Page:Narrative of a Voyage around the World - 1843.djvu/32

 xxvi No possible pains should be spared which may throw any light on the hitherto inexplicable form of" the curves which unite the degrees of equal magnetic variation, or on the annual motion of those curves to the east or to the west.

The diurnal arcs of variation should also occupy your attention in favourable situations ; and it will be very interesting, if, by multiplying observations, you can either confirm or refute the assertion that there is a constant difference between the variation on the east and west sides of an island, independent of that due to the space it occupies. The restrictions under which these delicate observations should be made will readily suggest themselves to you. No subject can be of greater importance to navigators than the laws which affect their compass, and none should be pursued with more perseverance; azimuths and amplitudes should be obtained every day, and under every variety of circumstance, as well on shore as on board ; and the latter, whenever practicable, should be made with the ship's head either north or south, or rather on the line of no deviation, as shown by the table which will have been formed in each vessel, of her local attraction.

The local attraction, however, varies in the ratio of the dip; it should, therefore, be carefully retried, (on every point of the compass,) at both extremes of the survey, as well as near the equator, and a full report of each trial transmitted to this office.

Observations for the dip and intensity should be made at different points of the coast, carefully avoiding the neighbourhood of any place which may be likely to influence the needle.

Nautical descriptions of the places comprised within the limits of the Survey, and clear directions for the