with a rule for empirically finding the amount of variation, for short periods at least, according to which the magnetic needle is found to vary from year to year at every spot along certain given lines, which it has been found possible to delineate upon the charts; thus showing at a glance the amount of variation to be allowed for at any given spot. As this is sufficient for all practical purposes of navigation, the seaman is, in most cases, relieved of the necessity of making for himself these observations and calculations, if only he can ascertain with anything like accuracy the position of his ship on the earth's surface, and has determined the amount of local variation on board.
These iso-magnetic lines are, however, susceptible of great improvement, and if they are ever to become practically and universally useful, repeated observations must not be neglected by such navigators as have the means and the requisite scientific knowledge to pursue such investigation.
On board the Novara not a single sunshiny day was suffered to pass without the variation being frequently determined, or such observations repeated as related to the determination of local attraction on board.
Under such circumstances, an unusual value attached to our ascertaining and following up so far as practicable the decrease in declination of the magnetic needle till it reached the zero point assigned to it, and comparing our own observations with the amounts stated on the charts.
It was, however, at least as regarded nautical matters, of
by no means special importance, that we should reach the very point of minimum declination,—it sufficed to ascertain that the observed diminution, as marked upon the charts, corresponded with our observations, which proved, in fact, to be the case.
This confirmation proved the more satisfactory, that when we reached the N.E. side of the Paomotu group (also called Pakomotu, lying between 13°-22° S., and 135°-150° W.) we found a fresh north-easter blowing, a phenomenon which during the fine season is due to the high temperature of these islands, and of course interposed a serious and persistent obstacle to our intended N.E. course.
Another impediment to our attempt to get nearer to the zero point of minimum declination presented itself in the far from healthy state of the ship's crew. A peculiar endemic colic,[99] called by the French at Tahiti colique sèche, or colique végétale (dry or vegetable colic), was rapidly extending among the men, and had already carried off one victim, a sailor, who died after a short illness on the morning of the 9th March, and was committed to the deep the same day with the customary solemnities.
By 17th March, in 15° 52′ S., and 137° 23′ W., the declination of the magnetic needle had diminished to 5 1⁄2° E., and thus far agreed pretty accurately with that indicated by the charts; it is not, however, likely that it actually falls to a
zero point, but rather diminishes gradually as the central point is approached, which would hardly be the case if the declination actually fell to zero.