Page:Popular Science Monthly Volume 26.djvu/158

146 hour-hands only, and the general use of the finer divisions into minutes and seconds is almost entirely the outgrowth of the requirements of modern civilization. Astronomical time-keeping is not here considered. By the Babylonian system of dividing the day, which was used by the Jews and other Oriental nations, the time between sunrise and sunset was portioned into twelve equal parts at all seasons of the year, the hour varying in length with the season. If this method of division still prevailed, the hours in New York city would vary in length from about forty-six to about seventy-five of our present minutes. In the Arctic regions the inapplicability of this system to general use would reach its climax of absurdity.

The general facts upon which all systems of time-keeping are based are commonly understood, but the details are seldom referred to.

The most primitive kind of timepiece is a sun-dial. Reduced to its simplest form, a sun-dial consists of a straight pole erected upon a permanently fixed circular plate, the shadow of the pole indicating midday when it coincides with a line drawn due north from the base of the pole, the pole being erected upon a line parallel with the axis of the earth. The other hours of the day are indicated by marks upon the circular plate upon which the shadow of the pole successively falls.

When the sun-dial was invented can not be stated. It was of very ancient origin, and is mentioned in the thirty-eighth chapter of Isaiah. The clepsydra, or water-clock, and the hour-glass, although very ancient, must from their nature have been invented subsequent to the sun-dial. But sun-dials, of which there are about a dozen different kinds, although common, were never in such general use as clocks are in modern times, and were philosophical rather than popular instruments. The clock was invented about 1379, and the pendulum as a regulating power in 1657.

The rapid development of the science of horology in the present century has been almost coincident with and in no small degree dependent upon the construction and operation of railway and telegraph lines. The needs of these great engines of modern civilization created a general demand for exactness in time reckoning which had never existed before. It was required both for the use of their employes and for the public which patronized their lines.

A sun-dial being stationary, when properly made and adjusted, exhibited solar time correctly, and a watch regulated from the dial by the equation of time would also be correct for that particular spot, but the moment the owner of the watch began to move east or west his time-piece no longer registered correct time, and when he traveled with the speed of a railway-train the error was rapidly exaggerated.

The necessity for exactness before mentioned, and the impossibility of adhering to local time, early attracted the attention of railway managers, and caused them much perplexity and annoyance. With the rapid construction of railway lines, the commingling of the various