Page:Popular Science Monthly Volume 3.djvu/744

726 The scene here described corresponds pretty closely, I have little doubt, with one actually witnessed by the novelist, except only that the captain or chief officer made the observations, and that either there had not been ten days' murky weather, or else that in the forenoon, several hours at least before noon, an observation of the sun had been made. The noon observation would give the latitude, and, combined with a forenoon observation, would give the longitude, but alone would be practically useless for that purpose. It is curious that the novelist sets the longitude as assigned much more closely than the latitude, and the value given would imply that the ship's time was known within less than a second. This would in any case be impracticable; but, from noon observations, the time could not be learned within a minute at the least. The real fact is, that, to determine true time, the seaman selects, not noon, as is commonly supposed, but a time when the sun is nearly due east or due west. For then the sun's elevation changes most rapidly, and so gives the surest means of determining the time. The reader can easily see the rationale of this by considering the case of an ordinary clock-hand. Suppose our only means of telling the time was by noting how high the end of the minute-hand was: then, clearly, we should be apt to make a greater mistake in estimating the time, when the hand was near XII., than at any other time, because then its end changes very slowly in height, and a minute more or less makes very little difference. On the contrary, when the hand was near III. and IX., we could in a very few seconds note any change of the height of its extremity. In one case we could not tell the time within a minute or two; in the other, we could tell it within a few seconds.

But the noon observation would be wanted to complete the determination of the longitude; for, until the latitude was known, the captain would not be aware what apparent path the sun was describing in the heavens, and therefore would not know the time corresponding to any particular solar observation. So that a passenger, curious in watching the captain's work, would be apt to infer that the noon observations gave the longitude, since he would perceive that from them the captain worked out both the longitude and the latitude.

It is curious that another and critical portion of the same entertaining novel is affected by the mistake of the novelist on this subject. After the scuttling of the Proserpine, and other events, Hazel and Miss Rolleston are alone on an island in the Pacific. Hazel seeks to determine their position, as one step toward escape. Now, "you must know that Hazel, as he lay on his back in the boat, had often, in a half-drowsy way, watched the effect of the sun upon the boat's mast: it now stood, a bare pole, and at certain hours acted like the needle of a dial by casting a shadow on the sands. Above all, he could see pretty well, by means of this pole and its shadow, when the sun attained its greatest elevation. He now asked Miss Rolleston to assist