"It opens like an umbrella," explained Deirdre. "We'll hoist it upright barely out of the water, and fasten on the weights. The canvas bag fits on that iron pipe. When you let it go, it sinks like an umbrella that's tightly closed, but when it touches bottom the weights spread it out and an explosive charge automatically goes off in that iron tube. It's special explosive. The gas it makes inflates the canvas bag, which can't burn underwater, and that floats the whole thing back up with the ribs of the umbrella stretched out and spreading the net between them. It should catch anything it encounters as it rises. As the pressure lowers, the excess gas can escape through a relief-valve. This dredge is experimental. If it works, it can be modified to do lots of things." "Such as poking at things we don't believe in," said Terry drily. "That explosion ought to stir up anything in its neighborhood. It'll be much more disturbing and audible than a few light taps against the _Esperance's_ hull!" Deirdre grinned ruefully and did not answer. The bulky tow slowed the yacht. She did not reach the position of the fish-filled circle until after nightfall, and it was necessary to have plenty of light by which to locate the inflated bag when it came to the surface, so nothing could be tried until the following morning. A short while before daybreak, lights appeared at the horizon. Red and green sidelights, and white central lights. It was a steamer. It came closer and closer. Presently, it turned and headed upwind and went dead slow, barely keeping steerage. It was the _Pelorus_. Dawn arrived in a golden radiance which thrust aside the night. The _Pelorus_ shone brightly in the first rays of the sun. A large object was hoisted out of her hold. Its shape was that of a gravid goldfish, with a smaller sphere hanging beneath it. It went overside, slowly, and there it floated, rolling wildly on the waves. For a very long time nothing seemed to happen. Then the water-level of the float sank a little. It was being filled with gasoline, which is lighter than water and practically incompressible. On the _Esperance_, the tow had been pulled alongside and the yacht's powerful winch hauled it upright. The yacht heeled over from the weight. The crew-cuts fastened the canvas sack in place, and Davis loaded the explosive charge into the iron tube. The crew-cuts cleared the nets. This preliminary operation seemed promising, and it was quite likely that the dredge would operate as it was designed to do. The _Pelorus_ whistled impatiently. Nick abandoned his job and went below to the short-wave set. He returned shortly after. "The _Pelorus_ says she'll be ready to send the bathyscaphe down for a test dive in two hours," he reported. "She says she will object if our gadget is floating free at the time, on the chance that it might interfere with the bathyscaphe. She asks if you can send our dredge down right away and get it over with."