It was hard work for the boys to keep their minds on their lessons.  Dick, especially, was very serious, and for a good reason.  Something was worrying him greatly—­ something of which Tom and Sam knew little.  What it was we shall learn later.

The boys had a quarter of an hour after classes before going to lunch, and they immediately sought out Doctor Wallington, whom they found in his private office.

“Yes, it is too bad that that rascal Crabtree escaped,” said the head of the college.  “I can well imagine that you are worried—­ since he has caused you and your friends so much trouble in the past.  Let us hope that the authorities will quickly recapture him.”

“Have you had any further word, sir?” asked Dick.

“I had word at eleven o’clock, from the newspaper office.  Up to that time he had not been located.”

“We wish to ask a favor,” went on Dick, and spoke about the proposed trip to Plankville.

“Very well, you may go, and in your biplane if you deem it safe,” said the worthy doctor.  Secretly he was quite proud of the students’ success with the Dartaway, as it had advertised Brill College not a little.

“Possibly we won’t be able to get back until to-morrow,” said Tom.  “We may be detained, or it may storm

“Take your time on the trip.  Only be careful that you have no accidents.”

“We’ll try to be careful,” answered Dick, with a grim smile.  “We don’t want a tumble if we can help it.”

“It is a grand sport,” answered the head of the college.  “Before long I expect to see aeroplanes in constant use.”

“Wouldn’t you like to go up with us some day, Doctor?” questioned Tom, slyly.

“Well—­ er—­ perhaps, Thomas.  But not just yet.  I wish—­ er—­ to see them more in general use first.”  And then the doctor bowed the students out.

The boys lost no time in preparing for the trip to Plankville.  After a somewhat hasty lunch they put on their flying suits and then went down to where the Dartaway was housed, in one of the buildings attached to the gymnasium.

“Looks to be all right,” remarked Dick, after an inspection of the flying machine, and while Sam and Tom were filling the gasoline tank and the oil distributor.

The engine was tried out for a minute, and found to be in perfect order.  As usual, as soon as the explosions of the motor were heard, a crowd commenced to gather, to see the start of the flight.

“Wish you luck!” cried Stanley.

“Say, look out that you don’t forget how to stop and sail to the North Pole!” sang out Spud Jackson.

“As if that could really occur!” murmured William Philander Tubbs, with a lofty look of, disdain.

“Sure it could happen,” returned Spud, good-naturedly.  Why, I heard of an airman who went up once and forgot how to turn his machine down, and he went around and around in a circle for sixteen hours.  And then he dropped ker-plunk right on top of a baker’s wagon and smashed twenty-six pies—­ all because his gasoline gave out.”