Measurement of Time.
At the very dawn of civilization, the day, however crudely, was divided into parts. These parts, long afterward, probably in Babylonia, became the twenty-four hours which have descended to us. The means of time-keeping came first, in all likelihood, from measuring the simple shadow of a stick, the gnomon, still set up as a sun-dial in our gardens. Next came an hour-glass with its falling sand; the clepsydra, with its water dropping from a jar; the burning of candles definite in length. At last came the supreme discovery that a pendulum, of given length, if kept in one place oscillates in an unvarying period, be its arc of motion long or short. Tradition has it that in Arabia, about the year 1000 A. D., the pendulum was used in time-keeping. Granting this to be true, we must nevertheless give Galileo credit for his independent discovery as he observed the swaying lamp of the cathedral at Pisa, early in the seventeenth century. In 1657 Huygens employed a pendulum in the construction of a clock which, of course, displayed a new approach to accuracy. In 1792 Borda and Cassini had improved their time-pieces so as to be correct within one part in 375,000, that is to one second in 104 hours. For the sake of portability, clocks were gradually reduced in size until they became watches. Instead of a pendulum they were furnished with its equivalent, a balance wheel, Pierre Le Roy having discovered that there is in every spring a certain length where all the vibrations, great or small, are performed in approximately the same period. For actuation, watches were provided with mainsprings which have steadily undergone improvement in quality and in placing.