Excerpt from The Improvement of Rivers, Vol. 2 of 2: A Treatise on the Methods Employed for Improving Streams for Open Navigation, and for Navigation by Means of Locks and Dams There are certain conditions to be determined with all structures for slackwater navigation, regardless of whether the dams are fixed or movable. The principal ones comprise the proper selection of locations, the determination of the level of the sills and of the minimum navigable depth, and the height to be given the lift, or vertical distance from pool to pool. Where the dam is fixed, its entire structure is brought to the full height to which it is desired to raise the water-level; when movable, the fixed part is merely a foundation on which to erect a suitable superstructure which can be raised or lowered as desired. The lock walls usually are built several feet high er than the crest of the -dam in order to enable lockages to be made when the water is considerably above the normal stage of the pool. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.