MATHEMATICAL INSTRUMENTS. 15 



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 con- 

 structing any proposed figure ; in plotting, or making plans. 

 The use of the compasses occurs in every branch of prac- 

 tical mathematics. 



The Drawing Compass. (Fig. 3, page 17.) These com- 

 passes 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 

 strew, may be separated from the other* and wheels mark- 

 ing- 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 



