DRAWING INSTRUMENTS. 



125 



Drawing 

 Instru- 



Spring di- 

 viders. 

 PLATE 

 r (XXXVII, 

 I-ig. 1. 



Hair com- 



Triangle 

 com ji.v.-.,. *. 

 Hig. 2. 



Proportion- 

 al ojinpas- 



MS. 



Fig. 6. 



adjustment, by turning die nut B, which gives a slow 

 progressive motion to the screw a, and thus to the 

 points. This -we consider as the best pair of dividers 

 we have seen, for the clamp is very stead)', and quite 

 convenient to use when great accuracy is required; but 

 for ordinary puqxwes, it may be quickly removed alto- 

 gether, by taking out the screw d, and the socket e. 

 The dotted lines shew another sort of clamp, which is 

 simply a curved piece of steel plate, with a slit through 

 it, for the reception of the screw d, which is tapped in- 

 to the leg of the compasses. The other end of the 

 clamp is fastened to the opposite leg by a single screw. 

 This makes a very effectual fastening, to preserve the 

 distance to which the points of the compasses are open- 

 ed, but is not so good as the preceding, because it wants 

 the adjusting screw. 



Fig. 1. is a small pair of dividers. These have no 

 joint, but are cut out of one solid piece of steel, and al- 

 ways tend to open by their own elasticity. They are 

 shut up and adjusted by the small screw a, which is 

 tapped into one leg, and passes through a hole in the 

 other. The upper part of the compasses forms a handle 

 for them. The points 6 and c are made to screw into 

 the legs, in order that they may be taken out to sharpen 

 them upon a hone ; and also because the points require 

 to be made of cast steel, and left of a hard temper, 

 whilst shear steel, and of a spring temper, is more fit 

 for the upper part, as it requires elasticity and tough- 

 ness. These form a most useful instrument for setting 

 out a great number of small equal divisions, not being 

 liable to alter whilst in use; and the screw gives the 

 means of adjusting them very accurately. Spring com- 

 passes are often made on a larger scale, but are not 

 then advantageous for drawing, because they take too 

 much time to set them by the screw. There are like- 

 wise what are called hair compasses, because they mea- 

 sure to a hair's breadth. They are a pair of common 

 dividers, but the steel part of one of the legs is attached 

 to the brass by a long spring, which constantly tends to 

 throw the point inwards, and is counteracted by a screw 

 passing through the brass, and tapped into the leg, so 

 that it draws the leg close to the brass. Having open- 

 ed the compasses nearly to the distance required, by 

 turning the screw, the point is accurately adjusted to it. 

 The only objection to this is, that the leg, depending 

 only upon the spring, is not strong as it should be, but 

 yields very materially when pressed into the paper, 

 and is not therefore to be depended upon for accu- 

 racy. 



Triangular compasses, (see Fig. 2,) are very useful in 

 taking the distances of three points at once. A is the 

 edge view of a pair of compasses, such as shewn in Fig. 

 .5, and B a third leg, attached to them behind by a 

 joint a, which is formed out of the pin b passing 

 through the centre of the principal joint. These are 

 chiefly of use to surveyors in transferring triangles : at 

 one operation they measure one side between the point 

 of the compasses A, and then adjust the point of the 

 leg B to the third angle of the triangle. By bending 

 the joint a, it will advance to or recede from both legs 

 together ; or by turning it sidewise on the centre pin b, 

 it may be brought nearer to either of the other points 

 at pleasure ; and thus the joint a having the properties 

 of an universal joint, the compasses will take in any 

 triangle. 



Proportional compasses, Fig. 6. These have points 

 at each end, the centre or joint being in the middle. 

 They are for reducing drawings in any required pro- 

 portion. Thus, any distance being measured between 

 the points A, B, by inverting the compasses, and era- 



Drawing 

 Instru- 

 ments. 



ploying the points a, b, they will measure out a distance 

 equal to one half of the former ; or the proportion may 

 be varied at pleasure, by altering the position of the 

 centre D. For this purpose, the limbs or legs have slits 

 rnade through them, which are cut dovetailed, and have al compas- 

 pieces of brass fitted into them ; and these two pieces ses. 

 are united together by a steel pin, which forms the P LA TE 

 joint. It is fitted through one piece with a round ccxxxvii, 

 shank and a conical head, and into the other with a Fig. 6. 

 square : beyond this it is formed to a screw, and has 

 a nut D fitted upon it, for the purpose of binding it 

 tight, and thus fixing the sliding pieces fast in any part 

 of the grooves where they are set. When this nut is 

 loose, and the compasses shut up, the sliders can be 

 moved in their grooves, by applying the finger and 

 thumb, to set the centre in a situation for any required 

 proportion ; and for this purpose divisions are made 

 upon the instrument, to set the sliders by; and these 

 are numbered to show the value they bear. Thus, on 

 one edge of the front side of the instrument are divi- 

 sions, entitled lines, numbered from 1 to 10; a line 

 which is marked across the slider being brought to 

 coincide with any of these divisions, will divide the 

 length between the points A a and B b in such propor- 

 tion, that the distance included between the points a, b 

 will divide the distance included between the great 

 points A, B into as many parts as the number shows. 

 The compasses will therefore reduce any drawing in 

 that proportion, by measuring the distances on the ori. 

 ginal by the great points, and marking them on the re- 

 duced copy by the points at the opposite end. It is evi- 

 dent from the Figure, that the two legs of the instru- 

 ment are not in the same plane; but one lying over the 

 other, they appear as one when they are shut up ; and 

 there is a small stud d fixed on one, which enters a 

 notch made in the other, and this keeps the two limbs 

 together, whilst the centre is moved, which cannot be 

 done except when the two legs are shut up, because it 

 is there alone that the grooves are parallel. On the op- 

 posite edge to that containing the divisions of lines is a 

 line of divisions, entitled circles. These are numbered 

 1 to 20 ; and the index being set to any number, the 

 points will open in the proportion of the radius of a 

 circle to the side of an inscribed polygon of that num- 

 ber of sides. Thus, if it is set to No. 8, and the points 

 A, B are opened to the radius oany circle, the opening 

 of the opposite points will divide the circumference into 

 eight equal parts. On the back face of the compasses 

 are two other lines of divisions, one entitled plans, and 

 the other solids ; these are in fact lines of square ami 

 cube roots, and are rather useful in calculation than in 

 drawing. The line of plans or squares, shows the pro- 

 portion between the areas of similar plane figures. Thus, 

 set the centre to No. 3, and measure the side of a square 

 in the long points, the short ones will then mark out 

 the side of a square, which will be one-third of the area 

 of the other; the same of triangles, circles, or any other 

 regular plane figures. The line of solids is in the same 

 manner, to express the proportions between cubes or 

 spheres. Thus, set the centre to No. 2, then measuring 

 the diameter of any sphere, or the side of a cube, with 

 the lon^j points, the others will show the dimensions of 

 a sphere or cube, which will have one-half the solid 

 content of the other. The best kinds of proportional 

 compasses are provided with an adjusting screw, clamp 

 and screw, like that which is applied to the measuring 

 compasses Fig. 5 ; and this is so adapted, that it may 

 be either extended between the legs, to adjust the open- 

 ing of the points, or it may be attached to the sliding 

 centre to give it a delicate motion, for the purpose of 



