_Correction of Mean-time Clock._—“First get its error on the observing, _i.e._ sidereal clock. This is always done by _coincidence of beats_, safe and certain to within one-tenth of a second, and with great ease and comfort by means of the loud-beating hammer which strikes the seconds of the sidereal clock on the outside of the case; one can then watch the neck-and-neck race which takes place every six minutes between the second of a sidereal clock and the second of a mean-time clock, the former always winning while you look at the motion of the mean-time seconds hand, and hear the seconds of the sidereal time.
Having got the error, say three (0·3)-tenths of a second slow, this is the arrangement for correcting it. The pendulum is suspended by a spring extra long, and a long arm goes across the clock pier, and the pendulum spring passes through a fine slit in the middle of it, and the left end (of said arm) turns on a pivot, while the right end rests on a cam, which can be turned by a handle outside the clock-case. Turning the handle one way raises the arm, and with that lengthens the acting length of the pendulum spring, and turning the other way, lowers it and shortens the pendulum, but so slightly that it takes fifteen minutes of the quickened rate of the pendulum, when shortened, to add the required 0·3 seconds to the indications of the clock.”[14]
The sidereal clock is used in many ways besides the purpose of giving a basis from which we can at any time get solar time, the distribution of which forms the subject of our next chapter.
This plan was devised and executed by Mr. Sang, C.E., Edinburgh.
CHAPTER XVIII. “GREENWICH TIME” AND THE USE MADE OF IT.
We have now described the method of obtaining and keeping true Greenwich time by means of transit observations, and the next thing is to distribute it either by controlling or driving other clocks electrically, or by sending electric signals at known times for persons to set their clocks right.
Nearly all, if not quite the whole, of the mean-time clocks in the Observatory are driven by a current controlled by the standard clock, as also is a seconds relay, _a_ Fig. 129. The clock controls, by currents sent every second by the relay, one or two clocks in London, by special wires.