The glass, which was to mark the time of the log’s running, was shaped like an hour-glass, but was much smaller, and the sand contained in it occupied only half a minute in running from one end to the other. Now, half a minute is the one hundred and twentieth part of an hour, and fifty-one feet is the same portion of a nautical mile, which is 6120 feet, or 840 feet longer than a geographical or land mile. Thus, when we say that a vessel sails six knots (or miles) an hour, we mean that six knots, or three hundred and six feet, of the log-line ran out in half a minute. The log-line is wound on a reel that turns very easily.
In the present instance Nimbus dropped the log into the water over the lee quarter of the brig, and held the reel in his hands. When the first fifty feet, which is called the “stray-line,” and is sufficient to carry the log clear of the vessel’s eddy, had run out, and Nimbus saw the first red rag touch the water, he sang out, “Turn!” Captain McCloud turned the half-minute glass, so that the sand in it began to drop to the other end, and answered, “Done!” The instant it stopped running he cried, “Stop!” and Nimbus held the reel, so that no more line should run out.
“Seben knot, five fadom, sah,” he reported to the captain.
“Very good,” said the captain; “reel in.” Then to Breeze and Wolfe he said, "That shows that we are running at the rate of seven and a half knots, or miles, an hour. By heaving the log every hour, and keeping note of all the courses steered, we shall not only know pretty nearly the distance run, but can determine our position at the end of each sea, or nautical, day, which is at noon. This is called ‘dead-reckoning,’ and is useful as a check on observations, and also when on account of cloudy weather no observation can be taken. Of course, for such reckoning we must have some fixed point to start from, or ‘point of departure,’ as it is called. Ours in the present case is the point, back here a few miles, that we established by finding its latitude and longitude, and marking it on the chart.
“There is one more thing to be thought of in our dead-reckoning, and that is the leeway. This may be caused by ocean currents, or by a beam wind, which not only acts upon the sails, so as to force the vessel ahead, but to a certain extent drives her sidewise. This must be allowed for, and every captain must use his own judgment to determine what leeway his vessel is making, and how much her course should be altered to allow for it. Now I am going to allow a couple of points for leeway, and instead of keeping her due west, Breeze, you may make it west-south-west.”
“Ay, ay, sir!” answered Breeze, promptly; “west-sou’west,” and he altered the brig’s course slightly in obedience to these instructions.
“At the same time,” continued the captain, “we shall mark the course on the chart, as though we were heading due west.”
All this had been so interesting to the young sailors that, though already quite hungry again, they were almost sorry to hear Nimbus announce dinner just at this point.
After dinner, and after Captain McCloud had rested for an hour in the cabin, the boys asked him to tell them how he escaped from his awful position in the forecastle of the capsized Sea Robin, and of his experiences, since that time.
“Well,” he replied, “of course I will tell you the whole story; but I hate so to think of that time that I shall make my yarn as brief as possible.”