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.

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