"What an expression," said Tom, imitating his sister's voice; "you girls are so common!" But the approach of the visitors made a truce a matter of necessity, and soon the project of the tree-house engrossed the entire attention. Boards were brought from the little tool-house, saws were in demand, and Gem was deputed to confiscate all the hammers and nails in the house for the use of the builders; the work went bravely on, and by noon the walls of the fortification were up, and the roof well advanced towards completion. A ladder brought from the barn, took the workmen half-way up the trunk; but the old tree was lofty, and a long space intervened between the end of the ladder and the lowest branches, which must of necessity be ascended in that squirming manner peculiar to boys, wherein they delight to bark their shins, tear their trousers, and blister their hands in the pursuit of glory. Gem, of course, could not hope to emulate the B. B.'s in this mode of progression towards the fortification, but she brought nails and carried boards with great energy. When there was no call for her services, she watched with intense interest the B. B. who happened to be squirming up. If there was no B. B. squirming up, there was sure to be one squirming down, for a principal part of the time seemed to be devoted to journeys below and aloft, besides elaborate contrivances for slinging boards and tools to the climbers' backs; indeed, to a looker-on, this seemed to be the chief interest of the fortification. At last it was done, all but the floor; Tom said it did not matter about that, as the boys could easily stand on the branches. Word was given to ascend, and, one by one, all the B. B.'s squirmed up the tree and took their places inside; nothing was to be seen but their feet, huddled together on the branches. It took ten minutes for all the band to assemble on high, but in less than two, down they squirmed again. "What is the matter?" said Gem in astonishment; she had not expected to see the B. B.'s for hours, absorbed as they would be in their leafy abode. "We're going to take up the dogs," said Tom, who came first; "we're going to sling 'em up in a basket. It will be such fun, and they'll like it first-rate." "Oh, don't, Tom!" exclaimed Gem; "Turk is too big, Grip will be sure to fall out, and it will make Pete Trone seasick."