The relict merely shrugged her shoulders, as much as to say that she knew no such thing; but Rose, who had been well taught, raised her serene eyes to her aunt’s face, and mildly said—­“All true, aunty, and that is owing to the fact that the earth is smaller at each end than in the middle.”

“Fiddle faddle with your middles and ends, Rose—­I’ve been in London, dear, and know that the sun rises later there than in New York, in the month of December, and that I know by the clock, I tell you.”

“The reason of which is,” resumed Mulford, “because the clocks of each place keep the time of that place.  Now, it is different with the chronometers; they are set in the observatory of Greenwich, and keep the time of Greenwich.  This watch chronometer was set there, only six months since; and this time, as you see, is near nine o’clock, when in truth it is only about four o’clock here, where we are.”

“I wonder you keep such a watch, Mr. Mulford!”

“I keep it,” returned the mate, smiling, “because I know it to keep good time.  It has the Greenwich time; and, as your watch has the New York time, by comparing them together, it is quite easy to find the longitude of New York.”

“Do you, then, keep watches to compare with your chronometers?” asked Rose, with interest.

“Certainly not; as that would require a watch for every separate part of the ocean, and then we should only get known longitudes.  It would be impracticable, and load a ship with nothing but watches.  What we do is this:  We set our chronometers at Greenwich, and thus keep the Greenwich true time wherever we go.  The greatest attention is paid to the chronometers, to see that they receive no injuries; and usually there are two, and often more of them, to compare one with another, in order to see that they go well.  When in the middle of the ocean, for instance, we find the true time of the day at that spot, by ascertaining the height of the sun.  This we do by means of our quadrants, or sextants; for, as the sun is always in the zenith at twelve o’clock, nothing is easier than to do this, when the sun can be seen, and an arc of the heavens measured.  At the instant the height of the sun is ascertained by one observer, he calls to another, who notes the time on the chronometer.  The difference in these two times, or that of the chronometer and that of the sun, gives the distance in degrees and minutes, between the longitude of Greenwich and that of the place on the ocean where the observer is; and that gives him his longitude.  If the difference is three hours and twenty minutes, in time, the distance from Greenwich is fifty degrees of longitude, because the sun rises three hours and twenty minutes sooner in London, than in the fiftieth degree of west longitude.”