The Best Clocks in the World.
At the Paris Observatory the standard clock, by Winnerl, is in a vault twenty-seven metres underground. At that depth the temperature changes are less than one fifth of a degree during the year, yet the effect of barometric changes on the rate of the clock have proved to be serious. This difficulty is avoided in the Naval Observatory at Washington, by enclosing the standard clock in an air-tight case within which the air is reduced to a pressure lower than that ever shown by a barometer at that level. To avoid risks of air leaking through this case were it to be pierced by a moving axle, this clock is actuated by weights lifted electrically by a small primary battery. The slight electric current required has no perturbing effect on the clock. This time-piece, provided with an escapement of great excellence, was manufactured by Clemens Riefler of Munich.
At the Observatory of the Case School of Applied Science, Cleveland, Ohio, another Riefler clock has a mean error of but .015 second per day. This means that in a year the total error is not more than 5.475 seconds, or one part in 5,760,000 of the 365 days. Such errors, minute as they are, give a good deal of trouble when they are irregular, that is, when the clock is sometimes slow, sometimes fast, in a fashion apparently lawless. When the divergences are fairly constant they can usually be traced to their source, making it feasible to apply a remedy.
Riefler clock.