“Mark, you and Washington, with Jack, start the engines. Turn on every bit of power you can. Fill the gas holder as full as it will hold, and use extra heavy pressure. I will see if I can not work the negative gravity apparatus to better advantage than we did before. We must escape if possible!” The boys, as was also Washington, were only too glad to have something to do to take their mind off their troubles. All three were much frightened, but Mark and Jack tried not to show it. As for Washington he was almost crying. Soon the whirr and hum of the machinery in the _Mermaid_ was heard. The craft, which was rushing in some direction, either downward, ahead or backwards within the unknown depths, shivered from the speed of the dynamos and other apparatus. Soon the boys could hear the professor starting the negative gravity engine, and then began a struggle between the forces of nature and those of mankind. Once more the adventurers anxiously watched the gages and indicators. For a while the ship seemed to be holding out against the terrible influence that was sucking her down. She appeared to hesitate. Then, as the downward force triumphed over the mechanical energy in the craft, she began to settle again, and soon was descending, if that was the direction, as fast as before. “It is of no use,” said the professor with a groan. “I must try our last resort!” He started from the engine room where Mark and Jack had gone. As he did so, he glanced at a thermometer hanging on the wall near the door. “Has any one turned on the heat?” he asked. “It’s shut off,” replied Mark, looking at the electric stove. “Then what makes it so hot?” asked the scientist. He pointed to the little silvery column in the tiny tube of the instrument. It registered close to one hundred degrees, though a few minutes before it had been but sixty. And the starting of the machinery could not account for the rise in temperature, since most of the apparatus was run by electricity and developed little heat save in the immediate proximity. The thermometer was fully ten feet away from any machine. “It’s the fiery furnace that’s doing it!” cried Washington. “We’re falling into th’ terrible pit an’ we’re goin’ t’ be roasted alive!” “It certainly is getting warmer,” observed Mark, as he took off his coat. Soon he had to shed his vest, and Jack and the professor followed his example. The others too, also found all superfluous garments a burden, and, in a little while they were going about in scanty attire. Still the heat increased, until it was almost torture to remain in the engine room. Nor was it much cooler elsewhere. In vain did the professor set a score of big electric fans to whirring. He even placed cakes of ice, from the small ice machine that was carried, in front of the revolving blades, to cool off the air. But the ice was melted almost as soon as it was taken from the apparatus.