The cinematograph is an instrument by which about fifteen photographs per second can be received on a film, each representing the photographed group at a different instant from the others. The advantages of this mode of photographing and of throwing pictures on a screen over the older methods are obvious. By controlling the rate at which the pictures are represented on the screen, movements too rapid to be analyzed by the eye may be made slow enough to permit observation; and, similarly, movements too slow for comprehension or rapid observation may often be quickened. The busy life of a city street, the progress of races or other competitions, many scenes in nature, and even the growth of a plant from seed to maturity, may be shown by means of a "moving picture." Photography is a noble servant of mind and soul. It brings to us likenesses of eminent persons and objects of nature and art which perhaps we should never be able to see otherwise. It has been used in measuring the velocity of bullets and in showing the true positions of animals in motion. Photography has created the "new astronomy." Immediately after its discovery, photography was applied to the science of the stars, and it has been ever since of incalculable service in this field of inquiry. Photographs of the moon were made as early as 1840, and much that is known to-day of the sun has been revealed by photography. So sensitive is the modern photographic plate to the influence of light, that photography has discovered and located stars which are invisible through a strong telescope. Astronomers are now engaged in making a photographic chart of the sky. CHAPTER XV CLOCKS The matters of every-day life, much less the affairs of a complex civilization, could scarcely be carried on without some accurate and uniform system of measuring time. Nature herself furnishes measurements for certain divisions of time. The "two great lights" that God made, as the Bible tells us, were designed "for signs, and for seasons, and for days and for years." The revolution of the earth around the sun marks the year; the revolution of the moon around the earth determines the month; the rotation of the earth on its axis causes and measures day and night. But no object of nature distinguishes the hours of the day or the divisions of the hour. Man requires a smaller unit of time than the day. He must divide the day into hours; the hours into minutes; the minutes into seconds. The division of the day into twenty-four hours is as old as authentic history. But the means for determining the hours and their subdivisions were at first quite crude and inefficient. [Illustration: A SUN DIAL]