They were now a hundred miles under the surface. But still the professor kept the _Mermaid_ sinking. Every now and again he would take an observation, but only found the impenetrable darkness surrounded them. “We must arrive somewhere, soon,” he muttered. It was about six o’clock that night that the alarm bell set up a sudden ringing. The professor who was making some calculations on a piece of paper jumped to his feet, and so did a number of the others. “We are nearing the bottom!” he cried. “The bell has given us warning!” CHAPTER XV IN THE STRANGE DRAUGHT The boys ran to attend to the engines and apparatus to which they had been assigned in view of this emergency. The professor, Washington, Bill, Tom and Andy, who had kept to themselves since the descent, came running out of the small cabin where they usually sat, and wanted to know what it was all about. “We may hit something, in spite of all precautions,” Mr. Henderson remarked. “Slow down the ship.” The _Mermaid_ was, accordingly checked in her downward flight, by a liberal use of the gas and the negative gravity machine. The bell continued to ring, and the dials pointed to the mark that indicated the ship was more than one hundred and fifty miles down. Mark, who had run to the engine room to check the descent, came back. “Why didn’t you slow her down?” asked the professor. “I did,” replied the boy. “The negative gravity and the gas machines are working at full speed.” “Then why are we still descending?” asked the scientist. “For a while our speed was checked, but now we are falling faster than before.” “I attended to the apparatus,” Mark insisted. Just then, from without the ship, came a terrible roaring sound, as though there was a great cyclone in progress. At the same time, those aboard the craft could feel themselves being pulled downward with terrific force. “We are caught in a draught!” Mr. Henderson cried. “We are being sucked down into the depths of the earth!” He ran to the engine room. With the help of the boys he set in motion an auxiliary gravity machine, designed to exert a most powerful influence against the downward pull of the earth. As they watched the great wheels spin around, and heard the hum and whirr of the dynamos, the boys watched the pointer which indicated how low they were getting. And, as they watched, they saw that the needle of the dial kept moving, moving, moving. “Our efforts are useless! We can’t stop!” the professor cried. Grave indeed was the plight of the adventurers. In their ship they were being sucked down into unknown regions and all their efforts did not avail to save them. It was an emergency they could not guard against, and which could not have been foreseen. “What are to do?” asked Mark. “We can only wait,” Mr. Henderson replied. “The terrible suction may cease, or it may carry us to some place of safety. Let us hope for the best.”