At length the man who holds the log throws it over into the water, and the ship, sailing onward all the time, leaves it there, floating edge upwards. The man who holds the reel lifts it up high, so that the line can run off easily as the ship moves on. As soon as the first rag runs off, which denotes the beginning of the marked point of the line, he calls out suddenly,--
This is the command to the man who holds the minute glass to turn it so as to set the sand to running. He accordingly instantly changes the position of the glass, and holds it perpendicularly, and immediately sets himself to watching the running out of the sand. The instant it is gone, he calls out,--
The man who is holding the reel, and another who stands by ready to help him, instantly stop the line, and begin to draw it in. They observe how many knots have run out, and they know from this how many miles an hour the ship is going. Each knot goes for a mile.
They do not have to count the knots that have run out. They can always determine, by the form and color of the last one that passed, what knot it is. One of the men goes immediately and reports to the captain that the ship is going so many knots, and the captain makes a record of it. The other men at once begin to draw in the line, which brings the log in also at the end of it. This line comes in very hard, for the friction of so long a cord, dragged so swiftly through the water, is very great. It generally takes four or five men to pull the line in. These men walk along the deck, one behind the other, with the line over their shoulders; and at first they have to tug very hard. The reel man winds the line upon the reel as fast as they draw it in. It comes in more and more easily as the part that is in the water grows shorter; and at length the log itself is soon skipping through the foam in the wake of the ship, until it comes up out of the water and is taken on board.
They heave the log every two hours,--that is, twelve times for every twenty-four hours,--and from the reports which the captain receives of the results of those trials, it is easy for him to calculate how far the ship has come during the whole period. As he knows, too, exactly how far the pilot has been steering by the compass all this time, he has both the direction in which the ship has been sailing, and the distance to which she has come; and, of course, from these data he can calculate where she must now be. This mode of determining the ship's place is called _by the reckoning_. The other is called _by observation_.