"That was his puzzle--his nut to crack. Throughout the ages it has been conundrums like these that have taxed human ingenuity and made of life such an alluring adventure. On the conquering of difficulties civilization has been built up. Well, man now attacked this problem of telling time. He did not aspire to narrow it down to any very fine point, for at that period of history one day was very like another, and he was a leisurely being with little to do but eat, sleep, fight or hunt. Notwithstanding this, however, he did want to know _when_ it was noon; _when_ it would be day. King Alfred, one of the English monarchs, hit upon a plan for telling the hours of the night by means of tall candles, made to burn a definite interval. When, for example, one of his candles burned out, he knew that four or six hours had passed. Other persons went further and had candles marked off into hours with black and white wax--" "That was a clever scheme!" "Clever, yes; and all very well for kings who could afford to burn wax tapers night after night. But there were, alas, many unfortunates who couldn't. Accordingly the obstacle persisted, and urged the world on to the next step up the time-telling ladder." "And what was that?" demanded Christopher with interest. "Telling time by water." "By _water_! But how?" "It was not so difficult as it sounds. In reality it was quite a simple plan. The ancients would take a jar, make a tiny hole in the bottom of it, fill it with water, and let the water drip slowly out. Having measured how long it would take to empty the jar, they had a sort of water clock." "Bravo! That was certainly easy." "Easy and far better than the sundial, too, for water would drip either in light or darkness, on cloudy days as well as bright ones. By means of marks on the jar, shorter intervals of time could also be determined. The receptacle, however, had to be kept filled and the hole free so there should be no variation in the regularity of the dripping. This water clock was called a _clepsydra_, the name being taken from two Greek words meaning 'thief of water.' Well, as you may imagine, the populace were delighted with this contrivance. It seemed as if now they certainly had the prize for which they had been searching. Moreover, with the water clock a new factor in time came into being. Instead of telling _when_, as the sundial did, the clepsydra, by measuring a given interval, told _how long_, which was a very different thing indeed. In other words it began to draw people's attention to the duration of time." "That is different, isn't it?" mused the boy.