“Rose Budd, you do not understand the uses of a chronometer, I see,” said the mate, firmly, “notwithstanding all I have told you concerning them.”

“It is to keep time, Harry Mulford, is it not?”

“True, to keep time—­but to keep the time of a particular meridian; you know what meridian means, I hope?”

Rose looked intently at her lover, and she looked singularly lovely, for she blushed slightly, though her smile was as open and amicable as ingenuousness and affection could make it.

“A meridian means a point over our heads—­the spot where the sun is at noon,” said Rose, doubtingly.

“Quite right; but it also means longitude, in one sense.  If you draw a line from one pole to the other, all the places it crosses are on the same meridian.  As the sun first appears in the east, it follows that he rises sooner in places that are east, than in places that are further west.  Thus it is, that at Greenwich, in England, where there is an observatory made for nautical purposes, the sun rises about five hours sooner than it does here.  All this difference is subject to rules, and we know exactly how to measure it.”

“How can that be, Harry?  You told me this but the other day, yet have I forgotten it.”

“Quite easily.  As the earth turns round in just twenty-four hours, and its circumference is divided into three hundred and sixty equal parts, called degrees, we have only to divide 360 by 24, to know how many of these degrees are included in the difference produced by one hour of time.  There are just fifteen of them, as you will find by multiplying 24 by 15.  It follows that the sun rises just one hour later, each fifteen degrees of longitude, as you go west, or one hour earlier each fifteen degrees of longitude as you go east.  Having ascertained the difference by the hour, it is easy enough to calculate for the minutes and seconds.”

“Yes, yes,” said Rose, eagerly, “I see all that—­go on.”

“Now a chronometer is nothing but a watch, made with great care, so as not to lose or gain more than a few seconds in a twelvemonth.  Its whole merit is in keeping time accurately.”

“Still I do not see how that can be anything more than a very good watch.”

“You will see in a minute, Rose.  For purposes that you will presently understand, books are calculated for certain meridians, or longitudes, as at Greenwich and Paris, and those who use the books calculated for Greenwich, get their chronometers set at Greenwich, and those who use the Paris, get their chronometers set to Paris time.  When I was last in England, I took this watch to Greenwich, and had it set at the Observatory by the true solar time.  Ever since it has been running by that time, and what you see here is the true Greenwich time, after allowing for a second or two that it may have lost or gained.”

“All that is plain enough,” said the much interested Rose—­“but of what use is it all?”