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.
The other dials on the time-desk are for various purposes connected with the signals. One little needle in a continual state of agitation shows that the electric current connecting the various sympathetic clocks of the Observatory is in full action. Another receives a return signal from various places after the despatch of the time-signal from Greenwich, and shows that the signal has been properly received at the distant station, whilst all the many electric wires within the Observatory or radiating from it are made to pass through the great key-board, where they can be at once tested, disconnected, or joined up, as may be required.
THE TIME-DESK.
The distribution of Greenwich time over the island in this way is thus a simple matter. The far more important one of the distribution of Greenwich time to ships at sea is more difficult. The difficulty lay in the construction of a clock or watch, the rate of which would not be altered by the uneasy motion of a ship, or by the changes of temperature which are inevitable on a voyage. Two hundred years ago it was not deemed possible to construct a watch of anything like sufficient accuracy. They would not even keep going whilst they were being wound, and would lose or gain as much as a minute in the day for a fall or rise of 10° in temperature. This was owing to the extreme sensitiveness of the balance spring—which takes the place in a watch of a pendulum in a clock—to the effects of temperature. The British Government, therefore, in 1714 offered a prize of the amount of £20,000 for a means of finding the longitude at sea within half a degree, or, in other words, for a watch that would keep Greenwich time correct to two minutes in a voyage across the Atlantic. In 1735, James Harrison, the son of a Yorkshire carpenter, succeeded in solving the problem. His method was to attach a sort of automatic regulator to the spring which should push the regulator over in one direction as the temperature rose, and bring it back as it fell. This he effected by fastening together two strips of brass and steel. The brass expanded with heat more rapidly than the steel, and hence with a rise of temperature the strip bent over on the steel side. This was the first germ of the idea of making watches 'compensated for temperature;' watches, that is, which maintain practically the same rate whether they are in heat or cold, an idea now brought to great perfection in the modern chronometer.
