On the morning of March 7, 1916, I took a sight of the sun when the chronometer showed it was 39 minutes and 1 second past 1. My sextant showed me, after I had made some corrections which I shall not attempt to explain, that the altitude of the sun was 24° 58′. From this, and other data that it is necessary to have, I worked out our time _when I took the sight_. The answer to my problem showed me that the time was 13 minutes and 4 seconds past 8 o’clock. Subtracting this time from the time shown by the chronometer I got 5 hours, 25 minutes, and 57 seconds. Because a difference of one hour of time represents a difference of 15 degrees of longitude, a difference of 5 hours, 25 minutes, and 57 seconds in time represents a difference of 81 degrees, 29 minutes, and 15 seconds in longitude. The Greenwich time was later than ours; therefore, our longitude was 81° 29′ 15″ west of Greenwich. I have purposely refrained from explaining the working of the problem, for that can only be done with such a reference book as Bowditch at hand, in order that the compiled logarithms may be looked up. Furthermore, the explanation is long, technical, and, to the beginner, tedious, and is beside the purpose of this book. I have given the incomplete explanation only to show that to find longitude one must find one’s “local mean time,” and must have a timepiece showing the “mean time” at Greenwich. In the foregoing explanation I have left out of consideration several factors vital to accuracy in navigation. For instance, I have not mentioned the fact that the sun is not so accurate in its movements as an accurate chronometer. Sometimes it is a few minutes ahead and sometimes it is a little behind time. From this, two expressions for time have come into use: “apparent time” and “mean time.” “Apparent time” is the time that is shown by the sun; “mean time” is the time shown by the clock. Because there is this difference there must be a correction made for it, and this correction is to be found in the Nautical Almanac, which is a valuable part of the navigator’s equipment. Again, the navigator takes the angle of the sun from the bridge or some other elevated part of his ship. The angle he gets from such a height is slightly different from the one he would get if he were at the water level. Therefore he must make a correction for the difference. This he finds by knowing his elevation above the water and looking up the correction. [Illustration: USING A PELORUS _This apparatus consists of a movable plate marked with compass bearings, set in a stand. The observer sets the plate to correspond to the standard compass, and then sights across it in determining the compass bearings of points ashore from which he wishes to learn his exact position._ ]