the station, he has only to ascertain the local time from a sun-dial, and the difference between this and the railway time gives the longitude. What the railway train is to the landsman, the moon is to the mariner, and while the landsman uses a dial to ascertain the local time, the sailor uses a sextant. The sailor may adopt the obvious plan of carrying a chronometer set to Greenwich time, and which, by comparison with the local time, at once gives him the longitude; but no chronometer can equal the moon as an indicator of time. The captain may forget to wind up the chronometer, some parts of the works may give way, the rate of going may change, and many other accidents may occur to render the chronometer useless; but the moon can ever be relied on. No winding up is needed, no danger of the mainspring of gravitation breaking. She no doubt changes her rate of motion, but the rate of change can be calculated so that her precise position at any moment can be predicted with absolute accuracy.
The moon would be useful to man for the division of time, though it was only monthly periods that she marked off, and though she only met the necessities of savage life; but the usefulness becomes more apparent when she serves to give the minutest divisions of time to meet the increasing necessities of man as he advances in civilisation. It is this development of adaptation, running parallel with the development
