Page:Narrative of a Voyage around the World - 1843.djvu/29
and of the shoals, is the precision and fulness of the soundings, with the quality of the bottom. There can be no doubt that the nature of the substances which are spread over the bottom of the sea does not depend on mere chance, but that they are in some measure connected with the adjacent shores, and sometimes with those of more distant parts of the coast, from which they have been swept by currents; and it would be a great benefit to navigation if this relationship could be satisfactorily traced. The transition also from one species of sand to another, and the link by which these beds or patches are connected, are subjects which would be well deserving the reflection and exertions of our scientific mariners. Besides the soundings to be carried along shore, (the breadth of which zone, proceeding from the back of the rollers, will depend on their depth and regularity,) the outer edge of the bank should also be laid down, as being a most important aid to the navigator, and a sufficient number of depths marked in the intervening space, to show the general slope of the bank. This outer edge may be assumed at a hundred fathoms, as in general it rapidly sinks from that depth, to one beyond the usual reach of the lead. Massey's machine should not be used in a greater depth than fifty or sixty fathoms.
In approaching the several groups of islands, and in the various runs which either vessel may have to make in the course of the survey, at a distance from the land, no opportunity should be missed of throwing down the deep sea lead. The negative language of no bottom soundings on the charts, is next in value to the real depth; and, unless when pressed in time by some paramount object, it should be the established practice of a surveying vessel, both night and day, though apparently