He picked up the model, and twisted the yards again so that they slanted sharply across the hull, making a small angle with its middle line. "Now she's braced sharp up, or close hauled—every sheet's hauled in—on the wind, and we'll start her heading north-east on the port tack. That is, the wind's on the port side of her, though we could have started on the opposite one heading north-west, if we had liked. Run that line along, and you'll find it makes an angle of four points of the compass, or forty-five degrees, with the wind, which makes it evident that by and by you come to the edge of the first quarter of the circle at east. Then, if we put the ship round with the wind on her opposite side, and sail at the same angle as far again, we come back to north, where the wind is blowing from, and when you grasp that you've got the principle of the whole thing. With the wind behind you all sails flowing, when you're working up against it, everything's flattened in, but you have to remember that all vessels don't sail equally close to the wind, and while a racing cutter will lie very close indeed, a shallow full-bowed hooker must have it almost on her side to keep her going. That's why I took four points as a handy example, because two tacks of forty-five degrees would bring us back again."
"But why doesn't the wind shove her away sideways when she's close-hauled?" asked Appleby.
Lawson nodded approval. "That shows you're following, it does," he said. "Still it don't amount to very much if the vessel's deep, because all of her that's in the water offers resistance to it. They all slide off a little, and that's the leeway."
"Well," said Niven, "when the wind's so to speak almost against her, what makes her go ahead at all?"
Lawson grinned. "What makes a kite go up against the wind? You see the sails of a close-hauled ship make about the same angle to it as a kite does. They didn't teach you that at school?"
"I think they did," said Appleby. "There's something very like it in the parallelogram of forces."
"The biscuit's yours," said Lawson. "Get that into you, and you know all the whys of sailing."
He yawned and bent over his book, repeating snatches of curious ditties about green to green and red to red, and steamers crossing, but Appleby remembered what he had heard, which was fortunate, because it was the only instruction that anybody ever gave him on board the Aldebaran. Then the cook banged on something in his galley, and Niven, who got up and stretched himself, went along to bring in the tea. He came back with a big steaming can and grinned at Appleby.
"They'll be getting very different tucker at home," he said. "Still, it will be beastly cold and wet up there just now."
His merriment was evidently a trifle forced, and another lad who lay poring over a book in a corner raised his head.