CHAPTER XIII.
THE CLOCK AND CHRONOMETER.
I. The Rise and Progress of Time-keeping.
When we dealt with the astronomical instruments of Hipparchus, we saw that although the astrolabe which that great observer used was the germ of our modern instruments, the time recorded by Hipparchus and those who lived after him down to the later times of the Roman Empire was, as they measured it, a time which would be entirely useless for us.
The ancients contented themselves with dividing the interval between sunrise and sunset, regardless whether this was in summer or winter, into twelve equal hours. Now, as in summer the sun is longer above the horizon than in winter in these northern latitudes, we have more time during which the sun is above the horizon in summer than in winter, and if that period of time is to be divided into twelve hours, the hours would be much longer in summer than in other seasons.
As we are informed by Herodotus, tables were made by which these varying lengths of hours might be indicated by the shadows of a pole, which they called a gnomon or style. This was placed in a given locality, and the hour of the day was determined by the position of the shadow of the gnomon; and we need scarcely say that as Hipparchus observed he was compelled to find the position of the sun in order to determine the absolute longitude of a star at night. The ancients were limited to such ideas of time as could be got from slaves, who watched the risings and settings of the constellations, and who tried to bring to their own minds and those of their masters some idea of the lapse of time; and this even a few centuries ago was ordinarily depended upon in several countries.
Then, a little later, we come to the time being measured by monks repeating psalms—a certain number perhaps in the hour; and there were the water and sand clocks dating from Aristophanes, which were the predecessors of our sand-glasses. Candles were also at one time used with divisions on them to show how long they had been burning. But when we come to clocks proper, the history of which is very imperfectly known, we find an enormous improvement upon this state of things; because the clock, being dependent upon a constant mechanical action produced by the fall of a weight, could not be got to imitate these varying hours.
Still the clock had to fight its own battle for all that; and the first clocks were altered from week to week, or from month to month, so that the time-keeper, which did its best to be constant, was made inconstant to represent the ever-varying hours.
Doubtless the history of the first clocks—by which we do not mean the sand clocks or water clocks of the ancients, but such as those used by Archimedes when he attached wheels together—is lost in obscurity; and whether clocks, as we have them, were suggested in the sixth (Boethius, A.D. 525) or ninth century matters little for our inquiry; but beyond all doubt the first clock of considerable importance that was put up in England was the one erected in Old Palace Yard in the year 1288, as the result of a fine imposed upon the Lord Chief Justice of that time.
Fig. 85.—Ancient Clock Escapement.