The larger periods of time are measured by the motion of the heavenly bodies. The year and the four seasons are marked off by the motion of the earth in its long journey around the sun; the months and the weeks are told by the changing moon; sunrise and sunset announce the coming and the going of day. The year and the seasons and the day were measured for primeval man by the great clock in the heavens, but how were smaller periods of time to be measured? How was the passing of fractional parts of a day, an hour or a minute or a second to be noted? An egg was to be boiled; how could the cook tell when it had been in the water long enough? A man out hunting wished to get back to his family before dark: how was he to tell when it was time to start homeward? [Illustration: FIG. 1.--A PRIMITIVE SUN-DIAL.] Plainly, the measurement of small portions of time was a very practical problem from the beginning. The first attempt to solve the problem consisted in observing shadows cast by the sun. The changing shadow of the human form was doubtless the first clock. As the shadow grew shorter the observer knew that noon was approaching; when he could reach out one foot and step on the shadow of his head he knew it was time for dinner; when his shadow began to lengthen he knew that evening was coming on. Observations of this kind led to the _shadow clock_ or _sun-dial_ (Fig. 1). You can make one for yourself. On a perfectly level surface exposed all day to the sun, place in an upright position (Fig. 1) a stick about three feet long, and trace on the surface the shadows as they appear at different times of the day. A little study will enable you to use the shadows for telling the time. Sun-dials have been used from the beginning of time and they have not yet passed out of use. They may still be seen in a few public places (Fig. 2), but they are retained rather as curiosities than as real timekeepers. For the sun-dial is not a good timekeeper for three reasons: (1) it will not tell the time at night; (2) it fails in the daytime when the sun is not shining; (3) it can never be used inside of a house. [Illustration: FIG. 2.--A MODERN SUN-DIAL.]