attempt to give lunar distances![1] 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.[2] 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.
Various useful rules and tables were appended to early volumes of the Almanac. Thus that for 1771 contains a method and table for determining the latitude by two altitudes and the elapsed time (first published by Cornelius Downes of Amsterdam in 1740). At the end of the Almanac for 1772 Maskelyne and Whichell gave three special tables for clearing the lunar distance; still their rule is neither short nor easily remembered. An improvement of Dunthorne’s solution is also given. In the edition for 1773 a new table for equations of equal altitude was given by W. Wales. In those for 1797 and 1800 tables were added by John Brinkley for rendering the calculations for double altitudes easier.
The plan of the Nautical Almanac was soon imitated by other nations. In France the Académie Royale de Marine had all the lunar distances translated from the British Nautical Almanac for 1773 and following years, retaining Greenwich time for the three-hourly distances. The tables were considered excellent, and national pride was satisfied by their having been formed on the plan proposed by Lacaille. They did not imitate the mode given for clearing the lunar distance, considering their own better.
Though the Spaniards were leaders in the art of navigation during the 16th and 17th centuries, it was not till November 4, 1791, that their first nautical almanac was printed at Madrid, having been previously calculated at Cadiz for the year 1792. They acknowledge borrowing from the English and French. The excellent Berlin Astronomisches Jahrbuch began to appear in 1776, the American Ephemeris in 1849. These two ephemerides and the French Connaissance des temps are independent and valuable works.
A book of Tables Requisite to be Used with the Nautical Ephemeris was published by Maskelyne at the same time as the first Almanac, and ten thousand copies were quickly sold. A second edition, prepared by Wales, appeared in 1781, an octavo of 237 pages, in the preface of which it is stated that it contains everything necessary for computing the latitude and longitude by observation. There are in all twenty-three tables, the traverse table and table of meridional parts alone being deficient as compared with modern works of the kind; dead-reckoning Maskelyne did not touch. He gave practical methods for working several problems; that for computing the lunar especially is an improvement on those by Lyons and Dunthorne, and a rule given for clearing the distance, called Dunthorne’s improved method, is remarkably short. Maskelyne’s rule for finding the latitudes by two altitudes and the elapsed time is also good. The third edition of the Tables was issued in 1802.
The publication of the Requisite Tables met a great want, and the existence of such accurate and conveniently-arranged mathematical tables for the special purposes of nautical calculations led to the more general use of many refinements which had been previously neglected. They formed the original of many subsequent and greatly extended collections, of which those by J. W. Norie are the more generally used in modern times in the mercantile marine, and the very accurate and comprehensive tables by James Inman (originally published in 1823) are constantly used in the British navy.
Until the middle of the 17th century mariners generally employed small collections of Dutch charts, known as “waggoners” from Waghenair, the name of a celebrated Dutch hydrographer in 1584. In 1671 appeared the English Pilot by John Sellers, who is styled the “Hydrographer Royal.” It forms a collection of rude sketches of the coasts of England, the North Sea, France and Spain, with sailing directions, and on its appearance the importation of Dutch charts was prohibited. Private enterprise, for many years after that, supplied both the British navy and the British mercantile marine with constantly improving charts, especially latterly, under the powerful patronage of the East India Company, whose hydrographer (Alexander Dalrymple), in 1795, was selected as the first hydrographer of the Admiralty. This post has since been occupied by a succession of distinguished naval officers under whom have grown up a large school of able nautical surveyors, the results of whose labours are now published in the well-known Admiralty charts.
Prior to the issue of charts by the Admiralty, the instructions to masters of vessels in the British navy enjoined them to “provide such charts and instruments as they considered necessary for the safe navigation of the ship,” while on the completion of a voyage of discovery it was customary for the results to be published for the Admiralty by private firms.
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.
A knowledge of the times and heights of high and low water and the directions of the tidal streams due to those phenomena are in many parts of the world (and especially round our own coasts) of vital importance to navigation. The theory of the tides was first laid down by Newton and Laplace, and in Phil. Trans., 1683. there is an account of Flamsteed’s tide table for London Bridge, which gave the times of each high tide on every day in the year. For a long subsequent period empirical tide tables for a few places in England were published by private individuals, but in 1832 the researches of Dr W. Whewell and Sir J. W. Lubbock enabled official tide tables to be issued by the Admiralty. These have steadily advanced in detail and accuracy, being now in many cases based on continuous tidal observations for a whole lunar period of 1814 years, and represent the practical epitome of our knowledge of the tides and tidal currents of the whole world. The formulae and tables on which these predictions are based are given in the introduction to each annual volume (see Tide).
Modern Navigation
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
- ↑ The French nautical almanac or Connaissance des temps appeared under letters patent from the king, dated 24th March 1679—seventeen years before the first issue. The following is a literal translation of its advertisement: “This little book is a collection of holy days and festivals in each month. The rising and setting of the moon when it is visible, and of the sun every day. The aspects of the planets as with respect to each other, the moon and the fixed stars. The lunations and eclipses. The difference of longitude between the meridian of Paris and the principal towns in France. The time of the sun’s entrance into the twelve signs of the zodiac. The true place of the planets every fifth day, and of the moon every day of the year, in longitude and latitude. The moon’s meridian passage, for finding the time of high water, ‘as well as for the use of dials by moonlight.’ A table of refraction. The equation of time [this table is strangely arranged, as though the clock were to be reset on the first of every month, and the explanation speaks of the ‘premier mobile’]. The time of twilight at Paris. The sun’s right ascension to hours and minutes. The sun’s declination at noon each day to seconds. The whole accompanied by necessary instructions.”
- ↑ Mayer’s tables were printed at London under Maskelyne’s superintendence in 1770.
