FIG. 25.—THE JEWELED WATER CLOCK BUILT BY CTESIBIUS ABOUT 250 B. C.
Figure 25 illustrates the most interesting clock he built and it was arranged to run year in and year out. The clock had a cylindrical face mounted on a hollow pedestal in which the mechanism was concealed. The column was divided off into twenty-four hours and a pointer that rose vertically marked off the lapse of time. But here he was faced with a serious complication. Hours in those days varied with the time of the year. A day from sunrise to sunset consisted of twelve hours. In summer, when the days were long, the hours were long, and in the short days of winter the hours were correspondingly shortened. To be sure, the variations in the length of the day are not so great in Egypt as they are in our latitude, because it is nearer to the equator; nevertheless there is a difference which the precise old mathematician had to take into account. In order to provide for variations in the hours, Ctesibius ran the lines spirally around the column and arranged his cylindrical clockface to turn slightly each day, so that in the winter months the clock hand or pointer moved over that part of the face where the daylight hour lines were closer together and the hours of night were farther apart, while in summer, the reverse would obtain. At the bottom of the column were two little cherubs. The cherub on the left was a sad little fellow who was constantly weeping. Tears trickled from his eyes and dropped into a basin. The tears passed into the hollow pedestal of the clock and gradually filled a cylinder formed in the base of the clock. A piston in this cylinder supported the other cherub. As the water gradually filled the cylinder this cherub was slowly raised and a wand he held in his hand pointed off the hours on the clockface. When the twenty-fourth hour was reached, a siphon came into play, which suddenly emptied the cylinder, permitting the pointer to drop. The siphon discharged its water into a small water wheel, which, by means of the system of gears, turned the column slightly to bring the hour lines in proper positions for measuring the time intervals of the next day. The column made one complete turn in 365 days. The jeweled bearings, referred to above, were placed in the eyes of the weeping boy, so that the holes that pierced them would not be enlarged by the constant wear of the water and thereby increase the rate of flow.
TIMING ANCIENT ORATORS
We have dwelt at considerable length upon this old clock of the pre-Christian era to show the ingenuity of inventors of that day, and also the careful study that was made of time by ancient mathematicians and astronomers. Of course water clocks were used before the time of Ctesibius. In fact, we read of them in the comedies of Aristophanes, written 400 B. C. Water clocks were used to limit the long speeches of orators at court, and in one place we find Demosthenes accusing a man of “talking in my water,” while at another time, when he was interrupted, he called to the officer to stop the water, showing that he valued every moment of time allotted to him for his speech.
In 807 Charlemagne was presented with a clock by the King of Persia. This consisted of an elaborate mechanism in which were all manner of wheels, and the clock would actually strike the hours. The driving power, however, was water.
But there were serious disadvantages in the use of water for the measurement of time. No great accuracy was ever obtainable with it, owing to the fact that its volume varied considerably with the temperature, and also with the dryness or moisture in the surrounding atmosphere. The idea of using a weight instead of water is claimed to have originated as far back as 990 A. D. The next important advance in the motive power of clocks was in 1500, when Peter Hele of Nuremberg invented the mainspring.
That was long before the pendulum made its appearance. The clock mechanism was slowed down and kept under control by what was known as a balance lever. (See Figure 26.) This was a horizontal lever mounted to oscillate in a horizontal plane. The lever was fitted with sliding weights, so that it could be carefully adjusted. The last wheel of the train of gears was provided with escapement teeth, somewhat similar to those used on our clocks and watches, but which would alternately move the lever this way and that. The inertia of the lever with the heavy weights on it was sufficient to prevent the mechanism from racing, and by this means, the motion was governed and slowed down, so that it measured time with a fair degree of accuracy.