On 8th March, Shrove Tuesday was celebrated on board. Several sailors had disguised themselves as Invalids, as Tahitians, as Nicobarians, &c., and played off all manner of pranks. Dolce, our cook, the merry-andrew of the vessel, figured as a troubadour, in which capacity he sang several heart-thrilling melodies. In the afternoon the band played
on deck, and in the evening the jolly tars, to their great gratification, received each a double allowance of grog.
It was our Commodore's intention to cross the shorter diameter of the almost elliptical curve of equal magnetic declination, which occurs in this vicinity, with the view, if possible, of ascertaining by observation by what law the "local variation" of the needle is diminishing within the curve of 5°, the latest indicated in the most recent magnetic charts.
This curve of 5° easterly magnetic declination lies, according to F. Evans,[98] between the parallels of 5° 30′ N. and 13° S. lat., and 120° W. and 134° 30′ W., north-eastward from the Marquesas Islands.
The magnetic needle, as is well known, does not point to the geographical poles, but is deflected from the due north and south meridian, in a direction eastward or westward according to locality, at an angle which, in the measure of the easterly or westerly magnetic variation of the plane, is called eastern or western declination or variation, and which not only gradually alters at every place with the lapse of time, but also is universally found to assume different values at different places, so that in certain lines, known as lines of equal declination, the variation remains the same for all places under that line during a certain given period.
As the compass is the sole reliable guide of the seaman
while traversing the ocean, and it is of the utmost importance to investigate and accurately lay down the ship's course for the port which is her object to make, it appears necessary to explain to the uninitiated how the local variation of the magnetic needle is determined, as thereby one can readily find the precise angle at which the magnetic meridian of any place is deflected from the true meridian.
The determination of this divergence is effected by means of observations of the sun, by the aid of which one can calculate at any moment its actual bearings, as seen from the deck of the ship, and this, compared with the true position of the sun, gives the amount of variation.
This apparently simple method of determination encounters in practice, owing to certain local influences, a variety of obstacles, for it is executed on board of a ship, which frequently contains within itself, at a greater or less distance from the binnacle, large superficies of iron, operating less or more prejudicially upon the needle, by deflecting it from the direction which it would actually have but for these masses of iron. Hence the variation is not even the same in all parts of the ship, nor does it follow the same direction, but varies according to certain laws, founded upon the intensity and direction of the magnetic attraction of the earth. It is therefore necessary to make allowance for these local deflections of the needle, in order to find the true variation of the needle.
So far as regards the last-named, many thousand observations, both by land and sea, have resulted in furnishing us