Of curious clocks there is no end, so I shall just refer to one invented by William Congreve, an Englishman, over one hundred years ago, and often coming up since as something new. A plate about 8 in. long and 4 in. wide has a long zigzag groove crosswise. This plate is pivoted at its center so either end can be tipped up a little. A ball smaller than a boy's marble will roll back and forth across this plate till it reaches the lower end, at which point it strikes a click and the mainspring of the clock tips the plate the other way and the ball comes slowly back again till it strikes the disk at the other end of the plate, etc. Every time the plate tips, the hands are moved a little just like the remontoir clock already described. Clocks of this kind are often used for deceptive purposes and those ignorant of mechanics are deceived into the belief that they see perpetual motion. The extent to which modern machine builders are indebted to the inventions of the ancient clock-maker, I think, has never been appreciated. [Illustration: Fig. 45--Domestic Dial by James Arthur] In its earlier stages the clock was almost the only machine containing toothed gearing, and the "clock tooth" is still necessary in our delicate machines. It is entirely different from our standard gear tooth as used in heavy machines. The clock-makers led for a long time in working steel for tools, springs and wearing surfaces. They also made investigations in friction, bearings, oils, etc., etc. Any one restoring old clocks for amusement and pleasure will be astonished at the high-class mechanics displayed in them--nearly always by unknown inventors. Here is an example: The old clock-maker found that when he wished to drill a hole in a piece of thick wire so as to make a short tube of it, he could only get the hole central and straight by rotating the piece and holding the drill stationary. By this method the drill tends to follow the center line of rotation; and our great guns as well as our small rifles are bored just that way to get bores which will shoot straight. The fourth and last chapter will deal with the astronomical motions on which our time-keeping is founded, our present hour zones of time, and close with suggestions for a universal time system over the whole world. CHAPTER IV ASTRONOMICAL FOUNDATION OF TIME