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_. The intelligent and reflecting boy who has carefully read and understood the preceding explanations will perceive that the two operations which we have been describing are in some sense the reverse of each other. By the former, the navigator ascertains by his measurements where the ship actually is to-day, and then calculates from that how far, and in what direction, she has come since yesterday. Whereas, by the latter method, his measurements determine directly how far, and in what direction, the ship has come; and then he calculates from these where she now is. Each method has its advantages. The former, that by observation, is the most sure and exact; but then it is not always practicable, for it may be cloudy. On the other hand, the latter--that is, by the reckoning--never fails, for the log can always be thrown, be the weather what it may; but it cannot be fully relied upon, on account of the currents in the water and the drifting of the vessel. Consequently, on board all ships they keep the reckoning regularly every day. Then, if they get a good observation, they rely upon that. If they do not, they go by the reckoning.