In antiquity, sand glasses were much less important than water clocks

In antiquity, sand glasses were much less important than water clocks, but their influence lasted longer. The middle ages took them, as the Greeks had taken the water clock--to time the length of speeches. Even in the sixteenth and seventeenth centuries they were to be found in every church. When the minister started preaching the clock was started, and when the sand had run out the minister must stop. There are certain common needs of humanity which reappear in every age.

We still see sand glasses today; they are sold as toys for children and as kitchen gadgets to time a boiling egg; but in only one place have they retained their original dignity. When a division is called in the English House of Commons, a two-minute sand glass is still turned; and, while the sand is running, the "division bells" are set in motion throughout the house.

The Greeks had yet another method of telling time. One which the Egyptians did not know, and which the Greeks probably invented themselves. There is a slight confusion about this, for Berosus (to whom the invention is attributed) seemed to have a bad habit of popping up at odd intervals in different countries and entirely different centuries, in a way that is quite extraordinary for a man who was himself a timekeeper. He is variously cited as a Chaldean who lived about 640 B. C. on the island of Cos, and as a Greek, living in Bithynia about 280 B. C. The latter, just to help out the confusion, wrote a history of Chaldea. Whatever his date, his fame at least has persisted, and at one time must have been greater than it is now. In Apian Caesars' Astronomy we find praise of the. author in a verse which reads:

"The Grecian boy Berosus has had statues to his name, May not the German master have a temple to his fame?"

The invention which gave rise to all this confusion is commonly known as the Hemisphere of Berossus (spelling to choice). It consists of a hollow hemisphere with hour gradations marked on the inside, and a small sphere set in the exact center. The shadow of the small sphere on the gradations gave the hour, and the lines can be arranged to show either equal or unequal hours. These hemispheres continued in use for many centuries and some Roman examples are still extant.

The dial of Thutmose employed a horizontal gnomon and determined the time solely by the altitude of the Sun. The Hemisphere of Berossus took into consideration the longitudinal position of the Sun in the heavens. The gnomon used was spherical.

A third style of sundial, which may have been invented by both the Chinese and the Greeks, employs both altitude and position. Its gnomon slants. It is the only kind of sundial which we ever see today.

All these modern sundials are based on a single common principle. The dial itself represents the Earth. The gnomon is the Earth's polar axis. There is, of course, a slight error; for the gnomon may be as much as four thousand miles away from the axis of the Earth, and our dial must be on the surface if it is to tell time at all; but that misplacement is so small when compared to the distance of 93,000,000 miles between the Earth and the Sun that the error is negligible.

As the Earth rotates, it carries with it everything placed upon its surface, buildings and trees, the stump with which primitive man told time, and the sundial of his later invention. As a result, each hour the shadow travels over one twenty-fourth of the equator. The equatorial sundial, which is the simplest of all kinds, makes strict, literal use of this principle.

The type of sundial which we commonly see today is made by projecting the equator of the Earth, and its divisions, onto a flat surface. Its variations are infinite. The plane surface may be horizontal, inclined or vertical; it may have four vertical faces, each one facing a cardinal point. In all ages the ingenuity and exactness required in making a sundial has challenged the finest craftsmen and artists. Thomas Jefferson, relaxing from his duties as President of the United States, amused himself by constructing sundials. Some craftsmen have devised fearful and wonderful instruments. A favorite variety is a single stand with sundials facing in every conceivable direction. Each one of these dials, however it is set, is only a projection of the Earth's equator, and each must have its gnomon parallel to the polar axis of the Earth if the shadow is to fall in the same direction at all times of year. And, of course, all sundials must be made to suit the latitude in which they are to be placed.