Now, some distance ahead, the peak of Mt. Katahdin, gloriously mantled in moonlight, rose before them. Their direct course lay over the summit of this eminence, and Mark decided that it would be better to rise to a higher strata and cross the mountain than to swing around it. Therefore Mark raised the bow of the flying machine and she darted upward on a long slant, drawing ever nearer to the shining peak of the great mountain. The night air was chill--it had been cool when they left the earth--and as they rose to the rarer ether it was evident that they would find a degree of temperature far lower than the usual summer heat. Mark kept the _Snowbird_ scaling swiftly upward, mile after mile; but the long tangent at which he had started to clear the summit of Katahdin did not prove sufficient, and by and by they found themselves within a very few yards of the rocky side of the peak. Out of a dark glen a spark of light suddenly shot--almost like a rocket in swiftness. Jack saw it first and cried: "See that! What is it? What do you make of it?" "A shootin' star, I declare!" said Andy Sudds. "Nothing of the kind," exclaimed Jack, quickly. "A star could not shoot up from the earth." "Wot's dat says somebody's a-shootin' at us?" gasped Washington White. "If dey punctuates our tire, we'll suah go down wid a big ker-smash!" The professor, however, watched the "shooting star" for some moments without speaking, and then rapidly made his way to Mark's side. "Send your 'plane up in spirals, boy!" he commanded. "Don't let that light rise over us. Be quick, now!" "What is it, Professor?" asked young Sampson, as he obeyed the scientist's injunction. "I am sure it is a light in the bow of another airship--but what manner of ship she is, or who drives her, I cannot guess," declared Professor Henderson, gravely. "Another airship!" cried Jack, who overheard him. "What do you know about that?" Mark handled the _Snowbird_ with great skill, and the powerful craft mounted much more swiftly than the distant spark of light. The spiral course the 'plane now followed carried it at times much farther from the mountain side than it had been when first the strange light was noticed. That light followed the _Snowbird_ up and up in similar spirals, and the boys were soon convinced that Professor Henderson's discovery was a fact. The lamp was in the bow of another air craft. "But why should we keep over them?" asked Jack. "There is no danger; is there?" "We do not know who they are," said the professor, shortly. "The craft came right out of a fastness in the mountain-side--a place difficult to reach, and which would not seem to attract aviators of the ordinary class." "I know what he is thinking of," cried Mark, suddenly. "I read in the paper that the Department of Justice officers are after some big smugglers and that it is believed the criminals, in going back and forth into Canada, use some kind of an aerial craft. Isn't that so, Professor Henderson?"