Two of these little dials are clock faces, electrically connected with the solar and sidereal standard clocks, so that, though these clocks are themselves a good way off, in entirely different parts of the Observatory, the time superintendent, seated here at the time-desk, can see at once what they are indicating. Between the two is a dial labelled 'Commutator.' From this dial a little handle usually hangs vertically downwards, but it can be turned either to the right or to the left, and when thus switched hard over, an electric current is sent through to the mean solar clock. If now we leave the computing room and cross the courtyard to the extreme north-west corner, we find the Mean Solar Clock in a little lobby, carefully guarded by double doors and double windows against rapid changes of temperature. Opening the door of the clock case, we see that the pendulum carries on its side a long steel bar, and that this bar as the pendulum swings passes just over the upper end of an electro-magnet. When the current is switched on at the commutator, this electro-magnet attracts or repels the steel bar according to the direction of the current, and the action of the clock is accordingly quickened or retarded. To put the commutator in action for one minute will alter the clock by the tenth of a second. As the error of the clock is determined twice a day, shortly before ten o'clock in the morning, and shortly before one o'clock in the afternoon, its error is always small, usually only one or two tenths. These two times are chosen because, though time-signals are sent over the metropolitan area every hour from the Greenwich clock through the medium of the Post Office, at ten and at one o'clock signals are also sent to all the great provincial centres. Further, at one o'clock the time balls at Greenwich and at Deal are dropped, so that the captains of ships in the docks, on the river, or in the Downs may check their chronometers. The Time-Ball is dropped directly by the mean solar clock itself. It is raised by means of a windlass turned by hand-power to the top of its mast just before one o'clock. Connected with it is a piston working in a stout cylinder. When the ball has reached the top of the mast, the piston is lightly supported by a pair of catches. These catches are pulled back by the hourly signal current, and the piston at once falls sharply, bringing the ball with it. But after a fall of a few feet, the air compressed by the piston acts as a cushion and checks the fall, the ball then gently and slowly finishing its descent. The instant of the beginning of the fall is, of course, the true moment to be noted.