Charts became greatly improved during the latter part of the 16th Century, owing to the studies of Mercator, after whom the “Mercator projection” is named. The Mercator projection is used in the type of map that shows the entire surface of the earth as if it were the unrolled surface of a cylinder, and is the type which is, perhaps, despite its errors, in commonest use to-day. But despite many improvements it was not until the 18th Century that modern navigation really began. Then, suddenly, both the sextant and the chronometer were invented in rapid succession—the one in 1731 and the other in 1735. The sextant is the instrument (now greatly perfected) that is used to measure accurately the angles between the horizon and the celestial bodies being observed, and the chronometer is the accurate timepiece (now also greatly perfected) used on practically all sea-going ships to keep a record of the time of the prime meridian of longitude—that is, the meridian numbered zero. Usually, nowadays, that meridian, as I have said, is the meridian of Greenwich, England, for it is at Greenwich that a British observatory is located, and at this observatory the vital data for seamen are compiled. With the introduction of the sextant and the chronometer the determination of longitude became simple. And latitude, too, because of the sextant, could more accurately be determined. It is not my purpose to go into detail in explaining the finding of one’s longitude, but I shall attempt to explain, simply, the theory. The sun, during a day of twenty-four hours, covers the 360 degrees of the circumference of the earth. That is, during every hour it passes 15 degrees. If you have a clock that tells you that it is 9 o’clock in the morning at Greenwich and you know that, according to the sun, it is 8 o’clock in the morning where you are, you know that because of that difference of one hour there is a difference of 15 degrees of longitude, and that you are 15 degrees west of the meridian of Greenwich. If you were 15 degrees east, your time would be 10 o’clock. Now if you have some accurate way of telling what time it is by the sun where you are, and you have a chronometer telling you the time at Greenwich, all you have to do is to subtract the earlier time from the later and work out how many degrees, minutes, and seconds of longitude are represented by the hours, minutes, and seconds of the difference. If it is later at Greenwich than where you are, you are west of Greenwich; if earlier, you are east.