The eastern turret is devoted to the Time Ball and its mechanism. Far out at sea—away from all sources of information but those to be asked of the planets, his compass, his quadrant, his chronometer, and his almanack, the mariner feels the value of _time_ in a way which the landsman can scarcely conceive. If his chronometer is right, he may feel safe; let him have reason to doubt its accuracy, and he knows how the perils surrounding him are increased. An error of a few seconds in his time may place him in danger—an error of a few minutes may lead him to steer blindly to his certain wreck. Hence his desire when he is leaving port to have his time-pieces right to a second; and hence the expenditure of thought, and labour, and money, at the Greenwich Observatory, to afford the shipping of the great port of London, and the English navy, the exact time—true to the tenth of a second, or six hundredth of a minute—and to afford them also a book, the Nautical Almanack, containing a mass of astronomical facts, on which they may base their calculations, with full reliance as to their accuracy. Every day for the last seventeen years, at five minutes before one o’clock, the black ball five feet across and stuffed with cork, is raised halfway up its shaft above the eastern turret of the Observatory;—at two-and-a-half minutes before that hour, it rises to the top. Telescopes from many a point, both up and down the river, are now pointed to this dark spot above the Greenwich trees, and many an anxious mariner has his time-pieces beside him, that their indications may be made true. Watch the Ball as you stand in the Park. It is now just raised. You must wait two minutes and a half, and as you do so, you feel what a minute may be. It seems a long, palpable, appreciable time, indeed. In the turret below, stands a clock telling the true time, gained by a laborious watching of the _clock-stars_; and beside the clock, is a man with a practised hand upon a trigger, and a practised eye upon the face of the dial. One minute—two minutes pass. Thirty seconds more, and the trigger has released the Ball. As it leaves the top of the shaft, it is one o’clock to the tenth of a second. By the time it has reached the bottom it is some five seconds later.