Page:EB1911 - Volume 19.djvu/308

 attempt to give lunar distances! In the English Nautical Almanac for 1767 we find everything necessary to render it worthy of confidence, and to satisfy every requirement at sea. The great achievement was that of giving the distance from the moon’s centre to the sun, when suitable, and to about seven fixed stars, every three hours. The mariner has only to find the apparent time at ship, and clear his own measured lunar distance from the effects of parallax and refraction (for which at the end of the book are given the methods of Lyons and Dunthorne), and then by simple proportions, or proportional logarithms, find the time at Greenwich. The calculations respecting the sun and moon were made from Mayer’s last manuscript tables under the inspection of Maskelyne, and were so continued till 1804. The calculations respecting the planets are from Halley’s tables, and those of Jupiter’s satellites from tables made by Wargentin and published by Lalande in 1759 (except those for the fourth satellite). The original Nautical Almanac contained all the principal points of information which the seaman required, but the great value of such an authentic publication to the whole astronomical world led soon to a considerable increase to its contents. As much of this was unnecessary for the ordinary requirements of navigation, since 1903 it has been issued in two forms, the larger for observatory purposes, the smaller for the class for whom it was originally intended.

The establishment of the Admiralty Hydrographic Office in 1795 marked a great step in the advancement of the art of navigation. On the 12th of August of that year an order in council placed all such nautical documents as were then in the possession of the Admiralty in charge of Dalrymple, whose catalogue, compiled for the use of the East India Company in 1786, contained 347 charts between England, the Cape, India and China; thus the germ of the present hydrographic department was established. The expense was then limited to £650 a year. The first official catalogue of Admiralty charts was issued in 1830, the total number being then 962.

After the close of the long devastating war in 1815 both trade and science revived, and several governments besides that of Great Britain saw the necessity of surveying the coasts in various parts of the globe; the greater portion of the work fell to the English hydrographical department, which took under its charge nearly every place where the inhabitants were not able to do it for themselves. Since that time its career of usefulness has steadily developed, and it not merely undertakes the constant improvement of the charts of the whole world, but periodically issues for the use of the seafaring community a vast amount of most accurate and practical nautical information on the various closely allied subjects of navigation, tides, compass adjustment and ocean meteorology.

Having thus sketched the progress of the art of navigation from an early period to the present time, we will now describe the modern methods by which it is brought into practical use