The sea became less and less smooth. There were little slapping sounds of waves against the yacht's bow. The muted rumble of her engine was not intrusive. The breeze increased. Davis gave a definite impression of having said all he intended to say for the time being. Terry stirred. "You want me to build a gadget," he said. "To drive fish. Would you want to give me some details?" Davis considered. A few drops of spray came over the _Esperance's_ side. "N-o-o-o," said Davis. "Not just yet. There's a possibility it will fit in. I'd like you to make one, and maybe it will fit in somewhere. But _La Rubia's_ the best angle we've got so far. There is one gadget I'd give a lot to have! You know, a depth-finder. It sends a pulse of sound down to the bottom and times the echo coming back. Very much like radar, in a way. Both send out a pulse and time its return." Terry nodded. There was no mystery about depth-finders or radars. "We've got a depth-finder on board," said Davis. "If I sail a straight course and keep the depth-finder running, I can make a profile of the sea bottom under me. If I had a row of ships doing the same thing, we could get profiles and have a relief map of the bottom." "That's right," agreed Terry. "What I'd give a lot for," said Davis, "would be a depth-finder that would send spot-pulses, like radar does. Aimed sound-pulses. And an arrangement made so it could scan the ocean bottom like radar scans the sky. One boat could make a graph of the bottom in depths and heights, mapping even hummocks and hills underwater. Could something like that be done?" "Probably," Terry told him. "It might take a good deal of doing, though." "I wish you'd think about it," said Davis. "I know a place where I'd like to use such a thing. It's in the Luzon Deep. I really would like to have a detailed picture of the bottom at a certain spot there!" Terry said nothing. He'd been made angry, then mollified, and now he felt tempted to grow angry again. There was nothing definite in what was wanted of him, after elaborate machinations to get him aboard the _Esperance_. He was disappointed. "Good breeze," said Davis in a different voice. "We might as well hoist sail and cut off the engine. Take the wheel?" Terry took the wheel. Davis went forward. Four dungareed figures came up out of the forecastle. The sails went up and filled. The engine stopped. The motion of the boat changed. More spray came aboard, but the movement was steadier. Davis came back and took the wheel once more. "I think," he said, "that we're acting in a way to--hm--be annoying. I ought to lay my cards on the table. But I can't. For one thing, I haven't drawn a full hand yet. For another, there are some things you'll have to find out for yourself, in a situation like this." "Such as--"