Before 169 37 35 ) E.
( 5 Ditto, At 169 48 48 ) Long
( 20 Ditto, After 169 47 22 1/2 )
( - - - - - - - -
( Mean Of These Means, 169 44 33
It is necessary to observe, that each set of observations, consisting of
between six and ten observed distances of
The sun and moon, or moon and
stars, the whole number amounts to several hundreds; and these have been
reduced by means of the watch to all the islands; so that the longitude of
each is as well ascertained as that of the two ports above-mentioned. As a
proof of this I shall only observe, that the longitude of the two ports, as
pointed out by the watch and by the observations, did not differ two miles.
This also shews what degree of accuracy these observations are capable of,
when multiplied to a considerable number, made with different instruments,
and with the sun and stars, or both sides of the moon. By this last method,
the errors which may be either in the instruments or lunar tables, destroy
one another, and likewise those which may arise from the observer himself;
for some men may observe closer than others. If we consider the number of
observations that may be obtained in the course of a month (if the weather
is favourable,) we shall perhaps find this method of finding the longitude
of places as accurate as most others; at least it is the most easy, and
attended with the least expence to the observer. Every ship that goes to
foreign parts is, or maybe, supplied with a sufficient number of quadrants
at a small expence; I mean good ones, proper for making these observations.
For the difference of the price between a good and a bad one, I apprehend,
can never be an object with an officer. The most expensive article, and
what is in some measure necessary in order to arrive at the utmost
accuracy, is a good watch; but for common use, and where that strict
accuracy is not required, this may be dispensed with. I have observed
before, in this journal, that this method of finding the longitude is not
so difficult but that any man, with proper application, and a little
practice, may soon learn to make these observations as well as the
astronomers themselves. I have seldom known any material difference between
the observations made by Mr Wales, and those made by the officers at the
same time.
In observing the variation of the magnetic needle, we found, as usual, our
compasses differ among themselves, sometimes near 2 deg.; the same compass too,
would sometimes make nearly this difference in the variation on different
days, and even between the morning and evening of the same day, when our
change of situation has been but very little. By the mean of the
observations which I made about Erromango; and the S.E. part of these
islands, the variation of the compass was 10 deg. 5' 48" E.; and the mean of
those made about Tierra del Espiritu Santo, gave 10 deg.
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