DRAWING INSTRUMENTS. 



127 



Drawing 

 Instru- 

 ments. 



to the holes made through all the parts of the joint. At 

 one end it has a head of brass or silver rivettetl fast upon 

 it, and on the other end a similar head is screwed, which is 

 therefore a nut, and being turned by a screw driver, binds 

 the joint tight to produce any degree of friction or stiff- 

 ness required ; for the object in making a joint is to give 

 it a great degree of friction, and at the same time to be 

 perfectly equable and regular in all parts. For this 

 reason the joints should always be double, that is, have 

 two pieces of steel in the joints, by which means there 

 are four separate surfaces in contact to produce friction, 

 and are therefore less liable to wear than if there were 

 only two surfaces, which is the case when only one 

 piece of steel is used. The joints of common instru- 

 ments not being so well fitted in the circular part, have 

 leaves or projecting parts on these steel pieces, which 

 enter notches at the upper part of the opposite leg when 

 the compasses are shut, but these leaves draw out from 

 the notches aa the compasses are opened. This is 

 not a good method, because the friction is then unequal 

 at the different degrees of opening, and such compasses 

 are liable to jump suddenly when shut up or opened by 

 the gradual pressure of the fingers. All the instruments 

 in Plate CCXXXVII. are made-to fit in the circular part 

 of the joint only, and this should be made large enough 

 to give as much steadiness as the instruments require. 

 The steel legs of compasses should be of cast steel, 

 hardened and well tempered at the points, so that they 

 will preserve a sharp point, without being so soft as to 

 turn up even when used onTjrass or copper, or so hard 

 as to be brittle. The upper part of the steel above the 

 points should be of a spring temper, and then they can- 

 not become bent by any accident, so as to prevent the 

 points meeting each other precisely as they should do 

 to measure minute distances. 



When the points by gradual wear become dull, they 

 should be repaired by sharpening on an oil stone or 

 hone. The insides of the points should first be rubbed 

 quite flat upon it, till on shutting them together the 

 two points meet as if they were one ; they are then 

 to be rubbed on the different sides till both are 

 brought to fine points, which should be tried on 

 paper, till they make very delicate punctures. The 

 strength of the legs should be such, that when the 

 pressure of the fingers is applied to the compasses to 

 open or close them, they will not bend or spring sensi- 

 bly by the force which is requisite to open or close the 

 joint ; if this is found to be the case, the joint must be 

 relaxed by turning the nut of the screw. No accurate 

 measure c.<n be taken in compasses which are so tight 

 in the joint as to cause the legs to spring in opening or 

 shutting, because when the points are brought by the 

 pressure of the fingers to the required opening, they 

 alter their distance the instant the pressure of the fingers 

 is relieved as much as they spring, which, in some com- 

 passes, is a very considerable quantity. 



For measuring very long distances, the beam com- 

 passes alone can be used, and also for drawing portions 

 of large circles. They are usually made with a wooden 

 beam, as shewn in Fig. ] 3. and two brass sliders A, B 

 fitted upon it. These have screws through the upper 

 part of them, by which they can be fixed fast upon any 

 part of the bar ; and in the lower part are sockets for 

 the reception of the different points a, b. These con- 

 sist of two plain steel points, a steel pen, and a port 

 crayon. A screw d is mounted in a socket or frame, 

 screwed to the end of the slider B, and this screw is 

 tapped into a piece of brass which is fixed at the end of 

 the woollen beam ; thrrt-lbre by turning the screw round 

 by its head d, it acts upon the slider B, and slowly ad- 



vances it upon the beam to the exact distance required. 

 In the best instruments, the head of the screw has a 

 circle fixed upon it which is divided, and thus the quan- 

 tity that the point is moved by the screw is ascertained 

 with great precision. The same compasses have gene- 

 rally several beams of different lengths, as it is inconve- 

 nient to have a beam of much greater length than the 

 distance between the points. 



When arches of circles of very great radius are tp be 

 drawn, the beam compasses are not the best instrument; 

 for even when they are extended to five feet in length, 

 the beams are very liable to bend by their own weight, 

 and this throwing the points farther from each other, 

 destroys all accuracy in drawing or measuring : Indeed, 

 in measuring as much as three or four feet, great care 

 must be used to handle them delicately and support 

 the beam in the middle. 



Many artists require very large circles, particular- Instrument 

 ly those who project maps of extensive countries. To f r < j^J' 

 these we can recommend the instrument Fig. 14. This aTtircIesT 

 consists of two rulers A, B, united by a joint at C, which 

 admits of placing the two at any required angle with (.^'xxxvir 

 each other. They have each a circular part behind the pj gi n t 

 centre, and in the upper one is cut a curved groove for 

 the reception of a screw d, which screws into the 

 lower one ; therefore by screwing this tight, the two 

 are immoveably fastened together at any angle at which 

 they may be set. The centre pin of the joint C is per- 

 forated, forming a tube to admit a cylindrical pen, on 

 the lower end of which the drawing point or pin is 

 fixed, and in such a position that its point is in the 

 intersection of the edges of the two rulers A, B. When 

 this instrument is used, two weights E, F are laid upon 

 the table, and they have sharp edges, against which 

 the rulers are applied, as in the Figure ; and when it 

 is slid along against them, the centre point C will de- 

 scribe an arch of a circle. The edges of the weights 

 E, Fare only necessary to represent a stationary support 

 against which the rulers can be applied ; and therefore 

 two pins or needles stuck in the table would have the 

 same effect, and indeed are often used. For the same 

 purpose, each of the weights must have three small pins 

 in the underside of it, which are pressed into the table 

 to hold it fast. By altering the angle of the two rulers, 

 the instrument will draw a segment of a circle of any 

 required radius ; and the mode of setting it is extreme- 

 ly simple : First, mark on the paper two points, which 

 are to be the extreme limits of the arch to be drawn ; 

 then, by drawing the chord and versed sine, an inter- 

 mediate point upon the curve between these two may 

 be found ; now fix two pins in at the extreme points, or 

 place the weights F, F so that their edges match these 

 points, and apply the instrument to them ; now loosen 

 the screw d, and open or close the angle of the rulers 

 A, B, till the centre point C exactly falls upon the inter- 

 mediate point in the curve. Here fasten them by the 

 screw, and the instrument is ready to draw the curve 

 by sliding it gradually against the pins, whilst the pen 

 in the centre traces upon the paper. 



Fig. 21. is an instrument for dividing circles into Farey's di- 

 various numbers, which was invented by Mr Farey. It viding in- 

 consists of a light brass or silver circle AA, having a itrnment. 

 cross bar a with a centre screw fixed in it ; and the Fl 8 4 21< 

 point of this screw is made very delicate, that it may 

 enter the paper without making an unseemly puncture. 

 Upon this centre the circle revolves, and its circumfe- 

 rence has several circles upon it, which are divided in 

 an engine, and holes drilled through the plate at every 

 division, in the same manner as the divided plates of 

 the engines used by watch-makers for cutting cog- 



