The professor’s going was not unaccompanied by difficulties.  Miss Melissa had insisted that if he was to accompany the expedition, she was going along, too.  This being manifestly impossible, the man of science was driven to the subterfuge of placing a bag of fossils in his bed to represent him.  On the night of the start, Miss Melissa looked into his room every few minutes to make sure he had not escaped.

It was not till morning that she discovered that the man of science had effected his escape through his bedroom window, climbing down a latticework to the ground.  At first she was half inclined to pursue him, but thought the better of it when she read the note the professor had left behind.

“Well,” said Miss Melissa to her little maid, “there’s one good thing—­he won’t be cluttering up the house with old stones and rocks for some time to come.”

“What shall I do with them fossils what he put in his bed to make believe it was him, miss?” asked the maid.

“You may throw them into the creek at the back of the house, Mary,” said Miss Melissa, and went placidly about her dusting and sweeping and “setting to rights.”

But of all this, the professor, on the train speeding westward, was blissfully unconscious.  Perhaps even if he had known it, he would not have cared much, for even his scientific mind was warmed and thrilled by the prospect of the aerial search for the mineral treasures of Rattlesnake Island.

CHAPTER XXIII.

ON THE BORDER LINE.

The long train of gray-coated coaches, filmed with the arid dust of the desert, rolled into Yuma, the little town at the junction of the Gila and Colorado River, popularly supposed to be the hottest place in America.  The boys, glad that their long journey had come to an end, felt that it was living up to its reputation as they alighted and stood in the blistering heat while their personal baggage was thrown off.

The professor, however, was quite oblivious to the scorching rays of the afternoon sun.  He darted about seeking specimens, and he had soon gathered up quite a collection of small rocks.  In the meantime Zeb Cummings, who was quite in his element, had helped the boys get their things together and see them loaded on a mule wagon which rattled them off to a small hotel, for they did not want to make themselves any more conspicuous than was necessary.

The boys wore gray flannel shirts, khaki trousers, stout high boots and broad-brimmed hats, and had fastened red handkerchiefs round their throats to keep off the sun from the back of their necks.  Zeb had a similar outfit.

The professor, however, still wore his baggy black garments, his only concession to the heat being a big green umbrella, which looked like a gigantic verdant mushroom.  As they drove off in a rickety sort of bus, having with difficulty persuaded the professor to leave off specimen hunting for a while, the boys did not notice that from the opposite side of the train three young men had alighted who, from a point of vantage behind a water tower, watched their movements.