Page:Popular Science Monthly Volume 3.djvu/735

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is the only form of material existence which can be realized in thought. In what sense, and to what extent, this assertion is well founded, will be my next subject of examination.


THE wreck of the Atlantic, followed closely by that of the City of Washington nearly on the same spot, has led many to inquire into the circumstances on which depends a captain's knowledge of the position of his ship. In each case, though not in the same way, the ship was supposed to be far from land, when in reality quite close to it. In each case, in fact, the ship had oversailed her reckoning. A slight exaggeration of what travellers so much desire—a rapid passage—proved the destruction of the ship, and in one case occasioned a fearful loss of life. And, although such events are fortunately infrequent in Atlantic voyages, yet the bare possibility that, besides ordinary sea-risks, a ship is exposed to danger from simply losing her way, suggests unpleasant apprehensions as to the general reliability of the methods in use for determining where a ship is, and her progress from day to day.

I propose to give a brief sketch of the methods in use for finding the way at sea, in order that the general principles on which safety depends may be recognized by the general reader.

It is known, of course, to every one, that a ship's course and rate of sailing are carefully noted throughout her voyage. Every change of her course is taken account of, as well as every change in her rate of advance, whether under sail or steam, or both combined. If all this could be quite accurately managed, the position of the ship at any hour could be known, because it would be easy to mark down on a chart the successive stages of her journey, from the moment when she left port. But a variety of circumstances renders this impossible.

To begin with: the exact course of a ship cannot be known, because there is only the ship's compass to determine her course by, and a ship's compass is not an instrument affording perfectly exact indications. Let any one on a sea-voyage observe the compass for a short time, being careful not to break the good old rule which forbids speech to the "man at the wheel," and he will presently become aware of the fact that the ship is not kept rigidly to one course, even for a short time. The steersman keeps her as near as he can to a particular course, but she is continually deviating, now a little on one side, now a little on the other, of the intended direction; and even the general accuracy with which that course is followed is a matter of estimation,