INSTRUMENTS, Hatkematical, a set of implements for describing mathematical diagrams and drawings of every description, when the figures or elementary parts are com posed of straight lines, circles, or arcs of circles. The most useful drawing instruments are the tbIlowing A drawing-pen—a pair of plain compasses, commimly called pair of drawing compasses, with a pen and pencil foot—a pair of bow-compasses—a pair of triangular com passes—a pair ofproportional compasses—a set ofspring-bows for small work, consisting of spring-dividers, spring-bow pen and bow-pencil—a protractor, in the form of a semici•cle, or of a rectangle—a plain scale—a a parallel rule.
Figure 1. Plate drawing yen. No. 1, the steel pen. No. 2, the same with a pointer, which is screwed into the upper end of the drawing-pen, and has its point enclosed in it, as in a case. The end which contains the ink consists of two thin plates, adjusted by a screw ; one of the plates is movable on a joint, for the purpose of being cleaned.
Figure 12. Plate II.—The dividers. The common sort.
Figure 9, No. 2, hair dividers, in which, by means of a screw, at a, the slighest alteration may be made in the extent, by turning it, one way or the other, as the distance requires to be shortened or extended.
Figure I2.—The drawing compasses. No. 1, the crayon foot. No. 2, the ink foot. Both these feet are tnovable upon it joint., so that when fitted in for use, they may be brought perpendicular to the surface of the paper, and thereby per form with greater accuracy. One leg of these compasses has a cylindrical socket, with a parallel slit on the outer side, into which the upper end of each foot is closely fitted : the slit permits the leg of the compass to expand at the socket, and thereby retains the foot with the greater force, in con sequence of the spring acquired by the surrounding parts of the socket in forcing in the foot.
one of small dimensions, and were so called frbin the handling point in the original construction being made in the form of a bow ; but which being Ibund inconvenient for the fingers, has been altered to the present shape.
Figure pair of triangular compasses ; consisting of three legs, two of which are movable at the head, in the same manner as the divider-4 or the drawing-compasses; the third leg, which is made of steel, is fitted into a socket, through a knob priljeeted the side of the head, by which means it is movable in almost any direction, while the other two remain stationary. Sometimes there is a joint in one of the legs, which permits its being lengthened or shortened at plea sure, so as to answer the expansion of the other two.
The plain proportional compu•ses are made in the form of a cross, haying a parallel slit cut down the middle of each part, and a centre piece fitted to each, so as to be movable round a pin, and to slide from one end of the slit to the other, or to any intermediate distance, to admit of the two points at one end being set in any ratio to the two points at the other. The centre piece is fhstened by means of a nut and screw.
Figure 10.—The improved proportional compasses. In these, the method of construction is in all respects the same as in the plain proportional compasses ; but the. adjusting screw is an addition, which admits of its being set with greater accuracy. The parts of the proportional compasses, when shut, mutually cover each other. Scales of different gradations are engraved on the margin of each part, and on both sides of the compasses, for certain uses, which will be afterwards shown.
The common protractor is in a semicircular form, with a scale upon one side ; Figure 11, Plate 1.—The best kind of protractor, in the form of a rectangle.
Figure 12.—A plain scale ; consisting of lines divided into equal parts, of various proportions.
Figure 13.—A sector, in the form of a joint rule. The figure represented is of the best kind, having a French joint. Figure 14.—The common parallel rule.