Philadelphia: Penn Publishing Company, 1912. Hardcover first edition - One of three titles in this series published in 1912. The five boys who make up the club, along with two other friends, return to Border City and the Circle T Ranch in Wyoming, to find the town booming, and a New York financier carrying on experiments with a dirigible balloon. Illustrated with a glossy frontispiece and three inserted plates by the author. Tissue guard at frontispiece. Lists thru the 1912 titles at the rear, with 2 from 1913 shown as "in preparation." 320 pp.