Suddenly Mark, who was looking through a telescope at the expanse of water stretched out under them, gave a cry.

“There’s a ship!  She’s on fire!”

“Where?” asked the professor, stretching out his hand for the glass.

“Just to the port of the forward tube.  See the smoke!” exclaimed Mark.

Mr. Henderson looked.  Through the lens he saw a column of black vapor rising skyward.  Mingled with it were red flames.

“Lower the Mermaid!” he cried.  “We must save those on board if we can!”

Mark ran to the conning tower, where Washington was, to give the order.  The colored man, who was looking ahead, intent on guiding the ship, did not at first hear what Mark called.

“Lower us!  Send the Mermaid down!” Mark cried again.

The sudden shout, and the excited voice of Mark so startled Washington, that, fearing some accident had happened, he pulled the lever, controlling the gas supply, with more force than necessary.

There was a loud explosion, followed by a crackling sound, a flash of light, and the Mermaid came to a sudden stop.

“What’s the matter?” cried Mark, feeling that something was wrong.

“I don’t know!” Washington replied, as he dashed toward the engine room.

The Mermaid, her forward flight checked, hung in the air, suspended, neither rising or falling.

“Why don’t we go on down?” the professor asked, hurrying to the tower.

“There has been an explosion—­ an accident!” exclaimed Mark.  “I guess we can’t go down!”

“But we must!” Mr. Henderson insisted, seizing the lever which should have produced a downward motion.  The handle swung to and fro.  It was disconnected from the apparatus it operated.

The ship was now stationary in the air, moving neither forward nor backward, neither rising nor falling.  Washington had stopped the air pumps as soon as he learned something was wrong.

When Mr. Henderson saw the useless lever, which had controlled the outlet of gas from the holder, he ran out on deck.  One glance told him what had happened.  One of the electric wires had become short-circuited,—­ that is, the insulation had worn off and allowed the current to escape.  This had produced a spark, which had exploded the gas which was in the pipe leading from the generator up into the aluminum holder.  Fortunately there was an automatic cut-off for the supply of vapor, or the whole tank would have gone up.

As it was, only a small quantity had blown up, but this was enough to break the machinery at the point where the lever in the conning tower joined the pipe.  If it had not been for the automatic cut-off all the gas in the holder would have poured out in a great volume, and the ship would have fallen like a shot.

“Can we do nothing to save those on the burning vessel?” asked Mark, pointing to where a cloud of smoke hung over the ocean.