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. The longitude is thus ascertained to such a nicety,
as to secure the navigator from any danger arising from the former
imperfect modes of finding it; "he is now enabled to make for his port
without sailing into the parallel of latitude, and then, in the seaman's
phrase, running down the port, on the parallel, as was done before this
method was practised. Fifty years ago, navigators did not attempt to find
their longitude at sea, unless by their reckoning, which was hardly ever to
be depended on."
Not long after the mariner's compass was employed, its variation was
noticed; as it is obvious that, unless the degree and direction of this
variation are accurately known, the compass would be of little service in
navigation, the attention of navigators and philosophers was carefully
directed to this point; and it was ascertained that the quantity of this
variation is subject to regular periodical changes.
Pages:
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858