DRAWING INSTRUMENTS. 



129 



Instru- 

 ments. 



Pr.ATE 

 CCIXXTII. 



Fig. 82. 



Pentagri>li 



PLATE 



CCXXITIII. 

 fig. 7. 



made between these two edges B and e is measured 

 by the divided arch and vernier ; but when it is used 

 for dividing, the point of the detent is set to the re- 

 quired circle, and the semicircle held down fast upon 

 the paper, whilst a mark is made by the edge of the 

 ruler, as shewn in the Figure. The tail of the detent is 

 then pressed to relieve the limb from the circle, and it 

 is moved another division ; and here another mark is 

 made by the rules, and so on, till as many divisions as 

 the semicircle contains are made. Having arrived at 

 the last, the ruler B is to be held fast, by pressing it 

 down upon the paper, and at the same time holding 

 down the tail of the detent. To release the circle from 

 it, the semicircle may be turned round on the centre 

 till the detent come to the commencement, and from 

 this set out again to complete the dividing of the cir- 

 cle, holding the semicircle fast, and turning tke ruler 

 round a division at a time, as before described. This 

 instrument is scarcely so convenient to use as the for- 

 mer, on account of the interruption in the middle of the 

 work to change the position of the semicircle ; but it 

 has one great advantage over them, viz. that it can be 

 extended to circles of larger radius than the instrument 

 itself, because the ruler B may be extended to any 

 length required, whereas the others cannot divide cir- 

 cles larger than the area of the opening within them. 



The pentagraph is an extremely useful instrument 

 to copy drawings, which it will do either on a reduced 

 or enlarged scale, or on the same size, at pleasure. For 

 copying maps or drawings, where almost all the lines 

 are irregularly curved, it is the most valuable, because 

 there is no other direct method of obtaining copies of 

 such lines. The principle of this instrument may be 

 easily explained by Fig. 7. AB and BC are two thin 

 brass rulers, united by a joint at B. DF and EF two 

 shorter rulers, united at F, and attached to the long 

 bars, by joints D and E. The proportions of the rulers 

 nre not important, except that they must form a paral- 

 lelogram, DF being = BE, and DB=FE; therefore in 

 all positions EF will be parallel to AB. The instru- 

 ment opens and shuts freely upon its joints; and at every 

 one of these is a small castor, or wheel, to support the 

 weight of the instrument, as it traverses horizontally 

 upon the table. A, G, and C, are three points upon the 

 rules, where tubes are fixed to receive the drawing 

 points. These three must be in a right line, as shewn 

 l-.y the dotted line, and so situated that the distance 

 ( G, will divide the distance C'A in the proportion in 

 which it is required to copy or reduce the drawing at 

 A, upon the paper at G, by means of a pencil or port 

 crayon, inserted into the tube G ; at A, a blunt tracing 

 point is fitted into the tube ; and the third point is kept 

 stationary, by means of a circular leaden weight placed 

 upon the table, and provided with small pins in the un- 

 dtTslde, to prevent it from moving. The point C, there- 

 fore, becomes the fulcrum or centre on which the whole 

 instrument moves ; and with this motion, combined 

 with that produced by extending and contracting the 

 instrument, the tracer A may be passed along all the 

 lines of the drawing, however irregular, and will com- 

 municate such a motion to the pencil at G, as to copy 

 it exactly in the proportion of the distances CG to 



i.. . I . 



It has been before mentioned, that the three points 

 must be situated in a right line; and it follows from the 

 arrangement of the rulers, that they will, in all the mo- 

 tions of the instrument, continue in a line with each 

 other, and will divide its length in the same proportion 

 in which they were first set upon the rulers. The in- 



voi.. viu. PABT i. 



strument acts to copy and reduce on the principle of 

 the lever ; considering the imaginary line AGC as an 

 inflexible lever, the fulcrum may be placed in either of 

 the points G or C, and in the other the pencil is to be 

 put. 



The point A is always used for the tracer, which is 

 applied to the original map or drawing, and the pro- 

 portion in which the instrument will reduce is thus : 

 As the distance between the tracer and the fulcrum is 

 to the distance between the pencil and the fulcrum, so 

 will the size of the drawing traced over be to the size 

 of the copy made by the pencil. Example, Put the 

 sliders G and C at such parts of their respective rulers 

 as will cause the distance CG (when they are in a 

 straight line) to be only half as much as AC; then if 

 the fulcrum is placed at C, the pencil at G, and the 

 tracer at A, it will reduce to one-third, because AC is 

 three times as great as CG. On the other hand, put 

 the fulcrum at G, and the pencil at C, and it will reduce 

 only one half; because in this instance AG, from the 

 tracer to the. fulcrum, is twice as much as GC, from the 

 pencil to the fulcrum. It must be remembered, that 

 when the fulcrum is placed between the tracer and the 

 pencil, the copy wiU be inverted with respect to the 

 original ; but in the former case, they will be parallel 

 to each other on the table, as shewn in the Figure. 

 The pencil and tracer should be lightly oiled before they 

 are used, to make them move freely in their tubes. 



The tubes are fitted upon the rulers by means of sli- 

 ders, (see Fig. 8,) which have a screw a, to fasten them 

 at any point. Trie tube 6 has the port crayon d accu- 

 rately fitted into it ; and this has a cup or box on the 

 top, by which it is loaded with shot or other small 

 weights, till a sufficient pressure is obtained to make it 

 draw a clear line. That part of the tube b which is 

 beneath the ruler, is made of a proper size, to fit into a 

 socket, which is formed in the lead weight, Fig. 9. at d, 

 and it is this which forms the centre or fulcrum. The 

 socket is very near the edge of the weight, that it may 

 be so placed as to admit the motion of the castor, which 

 is under the joint E, Fig. 7, when the slider is brought 

 up near to that joint, for reducing very small. The ac- 

 curate fitting of the points is very material for copying 

 correctly; and the instrument must be used upon a very 

 flat table, or the points will, by the flexure of the in- 

 strument, be thrown out of the perpendicular. To 

 avoid this danger as much as possible, all the joints are 

 made with short axes, as shewn in Fig. 10, wherey is 

 the end of .one of the rulers, having a short steel axis 

 screwed to it. The pivot at the lower end of this is 

 received into a tube g, which is fixed to the other ruler; 

 and the upper pivot is sustained by a cock or bridge k, 

 screwed to the same ruler. The tube g also receives 

 the spindle h of the castor, or wheel, on which the in- 

 strument travels. This has at the lower end a piece of 

 brass, into which the wheel is fitted, so as to turn upon 

 its own centre ; and by the spindle h turning within 

 its socket, the plane of the wheel always accommodates 

 itself to the direction of the instrument's motion. The 

 spindle h is retained in its place by the point of a small 

 screw entering a notch made round in the spindle. The 

 rulers o.'the pentagraph are marked at C and G, with 

 divisions by which the sliders are set; and these are 

 figured, to shew the proportions in which the instru- 

 ment will reduce when they are so placed. 



Fig. 6'. is an instrument, invented by Mr Farey, for 



the purpose of drawing lines converging to an inacces- 



sible centre. This is extremely useful to those who 



draw buildings, &c. in perspective, when die points to 



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