Perspective and geometrical drawing adapted to the use of candidates for second and third-class teachers' certificates . polygon straight lines be drawn tothe angles, the figure will be divided into as many trianglesas it has sides, and each triangle will contain two rightangles, but the angles around the common point within, to-gether make four right angles. Then if N represent thenumber of sides, the number of degrees in the angle of a regular polygon will be —^^ ^, that is, ^^ \ Now, 120 DRAWING. in the above figure the line DB may be called 180°, or two right angles. Then the angle EAB wil
Perspective and geometrical drawing adapted to the use of candidates for second and third-class teachers' certificates . polygon straight lines be drawn tothe angles, the figure will be divided into as many trianglesas it has sides, and each triangle will contain two rightangles, but the angles around the common point within, to-gether make four right angles. Then if N represent thenumber of sides, the number of degrees in the angle of a regular polygon will be —^^ ^, that is, ^^ \ Now, 120 DRAWING. in the above figure the line DB may be called 180°, or two right angles. Then the angle EAB will be represented by 5-2 —-—, or f of 180°; hence it is always necessary to draw 0 through the second point of division. (h) In a given circle. Let ABE be the given circle; draw any diameter FC, anddivide it into as many equal parts as the figure is to havesides (in this case five); with centre C and distance CFdescribe arc FG, and describe similar arc CG, intersecting atG. Draw GA from A, through second point of division; joinFA, and continue this around the circumference, completingthe polygon. (Fig. 89.). Ifo, 19,—To construct a regular pentagon on a given lineby a special method. Let AB be the given line; describe arcs CAD and CBD,with radius AB; join CD; with centre D and distance sameas AB describe arc EABF, cutting former arc in E and F. GEOMETRICAL DRAWING. 121 Join FG and EG, and produce them to meet arcs in H, K;join AH and BK; with centre H and distance HA describearc; and with centre K and distance KB, describe arc cuttingformer arc in L. Join LH and LK, completing the penta-gon. (Eig. 90.)
Size: 1738px × 1437px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, booksubjectgeometricald, booksubjectperspective
