Let the pupils set a stake, say about 5 feet high, at a point N on the school grounds about 9 A.M., and carefully measure the length of the shadow, NW, placing a small wooden pin at W. Then about 3 P.M. let them watch until the shadow NE is exactly the same length that it was when W was fixed, and then place a small wooden pin at E. If the work has been very carefully done, and they take the tape and bisect the line WE, thus fixing the line NS, they will have a north and south line. If this is marked out for a short distance from N, then when the shadow falls on NS, it will be noon by sun time (not standard time) at the school.
Problem. From a given point in a given line, to draw a line making an angle equal to a given angle.
Proclus says that Eudemus attributed to Œnopides the discovery of the solution which Euclid gave, and which is substantially the one now commonly seen in textbooks. The problem was probably solved in some fashion before the time of Œnopides, however. The object of the problem is primarily to enable us to draw a line parallel to a given line.
Practically, the drawing of one line parallel to another is usually effected by means of a parallel ruler (see [page 191]), or by the use of draftsmen's triangles, as here shown, or even more commonly by the use of a T-square, such as is here seen. This illustration shows two T-squares used for drawing lines parallel to the sides of a board upon which the drawing paper is fastened.[71]
An ingenious instrument described by Baron Dupin is illustrated below.
To the bar A is fastened the sliding check B. A movable check D may be fastened by a screw C. A sharp point is fixed in B, so that as D slides along the edge of a board, the point marks a line parallel to the edge. Moreover, F and G are two brass arms of equal length joined by a pointed screw H that marks a line midway between B and D. Furthermore, it is evident that H will draw a line bisecting any irregular board if the checks B and D are kept in contact with the irregular edges.
Book II offers two general lines of application that may be introduced to advantage, preferably as additions to the textbook work. One of these has reference to topographical drawing and related subjects, and the other to geometric design. As long as these can be introduced