96 



THE POPULAE EDUCATOE. 



silver, and other compound metals, such as compasses, drawing 

 pens, and protractors. We shall proceed to describe the most 

 useful, and afterwards to show their application. 



The Common Ruler or Straight-edge. This instrument gene- 

 rally consists of the bevelled edge of the plane or diagonal scale, 

 of the common Gunter's scale, of an ordinary foot rule, or of a 

 plain flat rule made with a fine straight edge, for the 

 sole purpose of drawing straight lines from one point 

 to another, or through any two points. It is some- 

 times made in the form of a right-angled triangle 

 (Fig. 1), with a similar edge, to serve the various pur- 

 poses of drawing straight lines, perpendiculars, right- 

 angled triangles, and parallel straight lines. In the 

 mechanical arts, a straight line is most readily ob- 

 tained by fixing a well-chalked string firmly at both 

 ends over the place where it is wanted, on a board or 

 stone, raising it, when tense (i.e., stretched), above the same, and 

 then letting ii drop suddenly, when the white or chalky trace of 

 the string will be marked on the board or stone as a straight 

 line. 



The Parallel Ruler. This very useful instrument is con- 

 structed in a variety of forms. Those represented in Figs. 

 2, 3, and 4, are the most common, and the cheapest. The 

 defect of the construction in Fig. 2 is, that in drawing a parallel 

 to a straight line through a given point, if the latter be at a 

 considerable distance from the former, the ruler may, from its 



Fig. 4. 



lateral motion, pass the point altogether, and render the 

 problem nugatory. This defect is obviated by the construction 

 in Figs. 3 aud 4, provided they be properly managed ; but this 

 management is the result of a little practice. 



The triangular ruler represented in Fig. 1 being made to slide 

 against a fixed ruler or straight-edge, as represented in Fig. 5, is 

 frequently employed for the purpose of drawing parallel straight 

 lines, In many cases this apparatus will be found even more 

 handy for this purpose than the parallel rulers represented 

 above. Fig. 5 represents the same triangle in two different 

 positions, and not two separate triangles. 



In order to test the accuracy of a 

 ruler, let it be applied to one eye, and 

 viewed along its edge from one ond to 

 the other ; the slightest departure from 

 the straight line will then become 

 visible. A good ruler, besides having 

 a straight edge, must be perfectly flat 

 and even, flexible, and made of well- 

 seasoned wood. Some are made of 

 ivory, bone, and metal ; these are less 

 liable to be affected by changes in tem- 

 perature, or by the humidity of the 

 atmosphere. Parallel straight lines are 



most easily drawn by artists and mechanics, with an F or a T 

 square, of which the form is distinctly noted by the name. 



The Compasses. Of compasses there are several kinds. This 

 instrument, which usually consists of two equal legs jointed at 

 one extremity, is employed for measuring the lengths of straight 

 lines, measuring and laying off distances, and describing circles 

 or arcs of circles in general. The Dividers, or compasses with 

 dry points, represented in Fig. 6, are chiefly used for dividing 

 straight lines into equal parts, or into parts having any other 

 proportion to each other. The best kind are furnished with 

 a turnscrew for tightei>ing the screw-axle at the joint. Others 



5. 



are furnished with an arc and tangent screw to fix the legs at 

 any required distance apart. 



The Socket Compasses, represented in Fig. 7, are furnished with 

 movable points, or pieces, which can be inserted in the socket at 

 pleasure, according to the use which is to be made of them. It is 

 chiefly employed in describing, that is drawing circles, in ink, or in 



o 



Fig. 8. Fig. 9. 



Fig. 10. 



pencil, or in mere trace. The tracing- 

 point in Fig. 7 is furnished with a joint 

 p. and a screw, in order to keep it perpen- Fig. 7. 



dicular to the paper when the legs are 



stretched to a great length. The ink-point, represented in Fig. 8, 

 is furnished with a screw, to admit more or less ink at pleasure, 

 with a joint for the same purpose as the tracer, and with a 

 joint in one of the leaves of the point to admit of its being 

 cleaned. The pencil or crayon-point, represented in Fig. 9, is 

 furnished with a joint for keeping the pencil or crayon per- 

 pendicular to the paper, and a socket or case for holding it. 

 The socket compasses are also furnished with a lengthening liar, 

 represented in Fig. 10, which is furnished with a socket exactly 

 the same as that of the leg, in order to admit of the 

 description, that is, the drawing of larger circles than 

 those which can be drawn only by the use of the 

 movable points and legs of the compasses. 



The Bow Compasses, so called because in their 

 first construction they could be shut up into a hoop, 

 which served as a handle to them ; or the Plug Com- 

 passes, represented in Fig. 11, and so called because 

 the stationary leg screws out and in like a plug, are 

 only used for describing circles of a very small size. 

 Such compasses are of the greatest utility to draughts- 

 men and engineers in drawing their plans. The 

 plug construction seems to present some advantages over the 

 old bows. 



Fig. 11. 



-"-* if if- 



Fig. 12. 



Beam Compasses are employed for describing circles of very 

 large radius, and such as are far beyond the reach of a case of 

 mathematical instruments. They consist of a long beam or bar, 

 carrying two brass cursors, that is, pieces on which it runs. One 

 of these is fixed at one end, and the other slides along 

 the beam, and is furnished with a screw to fix it at 

 any required distance. To the cursors may be screwed 

 points of any kind, whether steel tracers, pencils, or 

 crayons, or ink points. This apparatus is represented 

 in Fig. 12. To the fixed cursor there is sometimes 

 applied an adjusting or micrometer screw, as seen in 

 the figure, to enable a given distance or radius to be 

 taken with the greatest nicety. 



In a case of mathematical instruments are also con- 

 tained a Tracer and Draicing Pen, for drawing straight 

 lines in trace, or in ink. These two are usually 

 joined in one instrument, the tracing point being 

 screwed into the drawing pen ; this instrument 

 is represented in Fig. 13, where the ink-point is 

 constructed exactly on the same principle as that of 

 the socket compasses. In choosing a drawing pen, it 

 is better to select one which has an ink-point made of 

 German silver. The steel ink-points are apt to get 

 rusty if they are not kept carefully wiped, and lines ~ ' 

 drawn in red ink with a steel-pointed drawing pen soon get 

 discoloured, owing to the action of the ink on the metal while 

 in the pen. 



