LET. IX.- OF THE LATITUDE AND LONGITUDE. IZI

before, became now a matter, of absolute neces-sity. Floating in a frail vessel, upon an uncer-tain abyss, be has consigned himself to themercy of the winds and waves, and knowsnot where he is : an uniform simultaneous plane,and an uninterrupted horizon, is all that he feesaround him. The compass will direct him inhis course, but it shews him neither the coastshe has left behind, nor those he endeavours tofind.

In this situation of danger and distress, havingno other resource, he applies to the heavens forassistance: the same stars, which he saw in hisnative country, are still visible in the firmament,and these are his only guides. Being fixed andimmoveable, their situations are known; andrules have been derived from this property,which will enable him to find the latitude ofthe place at which he is arrived. But as manyplaces have the same latitude, this only informshim that he is somewhere in a certain circlewhich is parallel to the equator. To tell ex-actly in what part of this circle he is, he mustalso find the difference of longitude, and deter-mine the meridian which cuts the parallel in theplace occupied by his vessel.

The latitude of a place is easily discovered,but the longitude is a subject of the utmost diffirrusty. Many methods have been devised forthis purpose, which are sufficiently accurateL 4 upon