We Do Not Intend, By Any Means, To Enter At Any Length, Or Systematically, On These Subjects; But A Brief And Popular Notice Of Them Seems Proper And Necessary In Such A Work As This.

Astronomy here comes essentially to the aid of navigation:

We have already seen how, even in the rudest state of the latter, it derived its chief assistance from this sublime science, confined as it then was to a knowledge of the position of a few stars. Astronomy enables the navigator to ascertain his latitude and longitude, and to find the variation of the compass. The principal difficulty in ascertaining the latitude at sea, arose from the unsteady motion of the ship: to remedy this, several instruments were invented. We have already alluded to the astrolobe; but this, as well as the others, were imperfect and objectionable, till such time as Hadley's quadrant was invented, the principle and uses of which were first suggested by Newton.

To ascertain the longitude was a much more difficult task: there are evidently two methods of doing this, - by time-keepers or chronometers, and by making the motions of the celestial bodies serve instead of time-keepers. About the middle of the seventeenth century, Huygens proposed the pendulum clock for finding the longitude at sea; but it was unfit for the purpose, for many and obvious reasons. Watches, even made with the utmost care, were found to be too irregular in their rate of going, to be depended upon for this purpose. In the reign of Queen Anne the celebrated act was passed, appropriating certain sums for encouraging attempts to ascertain the longitude. Stimulated by this, Mr. Harrison invented his time-keeper, which on trial was found to answer the purpose with such tolerable accuracy, that he was deemed worthy to receive the sum awarded by parliament: it went within the limit of an error of thirty miles of longitude, or two minutes of time, in a voyage to the West Indies. Since this period, chronometers have been much improved, and excellent ones are very generally used: perhaps the most trying circumstances in which any were ever placed, existed during the voyage for the discovery of a northwest passage by Captain Parry; and then most of those he had with him were found to be extremely accurate.

It is evident, however, that chronometers are liable to a variety of accidents, and that in very long voyages the means of verifying their rate of going seldom occur. Hence the lunar method, or the method of ascertaining the longitude by means of the motions of the moon, is more useful and valuable. Here again, the profoundest researches of Clairaut, Euler, D'Alembert, and La Place, were brought practically to bear on navigation. Guided and aided by these, Tobias Mayer, of Gottingen, compiled a set of solar and lunar tables, which were sent to the lords of the admiralty, in the year 1755; they gave the longitude of the moon within thirty seconds. They were afterwards improved by Dr. Maskelyne and Mr. Mason, and still more lately by Burg and Burckhardt; the error of these last tables will seldom exceed fifteen seconds, or seven miles and a half. The computations, however, necessary in making use of these tables, were found to be very laborious and inconvenient; to obviate this difficulty, the nautical almanack, suggested by Dr. Maskelyne, was published, which is now annually continued.

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