Page:Popular Science Monthly Volume 80.djvu/285

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wooden frame in which was fastened a perpendicular cylinder closed at the bottom and open at the top. In it was placed a piston and rod, and on the rod a number of cogs. These cogs were geared into the cogs of a pulley. At the end of the axle on which the pulley was fastened was a hand behind which was attached the dial-plate to a wooden frame. On the dial, each numeral from I. to XII. was marked twice, and the hand moved round the whole face once in twenty-four hours. The contrivance was set in motion by starting a flow of water from a tank into the space between the bottom of the cylinder and the piston. As the piston-rod rose it turned the pulley and the shaft, and of course with it the hand at the end. By regulating the pressure of the water in the tank, the hand could be made to move faster or slower when it was desired to lengthen or shorten the hours to conform to the relative proportion of daylight and darkness in the twenty-four. Water-clocks were formerly much in vogue in the east and were sometimes very artistically constructed. Haroun al Raschid presented one to [[w:Charlemagne|Charlemagne] that was provided with a striking mechanism and adorned with movable figures such as are now quite common. The ancient Greek designations for the time of both the day and of the night were very vague: "the full market," "candle-lighting," "the first sleep," and so on. Herodotus says the troops that were dispatched by Xerxes to get in the rear of Leonidas left the camp "about the time of the lighting of the candles." It would have been more rational to say "about dark," but he evidently used the common phraseology. Cock-crowing was accepted as an indication of time. A well-known example is given in the story of Christ's trial. It is still much relied on by the peasants in some parts of Europe. In the nature of the case the Greek designations did not indicate the same actual time at all seasons of the year, as candle-lighting would be much earlier in the winter than in the summer. Soldiers divided the night into five watches, the length of which also varied with the seasons. It is not probable that they were accurately measured. This division of time is doubtless the oldest; it is several times referred to in the old testament. Sun-dials were a good deal used by the ancients. The Greeks seem to have received them from the Babylonians. Only the astronomers regarded the hours as of equal length. So far as can be known they depended upon water-clocks. But they were of much simpler construction than the one described above, usually consisting merely of two vessels each of which had a small orifice in or near the bottom. One of these vessels was placed above the other and the water which had been poured into it allowed to trickle slowly into the one underneath. When the lower vessel was full the orifice in the upper was closed, that in the lower opened and placed uppermost, when the same process would be repeated. The speakers in the assembly were timed by these clepsydræ, as they were called; they