9 Drawing pencil, 10 Ivory sector, 11 Ivory parallel rule, 12 Ivory protractor and plane scale combined. The most extensive sets contain, in addition 13 Pair of proportional compasses, 14 Pair of triangular compasses, 18 Fine steel bow pencil, 20 Double drawing pen, 21 Fine dotting instrument with set of movable rollers, 22 Needle holder. Price, mounted in German silver, $20.00. COMPASSES. COMPASSES are made of brass, or fine German silver, and with steel points. In good instruments the joints should be framed of different substances; one side or part should be of German silver or brass, and the other of steel, as the difference in the metals diminishes the wear and promotes uniformity in their motion; all shake and irregularity at the joint is a sign of imperfection. The points should be of steel, so tempered as neither to be easily bent nor broken; fine and tapering, and meeting closely when shut. Plain compasses are used to measure small distances, and for subdividing them; drawing circles, arches, or for constructing any proposed figure; in plotting, or making plans. The use of the compasses occurs in every branch of practical mathematics. The Drawing Compass.-(Fig. 3, page 17.)-These compasses are chiefly designed for drawing circles and circular arches; and it is often necessary they should be drawn with different materials, and therefore this pair of compasses has, in one of its legs, a triangular socket and screw, to receive and fasten the following parts or points for that purpose, viz. : 1. A steel point, which, being fixed in the socket, makes the compasses then but a plain pair for drawing blank circles, setting off lines, &c. 2. A pencil point (Fig. 4, page 17), for receiving a pencil or crayon, in using which the lines can be easily rubbed out if not right. 3. The dotting points (Fig. 5, page 17), or dotting pen, with a small indented wheel at the end, moving very freely, and receiving ink from the pen over it, communicates the same in equal and regular dots upon the paper, where dotted lines are chosen. In the most costly instruments one of the blades of this instrument is jointed, and by loosening the screw, may be separated from the other, and wheels marking different figures used; as a dot, a short line, a long line, a dot and a line, two dots and a line, &c. Also, by taking off the wheel it may be used as a pen for drawing very wide ink lines, the pen causing the ink to flow freely in a very wide line. 4. The ink point, or pen, (Fig. 6, page 17), for drawing and describing lines in ink; for this purpose the two blades or sides of the pen are opened or closed with an adjusting screw, that the line drawn may be as fine or as coarse as you please; in fine instruments, one of the blades is framed with a joint, that the points may be separated, and thus cleaned more conveniently. In the pencil point, dotter, and pen point, there is a joint by which you can set the lower part always perpendicular to the paper, which is necessary for drawing a line well in every opening of the compasses. 5. Lengthener.-(Fig. 7, page 17.)-One or two additional pieces are often applied to the best compasses; these by lengthening the leg enable them to strike larger circles, or measure greater extents than they would otherwise perform, and that without the inconvenience that would attend using long compasses. Compasses of the best kind are frequently framed at the end of the shank, so as to form a strong spring, and the points and lengthener slide into this socket, and are firmly held. The best description are furnished with joints in one or both legs, that they may be placed perpendicular to the paper. Price, in German silver, $12 to $18. Hair Compasses.-(Fig. 8, page 19.)—They are so named, on account of a contrivance in the shank to set them with greater accuracy than can be effected by the motion of the joint alone. One of the steel points is fastened near the top of the compasses, and may be moved very gradually by turning the screw either backwards or forwards. To use these compasses, 1st, place the leg to which the screw is annexed, outermost; 2d, set the fixed leg on that point from whence the extent is to be taken; 3d, open the compasses as nearly as possible to the required distance, and then make the points accurately coincide therewith, by turning the screw. Price, in brass, $2.00 to $3.00. The Drawing Pen.-(Fig. 9, page 19.)-This pen is used to draw straight lines; it consists of two blades with steel points fixed to a handle. The blades are so bent that the ends of the steel points meet, and yet leave a sufficient cavity |