128 



DRAWING INSTRUMENTS. 



wheel*. There are 6, 7, or 8, of these circles, each 

 containing a different nimilxT of holes or di\i- 

 thu, the outer oiu- maybe :f(iO. then 100, <>< ; . <>' 

 72, or it may have any othir mnnlx-rs at pleasure, 

 though these will be found sufficient for general pur- 

 pose*. These divisions are transferred to the paper at 

 any radius, by means of an index < <i. Tins is n piece 

 of steel attached to mi axis r /'l>y means of a thumb 

 screw g, which passes through a slit in the index, and 

 screws into the axis beneath, so as to bind the index 

 fact, and attach it firmly to the axis </ ; but when the 

 crew is loose, the index will slide, so as to bring the 

 crew ate nearer or farther froin the centre, to adjust it 

 to the radius of the circle it is intended to divide- ; for 

 it is the point of this screw which pricks into the pa- 

 per to mark the divisions, and it is for that reason 

 made very sharp. The axisc_/"is supported upon the 

 points of two centre screws, fixed through cocks pro- 

 jecting from die central bar a ; and there is a small 

 spiral spring, consisting of a slender piece of music 

 wire twi>ted round the axis, being secured to it at one 

 end by passing through a hole in the axis, and the other 

 end is turned up to rest on the central bar a ; by this 

 means it constantly tends to throw the pricking point 

 of the index upwards from the paper. The divisions 

 of the circle A are united by a steel point, at the end 

 of a small detent / m, which moves on a centre, and 

 has a spring under it at the end m, to throw the oppo- 

 site end down, and cause its point to enter the holes as 

 they pass in succession beneath it, when the circle 

 turns round. The centre of the detent is supported in 

 a piece of metal n, which has three very small points 

 in the under side of it, that it may fix itself firmly 

 upon the paper, when held down by the pressure of one 

 finger placed upon it. The manner of using the instru- 

 ment is this: Place its centre point in the prick which 

 has been previously made in the paper, as the centre 

 of the circle intended to be divided, and which may, 

 if thought proper, be drawn in pencil ; then loosen the 

 screw . and slide out the index till its point c reaches 

 the circle, and hero fasten it to the axis by the thumb 

 MTCW ; now take the detent, and choosing the circle 

 containing the number intended to be divided, put its 

 point into one of the divisions of that circle, and placing 

 the piece n at any convenient part of the circle, press 

 its three |x>ints into the paper to keep it steady, and 

 hold k down by placing the second finger of the left 

 hand upon it : The thumb of the same hand is to be 

 placed on the rim of the circle, near the point A, for 

 the purpose of moving the circle round upon its cen- 

 tre, and the fore linger is reserved to press down the 

 tail m of the detent, and thus raise its point out of the 

 divisions. Having with the thumb moved the circle 

 round, till, by the point of the detent dropping into 

 oneofthedivisions.it Ix-comes fast, and will turn no 

 farther, you press with the fore finger of the right hand 

 upon the head of the screw r, and cause its point to 

 prick into the paper, making the first division ; then 

 buffer the spiral spring to lift up tin- index c, and take 

 its point out of the paper, and at the same time press 

 down the tail m of the detent to relieve the circle from 

 its points, and, with the thumb, move the circle till the 

 point of the detent falling into another hole stops it at 

 the second division ; here prick the pin c into the pi- 

 per, then lift the detent, and move the circle another 

 division ; mark again, and so on, till the whole num- 

 bers are divided, which may be done very quicklv, and 

 with as great accuracy as it is possible to obtain upon 

 paper. The stud which supports the centre of tlc 



detent, turns round in a socket in the piece n, by which 

 means it can be turned to reach miy one of the circles 

 of divisions at pleasure. If a circle is to be divided into 

 oiif-half. one-third, or one-fourth, of any of the num- 

 bers divided on the 1'late. it can be done by suffering 

 the circle tu move 1,,', 3, or +, divisions every time, 

 b< -fore the priek is made, instead of once. The innn- 

 divided on the circle may be doubled thus : !!- 

 ving divided one circle as descril>cd, subdivide one 

 of the divisions ; then loosen the screw jr. and mo\e 

 the index sidewise the quantity of halt' a division, till 

 on trial the point will exactly enter the sulxli\ ision at 

 the time when the point of the index is holding the 

 circle by one of its divisions ; now fasten the MTCW, 

 and proceed to divide the circle over again ; then the 

 division made the second time will fall exactly between 

 those first made, and thus fill up the circle with double 

 the number first set out. Instruments on this plan 

 may be made to any size less than eight inches diameter. 



Fig. 20. represents another instrument by Mr Farcy, 

 which has the power of drawing lines, as well as marking 

 points. The circle of this is exactly the same ;i- the for- 

 mer, except that the radial bar has an arm behind it. The 

 instrument is provided with a similar detent to count the 

 divisions; but instead of the index for pricking, a bra-s 

 frame a b c is fixed UJHUI it by a screw at c. It supports 

 the ends of a steel wire a I, upon which is fitted a 

 sliding socket rf, adapted for receiving the leg of a pair 

 of compasses L, which are of the same structure as 

 Fig. 3 : the leg (its fast in the socket, which may 

 therefore, be considered us one piece with the leg. The 

 opposite leg of the compasses then stands in a proper 

 direction to draw lima on the paper, when motion 

 is given to it, by sliding the socket upon the wire. 

 This must be very accurately fitted, to be without any 

 shake, and it will then draw a straight line parallel to 

 the wire. The length of this is limited to what is n - 

 quired by adjustable stops, I and '-', which fit upon tilt- 

 wire, and by means of a small screw can be fastened 

 at any part : one of these stop the motion of the socket 

 each way. It i* plain that, by opening the com] 

 out, the lines may be drawn at any required distance 

 from the centre ; or, by t>\ isting them round on the 

 joints, which fix in the legs, they may be drawn at any 

 part of the area of the circle, and their length is deter- 

 mined by the stops. 



Fig. 22. is a very convenient protractor, combined 

 with a dividing instrument, which was invented by 

 Alexander Jafl'ray, E*<\. It is a semicircle AA gradu- 

 ated on the circumference M'ith degrees ; and within 

 this are several portions of circles divided into different 

 numbers, and drilled through the plate like the former; 

 the centre screw has a poirt to enter the pajwr. and it 

 is also the centre for an index or limb CD. This has a 

 vernier at the extremity C to apply to the graduated 

 arch AA, and within this it carries a detent <l, which 

 moves on two centre screws as an axis, and has a point 

 to enter the divisions of the M-\cral circles. This point 

 is fitted through a groove made in the detent, but can 

 be fastened at any part of the length of the groove by 

 a thumb screw, and in this manner it is set to any of 

 the dilferrnt rows of divisions. The tail of the deteii! 

 is brought very nrar to the centre, to be convenient 

 for the application of (hi- finger upon it.' The drawing 

 of the lines, or marks, is performed by means of a ruler 

 H, which is attached to the opposite end of the limb, 

 and traverse:- over the paper, having a thin edge which 

 points to the centre, as does likewise the edge i of tin 

 semicii -!<-. When it is used as a protractor, the angle 



1 



Drawing 



Instm- 



di. 



viilhip in- 

 Mruiucnt. 



Another 

 dividing in- 

 strument by 

 Mr Farcj. 



l'i MI: 

 ccxxxvii. 

 Fig. 80. 



Dividing 

 Fig. Z2. 



