Page:Philosophical Transactions - Volume 003.djvu/217

, or the conveniency of the place; and above this Frame let a Pin be put through the upper Pole, to hinder its getting out of the Circle B. and to the end 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. 3.) movable within the two Circles P. Q. very well fixt to the two arms ZZ. and let those two Circles be made parallel, hindring the Ball to get out. But the Office of these two Circles may be perform'd 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 flip out. 'Tis enough, if the Ball do but freely move in it; yet so as alwayes to touch it: which also is to be observed in the Ball with two Circles, by that means to keep alwayes the Center thereof, when it moves, just in the same situation.

Let the Plain of the Iron, or other Mettal, on which the Glass is to be ground (F. in Fig. 1.) be plac'd level upon the Form G. to do whic, I have practised the following Contrivance. Let there be prepared two square Planks of wood (FR) equally thick, long and broad; but in the undermost let there be fixed a square Ruler, solid and firm, as long as is the thickness of both the said Planks; and in the upper Plank let there be a square hole or groove so fitted, as that the Plank may steddily slide upon the said Ruler; And to such Rulers (which may be called the Regulators of the 2. Planks) let there be made a ledge to keep the board more steddy and stiff upon the Ruler.

Further, let these 2. Planks have two Gutters (R. V.) of a Figure, going a cross 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 said Gutters.) And then, when you will place the Plate FT. level on these Planks, FR, take a Pendulum, or other levelling Instrument, and adapt it upon the said Plate, which if it be not found level, observe on which side it leans, and adjust it by the wedges.

The use of this Instrument is very easie, since 'tis sufficient to guide with your hands the Mallet, fastned to the Pole upon the Plain, where the sand is spread; making such turns, as they use to doe in this work; and continuing so, till the Glass has taken Rh