Scarcely knowing whether to be amused or defiant at this attitude Mackinder chose the wiser course and refrained from further comment.
Although the boys could not be sure that the persons on the island were making efforts to hinder their escape they felt the better course was to remain beneath the surface until well out of the harbor.
Directly Jimmie called the attention of his comrades to an object in the water at no great distance ahead. The rays of the searchlight with which the submarine was provided indistinctly revealed a huge bulk slightly above the level at which they were traveling.
"What is that, a ship?" asked Harry wonderingly.
"I'll bet a cookie that's a mine!" declared Jimmie positively. "I'm going to give it a little more leeway, anyhow. It pays to be safe!"
Ned was working the periscope in an effort to locate the buoys as directed by Jimmie. Occasionally he turned the glass toward the group of buildings they had just left.
"They're shooting at us, I guess!" he stated presently. "I can see the flash of that cannon. It seems to be pointed this way!"
"Let 'em shoot, they can't see anything but the periscope and they'd have an awful time hitting that!" boasted Jimmie.
Scarcely had the lad spoken before the vessel was shaken by the force of a mighty concussion. A gigantic wave tossed the craft up and forward with a heave that threw the boys off their feet.
"What was that?" gasped Jimmie glancing at his companions with a terrified face. "Has one of the fuel tanks blown up?"
"Nothing doing!" replied Harry. "I think that cannon hit the mine we just passed. I'm glad we were no closer!"
"We'll soon be out of their range!" stated Jimmie. "Shut up your periscope and we'll go deeper. Give us a little more speed, Harry!"
In an hour Jimmie declared that they should be out of sight of the island altogether. The engines had been working at full speed ahead. Harry nursed the machinery constantly, knowing that it was new and would, therefore, require considerable care. Their urgent need for speed induced the lads to crowd the machinery to the limit, and Harry was gratified to note that every part responded properly to its task.
"Well, Mackinder," stated Ned as the tension on their nerves began to relax with the increasing distance traveled, "we're surely grateful to you for suggesting that we get away as you did!"