"I know, sonny," returned the old man with sympathy. "But do not imagine you are the only one who is not always able to put in the proper place the scraps of knowledge in his possession. Many an older person has wondered what part his learning had in the gigantic total of the ages. World history is conceived on a pretty big scale, you see. But that all we glean is somehow linked up with the rest, you may be very sure. Certainly this clepsydra was." "It's easy enough to see that _afterward_," asserted Christopher. "And so the Greeks managed to fix up their water clock to their satisfaction, after all." "Alas, not wholly to their satisfaction," was the answer, "for presently other difficulties concerning it arose. For example, unless the water poured into it was absolutely clean, the hole would fill up and the drip become slower; moreover, you must consider what happened in cold weather, for not only were these water clocks in unheated buildings, but you will recall they were set up in the market place or public square so the villagers might consult them. Here assembled the watch, whose duty it was to patrol the town and blow a horn for the changing of the guard; here, too, was stationed the officer whose duty it was at stated hours to refill the clepsydra." "Oh, I suppose the darn thing froze--that probably was the next obstacle," grinned Christopher. "It was," nodded McPhearson. "Then it couldn't have been much better than the old sundial," the lad sniffed, with contempt. "It had its outs. Nevertheless it held the front of the stage about two thousand years, and then I am sure you will agree it was high time a better device was substituted." "And what was that?" "The sand glass." "Our hourglass, you mean?" "Yes--or half-hour, quarter-hour--any fraction of an hour you choose. The idea of the sand glass was not entirely new, because some form of running sand had long before been used in the Far East. But the sand glass as we know it was new to the European world, and you cannot but agree it was a far more practical article than was the clepsydra for it neither froze nor had to be replenished. Moreover, it was lighter, less bulky, and could be carried about, and the old water clocks could not--that is, not without great inconvenience and danger of breaking. Oh, the sand glass was vastly better! Even now, after all these years, it is not entirely out of date, for it is still used to mark definite intervals of time." "I have one at home to practice by."