Page:Popular Science Monthly Volume 8.djvu/535

Rh speed of the vessel will permit, and at reasonable distances the deep-sea lead should be employed to obtain actual depths. Positive soundings exceeding 100 fathoms should be obtained as far to seaward as circumstances will permit the survey to be extended.

A difficult task of the hydrographic surveyor is, to search for the islands and dangers shown on the charts, or enumerated in nautical guides as uncertain in position or of doubtful existence.

Many facts show that the origin of a great number of these may be traced to deceptive appearances, to misplacement from faulty observations or reckoning, or to typographical errors in the reports published.

Reports of new dangers grow more frequent, as the sea-routes extend into regions heretofore but little traversed, and as the commercial navigator manifests a greater interest in hydrography. All these obstructions to navigation are placed on the charts, usually with queries, until they are verified and correctly located, or their nonexistence proved by professional authority through local search. Such dangers have frequently been found to exist at considerable distance from the positions given, from indifferent astronomical observations, or from reckoning referred to observations taken several days before or after their discovery; the search must, therefore, be extended over a considerable area. The search for islands is naturally less difficult than that for submerged dangers, which on the broad ocean can in some instances hardly be detected but by chance.

In causing reported dangers of this nature to be erased from the charts, on the strength of a search which has not been thorough in every particular, the hydrographer incurs a grave responsibility; there are a number of instances on record where dangers which had been searched for most carefully and by very competent authority, have been replaced exactly in the position from which they were erased, after they have been assured by the loss of a vessel on them, and the reëxamination of the position in consequence of it.

A correct representation of the character of the bottom of the waters is very important, not only for the selection of anchorages, but also as a guide to the navigator when he cannot otherwise obtain the position. of his vessel, especially when approaching a coast in fogs and thick weather, or when passing through channels not bordered by good landmarks; for this purpose specimens of the bottom should be brought up for examination, and every change of it noted.

The tidal relations, tidal hour, and the rise and fall at the various stages of the moon, and in the various seasons, the influence of the winds upon the tides, etc., can be deduced accurately only by observations continued through a longer period than the limited time of a running survey will generally permit. Observers should, if possible, be left for this purpose at the important points. A lunation is the shortest period in which approximate data can be arrived at, but