Lower and lower sank the submarine. There was a swirling¹ and foaming² of the water as she went down, caused by the air bubbles which the craft carried with her in her descent. Only the top of the conning³ tower was out of water now, the ocean having closed over the deck and the rounded back of the boat. Had any one been watching they would have imagined that an accident was taking place.  
In the pilot house, with its thick glass windows, Tom, his father and Captain Weston looked over the surface of the ocean, which every minute was coming nearer and nearer to them.
 
"We'll be all under in a few seconds," spoke⁴ Tom in a solemn voice, as he listened to the water hissing⁶ into the tanks.
 
"Yes, and then we can see what sort of progress we will make," added Mr. Swift. "Everything is going fine, though," he went on cheerfully. "I believe I have a good boat."
 
"There is no doubt of it in my mind," remarked Captain Weston, and Tom felt a little disappointed that the sailor did not shout out some such expression as "Shiver my timbers!" or "Keel-haul the main braces⁸, there, you lubber!" But Captain Weston was not that kind of a sailor, though his usually quiet demeanor⁹ could be quickly dropped on necessity, as Tom learned later.
 
A few minutes more and the waters closed over the top of the conning tower. The Advance was completely submerged. Through the thick glass windows of the pilot house the occupants looked out into the greenish water that swirled¹⁰ about them; but it could not enter. Then, as the boat went lower, the light from above gradually died out, and the semi-darkness gave place to gloom.
 
"Turn on the electrics and the searchlight, Tom," directed his father.
 
There was the click of a switch, and the conning tower was flooded with light. But as this had the effect of preventing the three from peering out into the water, just as one in a lighted room cannot look out into the night, Tom shut them off and switched on the great searchlight. This projected its powerful beams straight ahead and there, under the ocean, was a pathway of illumination for the treasure-seekers.
 
"Fine!" cried Captain Weston, with more enthusiasm than he had yet manifested. "That's great, if you don't mind me mentioning it. How deep are we?"
 
Tom glanced at a gage¹¹ on the side of the pilot tower.
 
"Only about sixty feet," he answered.
 
"Then don't go any deeper!" cried the captain hastily. "I know these waters around here, and that's about all the depth you've got. You'll be on the bottom in a minute."
 
"I intend to get on the bottom after a while," said Mr. Swift, "but not here. I want to try for a greater distance under water before I come to rest on the ocean's bed. But I think we are deep enough for a test. Tom, close the tank intake¹² pipes and we'll see how the Advance will progress when fully⁷ submerged."
 
The hissing stopped, and then, wishing to see how the motors and other machinery¹³ would work, the aged¹⁴ inventor and his son, accompanied by Captain Weston, descended¹⁵ from the conning tower, by means of an inner stairway, to the interior of the ship. The submarine could be steered¹⁷ and managed from below or above. She was now floating about sixty-five feet below the surface of the bay.
 
"Well, how do you like it?" asked Tom of Mr. Damon, as he saw his friend in an easy chair in the living-room or main cabin of the craft, looking out of one of the plate-glass windows on the side.
 
"Bless my spectacles, it's the most wonderful thing I ever dreamed of!" cried the queer character, as he peered at the mass of water before him. "To think that I'm away down under the surface, and yet as dry as a bone. Bless my necktie, but it's great! What are we going to do now?"
 
"Go forward," replied the young inventor.
 
"Perhaps I had better make an observation," suggested Captain Weston, taking his telescope from under his arm, where he had carried it since entering the craft, and opening it. "We may run afoul of something, if you don't mind me mentioning such a disagreeable subject." Then, as he thought of the impossibility of using his glass under water, he closed it.
 
"I shall have little use for this here, I'm afraid," he remarked with a smile. "Well, there's some consolation¹⁸. We're not likely to meet many ships in this part of the ocean. Other vessels¹⁹ are fo............