In one of his dialogues Plato declares that the philosophers are far more fortunate than the orators--“these being the slaves of a miserable water-clock; whereas the others are at liberty to make their discourse as long as they please.” To explain this passage, we must remember that it was the practice in the Athenian courts of justice, as subsequently in those of Rome, to measure the time allowed to the advocates for pleading by means of a water-clock. Three equal portions of water were put into it--one for the prosecutor, one for the defendant, and the third for the judge. A man was charged with the special duty of giving timely notice to each of the three speakers that his portion was nearly run out. If, on some unusual occasion, the time for one or other of the parties was doubled, it was called “adding water-clock to water-clock;” and when witnesses were giving evidence, or the text of some law was being read out, the percolation of the water was stopped: this was called _aquam sustinere_ (to retain the water). The hour-glass, which is still in use to a considerable extent for measuring short intervals of time, had great analogy with the water-clock, but was never susceptible of such regularity. In fact, at different periods important improvements were applied to the water-clock. Vitruvius tells us that, about one hundred years before our era, Ctesibius, a mechanician of Alexandria, added several cogged-wheels to the water-clock, one of which moved a hand, showing the hour on a dial. This must have been, so far as historical documents admit of proof, the first step towards purely mechanical horology. In order, then, to find an authentic date in the history of horology, we must go to the eighth century, when water-clocks, still further improved, were either made or imported into France; among others, one which Pope Paul I. sent to Pepin le Bref. We must, however, believe that these instruments can have attracted but little attention, or that they were speedily forgotten; for, one hundred years later, there appeared a water-clock at the court of Charlemagne, a present from the famous caliph Aroun-al-Raschid, regarded, indeed almost celebrated, as a notable event. Of this Eginhard has left us an elaborate description. It was, he says, in brass, damaskeened with gold, and marked the hours on a dial. At the end of each hour an equal number of small iron balls fell on a bell, and made it sound as many times as the hour indicated by the needle. Twelve windows immediately opened, out of which were seen to proceed the same number of horsemen armed _cap-à-pie_, who, after performing divers evolutions, withdrew into the interior of the mechanism, and then the windows closed.