Another of the high functions of the moon is that by which she subserves the wants of the navigator, and enables him to track his course over the pathless ocean. Of the two co-ordinates, Latitude and Longitude, that are needful to determine the position of a ship at sea (or of any standpoint upon the earth’s surface) the first is easily found, inasmuch as it is always equal to the altitude of the celestial pole at the place of observation. But the determination of the longitude has always been a difficult problem, and one upon which a vast amount of ingenuity has been expended. When it was first attacked it was soon discovered that the moon was the object of all others by which it could be most accurately and, all things considered, most readily determined. We must premise that the longitude of one place from another is in effect the difference between the local times at the two places, so that when we say that a place or a ship is, for instance, seven hours, twenty-four minutes, ten seconds, west of Greenwich, we mean that the time-o’-day at the place or ship is seven hours twenty-four minutes ten seconds earlier than that at Greenwich. Hence, finding the longitude at sea or at any place and moment means finding what time it is at Greenwich at that moment. Of course this could be most easily done if we could set a timekeeper at Greenwich and rely upon its keeping time during a long sea voyage; and this plan appeared so feasible that our Government long ago offered a prize of £20,000 for a timekeeper which would perform to a stated degree of accuracy after a certain sea voyage. One John Harrison did make such a timekeeper, that actually satisfied the conditions, and obtained the prize: and chronometers are now largely used for longitude, their construction having been brought to great perfection, especially in England, owing to a continuance (in a less liberal degree, however,) of Government inducement. But chronometers are not entirely to be relied on, even where several are carried, which in other than Government ships is rarely the case: recourse must be had to the heavenly bodies for check upon the timekeeper. And the moon is, as we have said, the body that best serves the requirements of the problem.