But various causes arise in the actual practice of navigation to prevent a ship keeping her true course. Winds may be contrary, and currents may drive her either to the one side or the other of it; while landpromontories, islands, and shallowscompel her to deviate from the direct line. A vessel also makes what is called leeway; which means that, when the wind blows on her side, she not only advances forward, but also slides through the water sidewise. Thus, in the course of a day, she may get a considerable distance off her true coursein sea parlance, make a good deal of leeway.
To perform the voyage correctly and safely in the face of these obstacles and hindrances is the aim and end of navigation; and the manner of proceeding is as follows:
The hour is carefully noted on setting sail, and from that moment, night and day, to the end of the voyage, certain observations are made and entered in the ships journal, called the log. Every hour the rate at which the ship is going is ascertained and carefully noted. The point of the compass towards which the ship is to be steered is given by the captain or officer in command to the steersman, who stands at the wheel with a compass always before him in a box called the binnacle. The course is never changed except by distinct orders from those in command; and when it is changed, the hour when the change is made and the new course to be steered are carefully noted down. Thus, at the end of the day, or at any other time if desired, the position of the ship can be ascertained by her course being drawn upon a chart of the ocean over which she is sailing,correct charts, or maps, being provided by the captain before starting.
The estimate thus made is, however, not absolutely correct. It is called the dead-reckoning, and is only an approximation to the truth, because allowance has to be made for leeway, which can only be guessed at. Allowance has also to be made for variations in the rate of sailing in each hour, for the winds do not always blow with exactly the same force during any hour of the day. On the contrary, they may vary several times within an hour, both in force and in direction. Those variations have to be watched and allowed for; but such allowance may be erroneous in a greater or less degree. Currents, too, may have exerted an unseen influence on the ship, thus rendering the calculation still less correct. Nevertheless, dead-reckoning is often the only guide the sailor has to depend upon for days at a time, when storms and cloudy skies prevent him from ascertaining his true position by other means, of which we shall speak presently.
Of course, in the early days of navigation there were no charts of the ocean. The navigator knew not whither he was hurrying over the wild waste of waters; but by observing the relative position of some of the fixed stars to his course while sailing out to sea, he could form a rough idea of the proper course to steer in order to return to the port whence he had started.
The compass, then, shows the sailor the course he has been going, and the log (of which more presently) enables him to ascertain the rate at which he has proceeded; while his chronometers, or time-keepers, tell him the time during which the course and rate of sailing have been kept up. And many a long cruise on the unknown deep has been successfully accomplished in days of old by bold seamen, with this method of dead-reckoning; and many a mariner at the present day depends almost entirely on it, while all are, during thick, stormy weather, dependent on it for days and sometimes weeks together.
The log, to which we have referred, is the instrument by which is determined the rate at which a ship is progressing. It is a very simple contrivance: a triangular piece of wood about the size of a large saucer, with a piece of stout cord fastened to each corner, the ends of the cords being tied together, so that when held up, the log, as it is called, resembles one of a pair of scales. One of the cords, however, is only temporarily attached to its corner by means of a peg, which when violently pulled comes out. One edge of the triangle is loaded with lead. The whole machine is fastened to the log-line,a stout cord many fathoms long, which is wound on a large reel.
Heaving the log, as we have said, takes place every hour. One sailor stands by with a sand-glass which runs exactly half a minute. Another holds the wooden reel; and a third heaves the log overboard, and pays out line as fast as he can make the reel spin. The instant it is thrown the first sailor turns the sand-glass. The log, being loaded on one side, floats perpendicularly in the water, remaining stationary of course; while the man who hove it watches sundry knots on the line as they pass over the stern of the ship, each knot representing a mile of rate of speed in the hour. As the last grain of sand drops to the bottom of the glass the first sailor gives a sharp signal, and the second clutches and checks the line, examines the knot nearest his hand, and thus knows at once how many knots or miles the ship is sailing at that time. The sudden stoppage of the line jerks the peg, before referred to, out of the log, thereby allowing the other two fixed cords to drag it flat and unresisting over the surface of the sea, when the line is reeled up and put by. The flight of another hour calls for a repetition of the heaving of the log.
As scientific knowledge advanced, instruments of peculiar and more complicated form were devised to enable navigators to ascertain more correctly their position on the surface of the sea; but they did not, and never will, supersede the method by dead-reckoningfor this reason, that the latter can be practised at all times, while the former are useless unless the sun, moon, or stars be visible, which in some latitudes they are not for many days and weeks, when clouds and fogs shroud the bright sky from view.
The first thing you have to do, my young boat-builder, then, is to go and visit a harbour or beach where varieties of boats are to be found, and, having settled in your mind which of them you intend to copy, make a careful drawing, in outline, of its form in four different positions. First, a side view, as in Figure 1. Then the stern, with the swelling sides of the boat visible, as in Figure 2. The bow, as in Figure 3; and a birds-eye view, as in Figure 4. The last drawing can be made by mounting on some neighbouring eminence, such as a bank or a larger boat, or, if that is impossible, by getting upon the stern of the boat itself, and thus looking down on it. These four drawings will be of great service in enabling you to shape your model correctly; for as you proceed with the carving you can, by holding the model up in the same position with any of the drawings, ascertain whether you are progressing properly; and if you get the correct form of your boat in these four positions, you will be almost certain to make a good boat. If, on the other hand, you go to work without drawings, the probability is that your boat will be lopsided, which will prevent it from floating evenly; or crooked, which will tend to check its speed in sailing, besides being clumsy and not ship-shape, as the sailors have it.