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 Oenopides 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 Oenopides, 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] [Illustration] [Illustration] An ingenious instrument described by Baron Dupin is illustrated below. [Illustration] 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.