( 30 ? ) 
Portion by a Spur, R, part of its upper End on the 
outlide is pared off toward the Edges, to form it into 
the Segment of a Cylinder, whofc Axis coincides with 
that of the Pillar. Its Ufe is to fupport a Reft, S3, on 
which the End of the fiat Arm moves backward and 
forward. This Reft being apply’d trarifverfely to the 
outer Part of the upright Board, where it is made Cy- 
iindrick, is bent into the fame Figure, by the means 
of four Screw-Pins, two of which pafling through each 
End of this, and of another Piece of the fame Length, T, 
(but fomethiiig narrower) placed over agaihft it on the 
infide of the Board, by their Nuts, draw them together, 
fo as to grafp the End of the upright Board between 
them the upper Edge of the Reft being firft (hot with 
a Plane very ftrait and fmooth. To render the Mo- 
tion of the Arm along the Reft fmooth and eafy, it 
has two Rollers lodged in a Box fix’d near to the End, 
on its underfide, V, to roll upon the Edge of the Reft, 
when the End of the Arm is moved along it. One of 
the Rollers is placed near each Edge of the Arm, and 
their Axes lye in Lines paffing through the Axis of the 
turning Pillar. The Reft is kept up to them, with a 
proper Degree of Force, by two Screws, W W, which 
run in two Plugs, XX, faftened on the Sides of the up- 
right Board, and bear againft the under Sides of two Pieces 
hx’d on the Infide of the Reft. 
The Motion of the Tube is governed by two Brafs 
Pegs, Y and Z. The firft of thefe, Y, is plac’d about 
10 or II Inches from the End of the Arm, and has a 
Line wound round it, which pafling under a fmall Pul- 
ley, f, fix’d in a vertical Pofition near the End of the 
Arm, is faftened to a Staple on the under fide of the 
Tube g. This Line, by the turning of the Peg, brings 
the fore End of the Tube to its due Elevation, being 
aded againft: by the Excefs of Weight in the hinder End 
C c c of 
