“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 any thing 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?” “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.