Summer found us again on Willow Clump Island with heads full of new ideas.  Bill had come across an old copy of Ewbanks’ “Hydraulics” in the school library.  It was a book describing machines of the ancients—­principally devices for raising water.  Rather dry reading, I thought, even though it was a wet subject; but Bill seemed to find it absorbingly interesting.  I came in late one afternoon, after a glorious game of baseball, only to find Bill poring over the yellowed leaves of the “Hydraulics” as fascinated as most fellows would be over a detective story.  It exasperated me to note that he thought more of this old book than he did of our baseball team.

“Bill,” I exclaimed, “what’s got into you?  I can’t for the life of me see what is so entertaining in that prehistoric book.”  “Oh, go way.  Don’t bother me,” was the surly reply.  But I wouldn’t be put off that way.  Quickly I snatched the book from his grasp and threw it out the window.

“Now, sir,” I cried, “maybe you will kindly explain to me why you persist in studying that old volume, to the neglect of our baseball team.”

“Don’t get so excited, old chap,” he replied.  “That book is all right.  I’m studying up some new schemes for next year’s expedition to Willow Clump Island.  Why, there are lots of things in that old book that we can make.”  And he proceeded to unfold his plans, sketching out some curious designs of water wheels and pumps.

By the time school closed for the summer Bill had thoroughly digested that volume, and was ready to reconstruct many of the ancient machines.

The Water Wheel.

Our first work on reaching the island was to erect a water wheel, or “noria,” as it was called in the book, in front of the camp.  It had been a great nuisance to keep our filter barrel full.  Every few days we would have to form a bucket brigade, passing pails of water up the line until the barrel was filled.  Now Bill proposed to do away with all this bother and let the river do the work for us.

Surveying for the Water Wheel.

[Illustration:  Fig. 250.  Surveying for the Water Wheel.]

We first determined the height of the upper filter barrel above the level of the river.  This was done with our surveying instrument, which was set level with the top of the barrel.  We sighted with the instrument to a long pole that was held upright at the edge of the water.  The pole had been marked off into feet with white chalk marks, and on sighting through the sight holes we found that the hairs came in line with the eleventh chalk mark.  The top of the filter was, therefore, 11 feet above the level of the river.  Bill figured that it would be necessary to construct a wheel about 15 feet in diameter in order to raise the water to the proper height.

Towers for the Water Wheel.

[Illustration:  Fig. 251.  Frame for Large Tower.]