"Likely you have. But whatever's the matter, you'll find it--I know that. I never see such a man for clocks as you in all my born days; an' the master, he say the same. 'Mr. McPhearson will soon get Seventeen into line,' he says, an' I know you will, sir. Don't you always?" In the meantime Christopher had peeped inside the clock. "Why, look at the great lead weight!" ejaculated he. "Yes. Many old clocks had weights such as this, which were pulled up when the clock was wound and gradually dropped as the clock ran down. Sometimes a stone was used; sometimes even a pail of small stones." "But where were springs and pendulums?" gasped the astonished boy. "Springs came a good deal later. Even pendulums were not introduced in any practical form until 1657. Up to that time a balance did the work. The advent of the pendulum, invented probably by Christian Huygens, a Dutch mathematician, opened up no end of complications for the early clockmakers. In the first place they could not decide where to put this new article. Some placed the pendulum at the front of their clock, letting it dangle down across the face; others tried to conceal it by hanging it outside the back. Still others made a dial that would project enough at either side to cover it up. "Nor did the novel innovation of the pendulum do much good at first, although theoretically makers of clocks conceded pendulums to be a scientific advance over older methods. Of course the theory of the pendulum had been for a long time in the minds of many thoughtful persons. Galileo had seized on its principle when observing the swinging of lanterns in the church at Pisa, and had written a scientific treatise on it. But to get an idea is one thing and to apply it is quite another. Pendulums were very complicated mechanisms. In the first place the length of the pendulum decides, you see, the rate of the clock's vibration; a short one resulting in a quick, nervous tick; and a long one in a slow, quiet one. Therefore pendulums meant more even vibration and more accurate time-keeping, and it was just when makers were rejoicing over these advantages that it was discovered the temperature of the place in which a clock stood affected the rod the bob hung on and threw the whole timepiece out of adjustment. Here was a pretty kettle of fish! A hot room, for example, would expand the rod and lengthen it." "And make the clock tick slower," put in Christopher eagerly. "Precisely." "Then the clock would go slower sometimes than others." "Exactly that! The variation was not great, of course, and we now have learned how to meet it by lengthening or shortening the pendulum by means of a screw placed near the bob. Nevertheless the variation is there. A common wire pendulum will vary approximately a minute a week; a brass rod will, on the other hand, vary that same minute in five days instead of seven. Wood, a material showing less change than metal, will vary only a minute in three weeks.