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. 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. By means, therefore, of
a table indicating those changes, under different latitudes and longitudes,
and of what are called variation charts, the uncertainty arising from them
is in a great measure done away. Another source of error however existed,
which does not seem to have been noticed till the period of Captain Cook's
voyages: it was then found, "that the variation of the needle differed very
sensibly on the same spot, with the different directions of the ship's
head." Captain Flinders attributed this to the iron in the ship, and made a
number of observations on the subject; these have been subsequently added
to and corrected, so that at present the quantity of variation from this
cause can be ascertained, and of course a proper allowance made for it. It
does not appear that any material improvement has been made in the
construction and use of the log, - that useful and necessary appendage to
the compass, - since it was invented about the end of the sixteenth century.
These are the most important improvements in nautical knowledge and
science, which renders navigation at present so much more safe and
expeditious than it formerly was; there are, however, other circumstances
which tend to the same object; the more full, accurate, and minute
knowledge of the prevalent winds at different times of the year, and in
various parts of the ocean; the means of foretelling changes of weather;
and, principally, a knowledge of the direction and force of the currents
must be regarded as of essential advantage to the seaman. When to these we
add, the coppering of ships, which was first practised about the year 1761,
and other improvements in their built and rigging, we have enumerated the
chief causes which enable a vessel to reach the East Indies in two-thirds
of the time which was occupied in such a voyage half a century ago.
Nor must we forget that the health of the seamen has, during the same
period, been rendered infinitely more secure; so that mortality and
sickness, in the longest voyages, and under great and frequent changes of
climate, and other circumstances usually affecting health, will not exceed
what would have occurred on land during the same time.
The great advantages which the very improved state of all branches of
physical science, and of natural history, bestow on travellers in modern
times, are enjoyed, though not in an equal degree, by navigators and by
those who journey on land. To the latter they are indeed most important,
and will principally account for the superiority of modern travels over
those which were published a century ago, or even fifty years since. It is
plain that our knowledge of foreign countries relates either to animate or
inanimate nature: to the soil and geology, the face of the surface, and
what lies below it; the rivers, lakes, mountains, climate, and the plants;
or to the natural history, strictly so called: - and to the manners,
institutions, government, religion, and statistics of the inhabitants.
Consequently, as the appropriate branches of knowledge relating to these
objects are extended, travellers must be better able, as well as more
disposed, to investigate them; and the public at large require that some or
all of them should at least be noticed in books of travels.
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