“To help mariners to find their longitude at sea, and thus know where they are.  As the sun passes so far north, and so far south of the equator each year, it is easy enough to find the latitude, by observing his position at noon-day; but for a long time seamen had great difficulty in ascertaining their longitudes.  That, too, is done by observing the different heavenly bodies, and with greater accuracy than by any other process; but this thought of measuring the time is very simple, and so easily put in practice, that we all run by it now.”

“Still I cannot understand it,” said Rose, looking so intently, so eagerly, and so intelligently into the handsome mate’s eyes, that he found it was pleasant to teach her other things besides how to love.

“I will explain it.  Having the Greenwich time in the watch, we observe the sun, in order to ascertain the true time, wherever we may happen to be.  It is a simple thing to ascertain the true time of day by an observation of the sun, which marks the hours in his track; and when we get our observation, we have some one to note the time at a particular instant on the chronometer.  By noting the hour, minutes, and seconds, at Greenwich, at the very instant we observe here, when we have calculated from that observation the time here, we have only to add, or subtract, the time here from that of Greenwich, to know precisely how far east or west we are from Greenwich, which gives us our longitude.”

“I begin to comprehend it again,” exclaimed Rose, delighted at the acquisition in knowledge she had just made.  “How beautiful it is, yet how simple—­but why do I forget it?”

“Perfectly simple, and perfectly sure, too, when the chronometer is accurate, and the observations are nicely made.  It is seldom we are more than eight or ten miles out of the way, and for them we keep a look-out.  It is only to ascertain the time where you are, by means that are easily used, then look at your watch to learn the time of day at Greenwich, or any other meridian you may have selected, and to calculate your distance, east or west, from that meridian, by the difference in the two times.”

Rose could have listened all night, for her quick mind readily comprehended the principle which lies at the bottom of this useful process, though still ignorant of some of the details.  This time she was determined to secure her acquisition, though it is quite probable that, woman-like, they were once more lost, almost as easily as made.  Mulford, however, was obliged to leave her, to look at the vessel, before he stretched himself on the deck, in an old sail; it having been previously determined that he should sleep first, while the wind was light, and that Jack Tier, assisted by the females, should keep the first watch.  Rose would not detain the mate, therefore, but let him go his way, in order to see that all was right before he took his rest.