Motor cars, bicycles, and many other machines are, or ought to be, made in this manner, so that if a driver at York breaks a part of the car he simply sends to London for another. It comes and fits into its place at once. But for this sort of plan you must do work true to much less than a thousandth of an inch, and, of course, no one must want to indulge his individual fancy as to the shape or appearance of the watch. The whole advantage consists in dead uniformity. But the cheapness is surprising. You can have a better watch now for 30_s._ than could have been got for £30 twenty years ago. Artistic people are in the habit of condemning this uniformity as though it were inartistic and degrading. In truth, it is not degrading to get a machine to do what you want at the expense of as little labour as possible. You pay 30_s._ for the watch, but you have £28 10_s._ left to spend on pictures. Only one ought not to confuse industry with art. Watches made in this way have no pretence to be artistic products. They are simply useful. To rule them all over with machine lines or to put hideous machine ornament on them is purely and simply base and degrading. Let your _ornament_ be hand work, your utility machine work. Thus then I have endeavoured to give a very brief sketch of the modes of measuring time, and incidentally to introduce my readers to those laws of motion which are the foundation of so large a part of modern science. It only remains that I should shortly describe modern apparatus by means of which it is possible to measure with accuracy periods of time so short as to appear impossible. But when you see how it is done the method seems easy enough. It is still by means of a pendulum, only a pendulum beating time not once, but hundreds and even thousands of times in a second. And such pendulums, instead of being difficult to make, are remarkably simple, and present no difficulty whatever. For we have only to use the tuning fork which has been previously described. The tuning fork consists of a piece of steel bent into a U shape. The arms are set vibrating so as alternately to approach and recede from one another. The reason why there are two arms is that, if they come together and recede, they balance, and hence the instrument as a whole does not shake on its base. This balance of moving parts of a rapidly moving machine is very important. Some motor cars are arranged so that the engines are “balanced,” and the moving parts come in and out simultaneously, leaving the centre of gravity unchanged whatever be the position of the motion. This makes the vibration of the car very small. The tuning fork is therefore balanced. Being elastic, it obeys Hook’s law, “As the force, so the deflection.” And therefore, as we have seen, the vibrations of the fork are isochronous.