This plan of finding out the ship's place every day, and of ascertaining the distance which she has sailed since the day before, would be perfectly successful, and amply sufficient for all the purposes required, if the sun could always be seen when the hour arrived for making the observation; but this is not the fact. The sky is often obscured by clouds for many days in succession; and, in fact, it sometimes happens that the captain has scarcely an opportunity to get a good observation during the whole voyage. There is, therefore, another way by which the navigator can determine where the ship is, and how fast she gets along on her voyage. This second method consists of actually measuring the progress of the ship through the water, by an instrument called the log and line. The log--which, however, is not any log at all, but only a small piece of board, loaded at one edge so as to float upright in the water--has a long line attached to it, which line is wound upon a light windlass called a reel. The line, except a small portion of it at the beginning is marked off into lengths by small knots made in it at regular intervals. There are little rags of different forms and colors tied into these knots, so that they may easily be seen, and may also be distinguished one from the other. When the time comes for performing the operation of _heaving the log_, as they call it, the men appointed for the purpose bring the log and the reel to the stern of the ship. One man holds the log, and another man the reel. There are two handles, one at each end of the reel, by which the man who serves it can hold it up over his head, and let the line run off from it. Besides the two men who hold the log and the reel, there is a third, who has a minute glass in his hand. The minute glass is like an hour glass, only there is but just sand enough in it to run a minute. The man who has the minute glass holds it upon its side at first, so as not to set the sand to running until all is ready. [Illustration: HEAVING THE LOG.] 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,-- "Turn!" 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,-- "Stop!" 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.