VOL. in.] PHILOSOPHICAL TRANSACTIONS. ^QQ 



you intend to work, let there be put a plane of iron or other metal horizon- 

 tilly ; and perpendicularly over this plane let the pole be fastened to the ceiling 

 of the room, if it be high enough, othenvise to another steady fastening lower 

 than the ceiling, in this manner : about the head of the said pole let there be 

 fastened a frame, made of two concentric rings or hoops, so that the one be 

 moved within the other on two poles, and this other on other poles, moveable 

 between two small arms fixed to the ceiling : which frame you may imagine 

 like that by which the mariners compass is kept horizontal, or that which they 

 use in Italy for carrying oil-lamps by night horizontally : or the same may be 

 done with a ball moveable within two circles fastened on the top of the pole. 

 All which will be better understood by the figures, where, in fig. 9, pi. 7, T 

 is the lens, cemented to the piece E, fastened to the pole S, which passes 

 through the centre of the inner circle B, moving on the pivots I, H, (fig. 10,) 

 in the outer circle A ; and this is fastened in a frame on the pivots L, M, in the 

 arms C, D, (fig. Q,) in a wall, or above in the ceiling as was said ; and above 

 this frame let a pin be put through the upper pole, to hinder its getting out of 

 the circle B, and that it may be raised a little, but not to be made lower by the 

 workman. Or else, let the pole S be thrust into the ball O, N, (fig. 11,) 

 moveable within the two circles P, Q, very well fixed to the two arms Z, Z; and 

 let those two circles be made parallel, to prevent the ball from getting out. But 

 the office of these two circles may be performed by one alone, but larger, in the 

 manner of a socket, which may gird about such a part of the circumference of 

 the ball in the middle as not to let it slip out. It is enough, if the ball do but 

 move freely in it ; yet so as always to touch it : which also is to be observed in 

 the ball with two circles, by that means to keep always the centre when it 

 moves just in the same situation. Let the plane of the iron, or other metal, 

 F, (fig. 9,) on which the glass .is to be ground, be placed level on the form G, 

 to do which I have practised the following contrivance : let there be prepared 

 two square planks of wood, F, R, equally thick, long and broad ; but in the 

 undermost let there be fixed a square ruler firm and solid, of a length equal to 

 the thickness of both the planks ; and in the upper plank let there be a square 

 hole or groove so fitted, as that the plank may steadily slide on the ruler ; and 

 to such rulers, which may be called the regulators of the two planks, let there 

 be made a ledge to keep the board more steady and firm upon it. Further, let 

 these two planks have two gutters, R, U, going across from end to end ; into 

 which may pass two wooden wedges like Y, of which may be made four, to 

 put one of them against another in the gutters. And then placing the plate 

 F, T, level on the planks F, R, take a pendulum or other levelling instrument, 

 and fit it on the said plate, and adjust it by the wedges to a level position. 



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