He glanced at the depth gage and noted that they were now four hundred feet below the surface. Then he consulted some charts. "There is a depth of one thousand feet about here," he remarked. "Lower the ship, Washington. Let us see if by getting on the ocean bed we can get away from this obstruction." The colored man opened wider the sea cocks by which the tanks were filled. The increased ballast sunk the _Porpoise_ still lower, and, in a few minutes a slight jar told the navigators that they were on the bottom of the ocean. "Now we will see if we have cleared the obstruction," said the professor. He started the big screw to revolving, but the ship did not move. It shivered and trembled throughout its length but remained stationary. "Maybe dar's a debil fish what hab circumulated dis ship in de exteror portion ob his anatomy," suggested Washington, rolling his eyes until only the whites were visible. "I presume you mean that a giant squid or cuttle fish has attacked us," spoke the professor. "Yas, sir," replied Washington. "That's nonsense," went on the inventor. "However, we must make an investigation." "How are you going to do it?" asked Mark. "You can't see the end of the tube from inside the ship, and, even if we went to the surface it would still be under water." "We are going to look at it while here, under the ocean," said the inventor. "Well, maybe you're a good swimmer," put in Jack, "but I don't believe you can stay under, in this depth of water, long enough to see what the trouble is." "I think I can," answered Mr. Henderson. "How?" "I'll show you. Washington, bring out the diving suits." The colored man, his eyes growing bigger every minute, went to a locker and brought out what seemed quite a complicated bit of apparatus. "With the aid of these," said the professor, "I will be able to go out, walk along the ocean bed, and investigate the mystery. Do you boys want to come along?" "Is it safe?" asked Mark, who was inclined to be cautious. "As safe as any part of this under-sea voyage," replied the professor. "These diving suits are something I have not told you about," he went on. "They are my own invention. Besides the regular rubber suits there is an interlining of steel,--something like the ancient suits of chain mail--to withstand the great pressure of water. Then, instead of being dependent on a supply of air, pumped into the helmet from an apparatus in a boat on the surface, each person carries his own air supply with him." "How is that?" asked Jack, and Mark also asked the question. "Simply by attaching a little tank of the compressed gas to the shoulder piece of the suit," said the inventor. "There is enough air in the tank to last for nearly a day. It is admitted to the helmet as needed by means of automatic valves. In other respects the diving suit is the same as the ordinary kind, except that there is a small searchlight, fed by a storage battery, on top of the helmet."