield downwards. Owing to its incUnation when a weight is put on the plat- 

 form a', it innni'diately moves from /; toward </ and tlie ])and-wheel turns. By a 

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

 more than sufficient to jxilisji a lol incli mirror is obtained. This machine, 

 in which very little force is lost in o\('rcoming friction, is frequently emploved 

 for dairy use, and is moved commonly in the State of New ^'ork bv a sheep. I 

 have generally myself walkfxl in the one used l)y me, and have travelled some days, 

 during tixc hours, more than ten miles. 



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

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



"A disk of plate glass 15^ inches in diametcu-, and \\ inch thick was pro- 

 cured. It had been })olished Hat on both sides, so tliat its interniil constitution 

 might be seen.' It was fastened upon tin' tal)le A of the machine, l)\ four blocks 

 of wood as at c. Fig. 21. Undern(-ath the glass were thrrv 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 4(1 poinids ha\ing been cast, was laid 

 upon the upper surface of the glass, and then tlie screw / ^\•as depressed so as to 

 catch in a perforated iron ])late //, at tlie back of tlie lead ///, and ])ress downward 

 strongly. 



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

 m the lead, motion was commenced and continued for Indf an hour, an occasional 

 supply of emery being gi\-en. The macliine made lot) eighl-incii cross strokes, and 

 the mirror 50 re^■olutions ])er minute. The grind(>r iii was ot-casionallj' restrained 

 from turning by th(> hand. At the end of the time the detritus was washed awav, 

 and an examination witli the gauge made. A s]iot 1 1 inches in diameter, and J, 

 of an inch deep, was found to have been gromid out. The same ])rocess was con- 

 tinued at intervals for ti'u hours, measurements with the gauge being fret[uentlv 

 made. The concave was then suffici(>ntly deep, 'llw \viu\vn grinder was kept of 

 the right convexity l)y beating it on the back when necessary. A finer varietv of 

 coarse emery, and after that Hour emery A^-ere next ])ut on, (^ach for an hour. 'I'hese 

 left the surface moderately smooth, and n(>arly of the right focal length. The 

 leaden "Tinder was tlien dismissenl, and the iron one. I'ii;-. (i. i)nt in its stead. The 



' The glass that I have used has generally been such as was intended Inr di'ad-h'ghts and sky- 

 lights ill ships. 



