When the board is turned edgewise toward the sun so that its shadow is as thin as possible, rule a pencil line alongside it on the horizon plane. The angle which this line makes with a line pointing due south is called the sun's _azimuth_. When the sun is south, its azimuth is zero; when west, it is 90°; when east, 270°, etc. EXERCISE 5.--Let a number of different students take the sun's altitude during both the morning and afternoon session and note the time of each observation, to the nearest minute. Verify the setting of the plane of the horizon from time to time, to make sure that no change has occurred in it. 6. GRAPHICAL REPRESENTATIONS.--Make a graph (drawing) of all the observations, similar to Fig. 5, and find by bisecting a set of chords _g_ to _g_, _e_ to _e_, _d_ to _d_, drawn parallel to _B B_, the time at which the sun's altitude was greatest. In Fig. 5 we see from the intersection of _M M_ with _B B_ that this time was 11h. 50m. The method of graphs which is here introduced is of great importance in physical science, and the student should carefully observe in Fig. 5 that the line _B B_ is a scale of times, which may be made long or short, provided only the intervals between consecutive hours 9 to 10, 10 to 11, 11 to 12, etc., are equal. The distance of each little circle from _B B_ is taken proportional to the sun's altitude, and may be upon any desired scale--e. g., a millimeter to a degree--provided the same scale is used for all observations. Each circle is placed accurately over that part of the base line which corresponds to the time at which the altitude was taken. Square ruled paper is very convenient, although not necessary, for such diagrams. It is especially to be noted that from the few observations which are represented in the figure a smooth curve has been drawn through the circles which represent the sun's altitude, and this curve shows the altitude of the sun at every moment between 9 A. M. and 3 P. M. In Fig. 5 the sun's altitude at noon was 57°. What was it at half past two? [Illustration: FIG. 5.--A graph of the sun's altitude.] 7. DIAMETER OF A DISTANT OBJECT.--By sighting over a protractor, measure the angle between imaginary lines drawn from it to the opposite sides of a window. Carry the protractor farther away from the window and repeat the experiment, to see how much the angle changes. The angle thus measured is called "the angle subtended" by the window at the place where the measurement was made. If this place was squarely in front of the window we may draw upon paper an angle equal to the measured one and lay off from the vertex along its sides a distance proportional to the distance of the window--e. g., a millimeter for each centimeter of real distance. If a cross line be now drawn connecting the points thus found, its length will be proportional to the width of the window, and the width may be read off to scale, a centimeter for every millimeter in the length of the cross line.