There are two great standard clocks at the Observatory: the mean solar clock and the sidereal clock. The latter registers twenty-four hours in the precise time that the earth rotates on its axis. A 'day' in our ordinary use of the term is somewhat longer than this; it is the average time from one noon to the next, and as the earth whilst turning round on its axis is also travelling round the sun, it has to rather more than complete a rotation in order to bring the sun again on to the same meridian. A solar day is therefore some four minutes longer than an actual rotation of the earth, _i.e._ a sidereal day, as it is called, since such rotation brings a star back again to the same meridian. The sidereal clock can therefore be readily checked by the observation of star transits, for the time when the star ought to be on the meridian is known. If, therefore, the comparison of the transit taps on the chronograph with the taps of the sidereal clock show that the clock was not indicating this time at the instant of the transit, we know the clock must be so much fast or slow. Similarly, the difference which should be shown between the sidereal and solar clocks at any moment is known; and hence when the error of the sidereal clock is known, that of the solar can be readily found. It is often quite sufficient to know how much a clock is wrong without actually setting its hands right; but it is not possible to treat the Greenwich clock so, for it controls a number of other clocks continually, and sends hourly signals out over the whole country, by which the clocks and watches all over the kingdom are set right. In the lower computing room, below the south window, we find the Time-Desk, the head-quarters of the Time Department. This is a very convenient place for the department, since one of the chronometer rooms, formerly Bradley's transit room, opens out of the lower computing room; the transit instrument is just beyond; it is close to the main gate of the Observatory, and so convenient for chronometer makers or naval officers bringing chronometers or coming for them, whilst just across the courtyard is the chronograph room, with the Battery Basement, in which the batteries for the electric currents are kept, and the Mean Solar Clock lobby, with the winch for the winding of the time-ball at the head of the stairs above it. These rooms do not exhaust the territory of the department, since it owns two other chronometer rooms on the ground floor and first floor respectively of the S.-E. tower. At the time-desk means are provided for setting the clock right very easily and exactly. Just above the desk are a range of little dials and bright brass knobs, that almost suggest the stops of a great organ.