The Young Volcano Explorers by Edward Stratemeyer

For four days the boys traveled around Ponce with the professor, taking in all the sights. They also took a trip on the railroad to Guayanilla and Yauco, and likewise down to several small villages along the seacoast. They were particularly interested in the American government of the island, and spent several hours at the various departments. Here the professor met two officials whom he knew, and all were made to feel thoroughly at home.

In those trips Hockley had little to say, and the other boys noticed the change in his manner.