Frank and Fred were up early on the morning of their arrival at Aspinwall, and as soon as the gang-plank was out they hurried on shore, accompanied by the Doctor. Tropical verdure greeted their eyes as they looked inland, and the open sheds and slightly built houses told very plainly that they had reached a region where frosts were unknown. The wharf where the steamer lay was more than a thousand feet in length, and, on inquiry, they learned that it was built on a coral reef, which formed an excellent foundation. "You observe," said Dr. Bronson, "that the piles resting in the water are covered with copper, to resist the teredo, a tropical worm which is very destructive to wood. Perhaps you would like to know something about him. [Illustration: THE SHIPWORM AND HIS WORK.] "Well," the Doctor continued, "the teredo is better known as the ship-worm, a name he has obtained from his habits of attacking the timber of ships in tropical countries, and also in the warmer parts of the temperate zones. He is a long worm with a boring head; imagine an auger endowed with life, and you have a very good idea of what the teredo is. He enters the wood when young, and keeps on boring all his life; he goes in the direction of the grain of the wood, and only turns aside for hard knots or for a fellow-worm, whose presence he seems to detect by the sound of his work. The teredo attacks wood immersed in salt water, and hence his destructiveness to ships and to the piles that support docks and other marine structures. The timber is perforated and riddled so much that it crumbles to pieces in the course of time, and not a very long time either. Millions of dollars have been lost in consequence of the worm's performance, and not a few human lives. Ships lying in tropical harbors have been ruined by the teredo, and the injury has remained unknown until the vessels went to sea and were lost in the first gale that blew. "But he has not been without his uses," said the Doctor, with a smile. "It was the teredo that gave Brunel his idea of a machine for tunnelling under the Thames River, and since his time most of the machines for tunnelling in soft earth have been made on the teredo principle. The head of the worm has a series of cutting disks that eat away the wood; Brunel made a gigantic worm with windows in front, and each window was occupied by a man who removed the earth before him and thus made way for the machine to be pushed forward. The progress of Brunel's worm under the bed of the Thames was exactly like that of the teredo in a piece of wood." [Illustration: THE DONKEY'S DESCENT.]