( ) 
to the Figure, and their Diftance depends on the 
Ratchet-Teeth (Fig. 4.) in the Brafs-Bottom. 
On the Axis of each uppermoft Wheel a (which 
Axis muft be made fquare as far as it projedls over 
the Covering- Plate) is fixed a thin round Silver-Plate 
x x (in the Syftems BB and CC) or ad in Fig . 3. 
yet fo that it may not rub againft the Covering Plate* 
It has a hollow Axis be (Fig. 3.) on which is a right 
or left Skrew, according to the Syftem it belongs to, 
and a fmall Shoulder c. To the Skrew is skrewed 
the Handle fs (Syftem BB and CC, Fig . 1.) which 
is vertically flat on- the Extremity, in order to turn 
by it the Plate and the Wheels. The Plate (as ap- 
pears by the Figure) is divided by 3 concentiick 
Circles into two Rings, in the outmoftof which are 
engraved the Numbers for Addition , in the inmoft 
thofefor Subjlra£tion. I will hereafter call this Plate 
only the Silver-F late , the firft Ring the Addition- 
Ring , thefecondth zStibflr action- Ring : Moreover 
two Indexes w andjy are skrewed to the Covering- 
Plate 5 w Ihews the Numbers of the outmoft or 
Addition-Ring, , and y thofe of the Subtraction- 
Ring. They have Hinges, that they may be lifted 
up, and the Silver-Flatc taken out or put in again : 
Their Curvature ferves for a Direction, which way 
the Plates ought to be turned. A skilful Artificer 
will be able to give them a neater and handfomer 
Shape, than here in the Draught, where I would not 
cover the Numbers. 
Ail this being done, there remains now the 
Figures or Numbers to be engraved, in the manner 
following : Place each uppermoft Wheel ^(Syftem A A) 
fo, that the Tooth of Communication be ready to 
M % catch 
