Page:The Boy Travellers in South America.djvu/32

 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 Brunei 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 ; Brunei 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 Brunei's worm under the bed of the Thames was exactly like that of the teredo in a piece of wood,"

The Doctor delivered his improvised lecture amid the rattle of boxes that were sliding down the sloping gangway from the side of the steamer, as the process of unloading began almost immediately on her arrival. The lecture was suddenly terminated by the inattention of the audience, the antics of a donkey in a portable stall having caught their eyes. The animal did not relish the rapid descent along the gangway, as his progress easily averaged a mile a minute, and the momentum acquired in the slide carried him far out upon the wharf. He reared and plunged as he was going downwards, and in his struggles one of the upper slats of his cage was torn off. But at this point he became discreet, and carried his protests no further than to lift up his voice in its loudest tones.

Threading their way through the mass of bales and boxes that covered the wharf, our friends were soon on solid earth at the end of the coral reef already mentioned. Here the tropical forest was visible in all its luxuriance, and not very far away, as the city does not cover a large area, and the trees grow luxuriantly wherever they are not kept down by the hand of man. Dr. Bronson explained to the youths that Aspinwall is built upon the island of Manzanillo, which is about three miles long by a