Page:Popular Science Monthly Volume 26.djvu/684

666, better than many regulators do; hut, while it does not make an average variation of a second a day, it is far enough from making an actual variation of less than a second a day. Something better is obtainable from the very best astronomical clocks, which, indeed, are found to keep a uniform rate, from which they will vary only three or four hundredths of a second daily. But astronomical clocks, as is well known, are not required to indicate the exact time, mean or sidereal, but only to go at a uniform rate, which, if it be found to be practically invariable, is corrected at the time of the observation in which it is employed. The most accurately running large clock in the world, which has been regulated to keep diurnal time, is the Westminster clock in England. When the contract for the building of that clock was given out, it was stipulated that it must come within a second a day. It, in fact, does much better than this, for it is found that its variation is usually less than a second a week. It telegraphs its time daily and automatically to Greenwich, and the astronomer royal has said of it, "The rate of the clock is certain to much less than a second a week."

The other practical method of regulating the escape of a timepiece is through the balance-wheel, which, of course, must be resorted to in case of watches, ship-chronometers, and all clocks which are to be moved or carried about.

The method of regulation now in use in the watch is the result of long study of ingenious men. Starting with the discovery of the balance-wheel, carried back and forth with a diminishing oscillation by means of the hair-spring, we have before us the question of how to unlock the scape-wheel with each swing of the balance. In hunting through all classes of watches which find their way into the jeweler's refuse-box, after having served out their period of usefulness, we shall not be likely to come upon more than three (and certainly not more than four) styles of escapement. The oldest watches in the box will have what is termed the horizontal or barrel escapement. This is the escapement of the so-called "bull's eye" watch of our grandfathers. These watches always had a great reputation as time-keepers, yet I presume it is safe to say that there never was one in existence which could be relied upon to keep within a minute a day in the pocket, and most of them needed a much larger allowance. This escapement had a small wheel, with its axle parallel with the plates of the movement, and its teeth acted upon the pallets, which were little, flag-like appendages to the staff of the balance, and set at right angles to each other. The chief defect of it was, that a slight variation in the power, acting directly as it did upon the balance, affected very materially the rate of the watch. So that, while our grandfather's clock was even a better time-keeper than clocks of the same grade manufactured to-day, our grandfather's watch is not to be named in comparison with the cheapest modern watches.