"Give me the course, if you please, Captain," said Louis, as he took the spokes.
"North-east and a quarter north," replied Scott, who had evidently taken it from the chart before; and for the want of a parallel rule he was compelled to resort to expedients in order to find it.
"North-east and a quarter north," repeated Louis; and he watched the compass till he had the steamer on the course indicated.
Possibly some non-nautical readers would like to know how the skilful commander of the Maud had taken from the chart the course he had given out. A parallel rule is used in obtaining it. This is two rules, each an inch or less in width, with a brass piece connecting them, with pins at each end of it having play enough to permit the two wooden parts to be spread out. When the parallel rules are together, the brass bar joining them lies at a sharp angle with their length, so that one of the parts may be moved out from the other till the brass bar is at right angles with the length.
Captain Scott laid the outer edge of the right-hand rule on the course he had marked on the chart with a pencil, from Point Al Boassa to Tarifa. On the chart before him was a diagram of the compass. It was divided into four quarters by two heavy black lines. The one within two points of perpendicular had an arrow at the upper end, which pointed to the magnetic north, though on some American charts the true north is indicated. On this diagram the thirty-two points of the compass are marked; the heavy black line across the figure showed the east and the west.
Putting one or more fingers on the right-hand rule, the navigator of the Maud held it fast in the position in which he had placed it. With the left hand he moved the other rule out as far as he could, which left an opening two and a half inches wide, more or less, between the two parts of the implement. Holding the left-hand rule fast to the paper, he moved the right up to it. Then the whole rule had been moved over two inches. He repeated these movements till he had brought the edge of the ruler on the centre of the compass diagram. If this edge had rested on the north-east and south-west marks, the course would be either one or the other of these two. The navigator knows that his general course is to the northward, and he has accurately obtained the direction in which he is to sail.
But Scott found that the edge of his rule came a little to the left of the heavy mark for north-east, and the same to the right of south-west. He had to estimate that it was a quarter of a point beyond the line. The points on some compasses are divided into halves and quarters, so that the helmsman has no difficulty in keeping the point he is to steer on the notch.
Louis moved the wheel till he brought the line of north-east a quarter north on the notch, which is made in the immovable part of the compass. The pilot-house of the Maud was small, but it was large enough to contain a sofa, or divan, across the back; and here the captain seated himself to figure up the present speed of the steamer. Three and one-half miles in seventeen minutes was an easy problem to solve.
"Twelve and two-tenths knots an hour!" he shouted in his delight at the result, which he had obtained in three repetitions of the calculations; and this time he was sure there was no mistake, for it was dead low tide, and there were no allowances to be made.
"Then we are certainly all right, and we shall not fall into the clutches of the Grand Mogul," replied Louis.