FIG. 129.—Arrangement for correcting mean solar time clock at Greenwich. ] At Greenwich there is, as we have already seen, a _standard, sidereal clock_, that is, a clock keeping sidereal time; and regulated from this is the _standard solar time clock_, giving the time by which all our clocks and watches are governed. In practice at Greenwich the solar clock is regulated as follows: in the computing room are two chronometers, _c_ and _b_, Fig. 129, the one, _c_, regulated electrically by the mean-time clock, and the other, _b_, regulated by the sidereal clock—the error of the latter being known by transit observations of stars on the Nautical Almanac list, the difference between the observed time of transit and the right ascension of the star being the error required. The proper difference between the two clocks is then calculated and the error allowed for, which shows whether the solar clock is fast or slow; to correct it the following method is adopted: Carried on the pendulum of the solar clock is a slender bar magnet, about five inches long, and below it, fastened to the clock-case, is a galvanic coil; the magnet passes at each swing over the upper end of the coil; if now a current is sent through this coil in one direction repulsion takes place between the magnet and coil, and the clock is slowed; if, on the other hand, the current is reversed, the clock is made to gain. Now between the two chronometers is a commutator, _d_, which, by moving the handle to one side or the other, sends the current through the coil in such a manner that the clock is accelerated or retarded sufficiently to set it right; when the handle of the commutator is in the position shown in the drawing no action takes place. As an instance of another method of regulating one clock from another, we will quote what Professor Piazzi Smyth says of the clock arrangements at Edinburgh. _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.