Fig. 32—Antique Clock, Entirely Hand-Made
Fig. 33—Antique Clock, Entirely Hand-Made
It runs with an error under one second a week. This is surpassed only by some of the astronomical clocks which run sometimes two months within a second. This wonderful timekeeping is done with seconds pendulums of about 39 in., so the theoretical advantage of long pendulums is lost in the difficulties of constructing them. Fractions are left out of these lengths as they would only confuse the explanations. At the Naval observatory in Washington, D. C., the standard clocks have seconds pendulums, the rods of which are nickel steel, called “Invar,” which is little influenced by changes of temperature. These clocks are kept in a special basement, so they stand on the solid earth. The clock room is kept at a nearly uniform temperature and each clock is in a glass cylinder exhausted to about half an atmosphere. They are electric remontoirs, so no winding is necessary and they can be kept sealed up tight in their glass cylinders. Nor is any adjustment of their pendulums necessary, or setting of the hands, as the correction of their small variations is effected by slight changes in the air pressure within the glass cylinders. When a clock runs fast they let a little air into its cylinder to raise the resistance to the pendulum and slow it down, and the reverse for slow. Don't forget that we are now considering variations of less than a second a week.
The clock room has double doors, so the outer one can be shut before the inner one is opened, to avoid air currents. Visitors are not permitted to see these clocks because the less the doors are opened the better; but the Commander will sometimes issue a special permit and detail a responsible assistant to show them, so if you wish to see them you must prove to him that you have a head above your shoulders and are worthy of such a great favor.
Fig. 34—Triple-Case Turkish Watches
The best thing the young student could do at this point would be to grasp the remarkable fact that the clock is not an old machine, since it covers only the comparatively short period from 1364 to the present day. Compared with the period of man's history and inventions it is of yesterday. Strictly speaking, as we use the word clock, its age from De Vick to the modern astronomical is only about 540 years. If we take the year 1660, we find that it represents the center of modern improvements in clocks, a few years before and after that date includes the pendulum, the anchor and dead beat escapements, the minute and second hands, the circular balance and the hair spring, along with minor improvements. Since the end of that period, which we may make 1700, no fundamental invention has been added to clocks and watches. This becomes impressive when we remember that the last 200 years have produced more inventions than all previous known history—but only minor improvements in clocks! The application of electricity for winding, driving, or regulating clocks is not fundamental, for the timekeeping is done by the master clock with its pendulum and wheels, just as by any grandfather's clock 200 years old. This broad survey of time measuring does not permit us to go into minute mechanical details. Those wishing to follow up the subject would require a large “horological library”—and Dr. Eliot's five-foot shelf would be altogether too short to hold the books.