Addition of Numbers with Accuracy and Dispatch. 169 



centre index CL slips over the sloping side of these teeth 

 ■when raoved in one direction ; but when moved in the 

 other, its edge c catches into the perpendicular sides of the 

 teeth, and carries the circle round with it. EE, fig. 3, are 

 two brass cocks, screwed to the side of the lower circle 

 BB, and projecting from it beyond the circumference M 

 of the upper circle ; the ends of ihem support a flat circular 

 wooden or brass limb, FF, which (as shown in fig. 1.) has 

 other correspondent divisions and figures upon it, over 

 which the inde.x passes : at one end of the limb a wire 

 stop, b, is fixed; and when the index is pressed against this, 

 its edge c will stand upon the figure 1. of the limb FF, 

 which is numbered on progressively, 1, 2, 3, 4, 5, G, &c. to 

 50; which numbers are the same distance apart as the teeth 

 upon the edge of the great circle A; so that, by moving the 

 index to any of these numbers, its edge C will have passed 

 over the same number of teeth of the circle, as the number 

 of the limb which it is carried to denotes; but in passing 

 in that direction it slips over the sloping edges of the teeth 

 without moving the circle : now, the edge C having ar- 

 rived at any intended number, as 19, for instance, the edge 

 of the lever is pressed into the teeth ; and being brought 

 back again till it touches the stop b, it will have moved 

 the circle A round I9 teeth. At the extreme end of the 

 limb FF, a piece of brass, PP, is fixed, so as to form a 

 reading-index for the numbers on the several circles, which 

 are described on the face of the great circle AA : these are 

 four in number, viz. one for the pence, one for the shillings, 

 and two circles for the pounds : the external circle, which 

 is the pence, is numbered 1, 2, 3,4, 5, 6, 7j 8, 9, 10, 11, 

 then 1, marked in red, to denote 1 shilling* : then 1, 2, 3, 

 4, 5,6, 7, 8,9, 10, 11, again, and 2, in red, to denote 2 

 shillings, and so on, up to 180, which will be 15 shillings: 

 the next circle towards the centre is for the pounds, this is 

 numbered 1, 2, 3, 4, 5, 6, &c. up to 19, then one, in red, 

 for one pound: then I9 numbered successively again, and 

 2, in red, for two pounds, and so on till 9 pounds, which 

 fills the circle, because 9 pounds contain 180 shillings : the 

 third circle towards the centre is for the addition of pounds, 

 or any other whole numbers : the circle therefore is num- 

 bered in regular ascending series, from 1 to 180; but to 

 enable the instrument to count higher than 180, the fourth 

 circle is introduced : this begins at ISl, and proceeds, by a 

 regular increase, to 360; G, figs. 3 and 4, is a detent, 



• Thor.e flp^res. which in the instrument arc marked in red, the engraver 

 has (lisiinguiihcd by including them ia a tmiill circle. 



moveable 



