In the case of clocks such delicacy of adjustment is hardly necessary. Pendulum clocks, if they are to run accurately, must have their pendulums automatically adjustable for variations in temperature, because the longer the pendulum the more slowly it oscillates. Many years ago a grid type of pendulum was invented in which various alloys were used, which reacted one against the other and preserved the center of gravity always at the same distance from the center of oscillation. The action of a pendulum movement is similar to that of a balance-wheel movement described above. The pendulum operates a pallet lever similar to that shown in Figure 29. The teeth of an escape wheel, which are commonly somewhat different from those of the watch movement, strike the pallets of the pallet lever, and the escape wheel is intermittently stopped and permitted to proceed. The pressure of the escapement teeth against the pallets is just enough to keep the pendulum swinging and the speed of the clock is regulated by lengthening or shortening the pendulum. We have added little to clock or watch movements in recent years. About the only conspicuous modern invention is the torsion pendulum. The pendulum in this case is a heavy horizontal disk suspended by a wire, and the disk rotates first in one direction and then in the other, twisting and untwisting the wire. The advantage of this pendulum is that the oscillations are very slow, and hence it is possible to keep the clock going for a year at a single winding. The electric regulation of clocks is another important improvement. A clock need not be a very perfect timepiece, but if provided with an electric regulator its hands are brought up to the correct time every hour in response to an impulse sent from an accurate master clock which in turn has its time corrected daily from the National Observatory in Washington. CHAPTER V PUTTING RIVERS TO WORK FLOWING water exerts a strange fascination upon mankind, even to the present day. Tourists travel hundreds of miles to view the glorious spectacle of a riotous tumbling cataract. Is it strange, then, that in the olden times, when the world was peopled with gods and genii and strange spirits, the ancients looked upon ceaselessly flowing rivers as the symbol of life? It was most natural for them to covet the endless power of a river and eventually, despite their superstitions, to try to utilize some of its energy. It may be that sailboats antedate the first water wheel, but it seems much more probable that flowing water was the first inanimate power harnessed by man. Windmills were certainly a later development. They possessed the advantage that they could be located anywhere while the water mill had of necessity to be built along the bank of a stream. However, the power of the wind is so unreliable and fluctuates so widely that it was little used, except in flat countries, where there was little if any available water power.