Page:An introduction to linear drawing.djvu/48

 ﻿10. After the pupil has drawn the semicircle, the monitor must require him to draw angles of various si- zes, from 1 to 180 degrees. Then, laying aside the semicircle, let him draw angles of various degrees, which the monitor will test by his brass semicircle, or by an- gles of pasteboard previously prepared : the latter art; the handiest if well cut.

11.	Make a sphere and Us meridians. (fig. 5.)

Describe a circle and draw two diameters perpendic- ular to $ach other ; one for the axis, and the other for the equator, (a circle which goes round the earth at an equal distance from the ends of the axis, which ends are called poles.) Then draw arcs of a circle, all pass- ing through the poles, and whose centres are conse- quently on the perpendicular to the axis (that is, the equator) prolonged to the right or left hand. See Class III, problem 35. fig. 21.

These arcs have their centre as much farther off as they are nearer the axis. Their number is not import- ant, but if five be made on each side of the axis, as in the figure, each of file spaces between them will be just 15 degrees, or one twenty-fourth part of the whole sphere. These arcs in geography are called meridians..

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