Earth materials do not unite or combine with timber or masonry, but there are no such distinct lines of transition and separation between different earth materials themselves as Fig. 16 would seem to indicate. Puddle Trench. In the last three dams mentioned (Figs. 14, 15, 16) the puddle trenches are made with vertical sides or vertical steps and offsets. A wedge-shaped trench certainly has many advantages over this form. Puddle being plastic, consolidates as the dam settles, filling the lowest parts by sliding on its bed. It thus has a tendency to break away from the portion supported by the step, and a further tendency to leave the vertical side, thus forming cracks and fissures for water to enter. The argument advanced by those holding a different view, namely, that it is difficult to dress the sides of a trench to a steep batter and to timber it substantially, has in reality little weight when put to practical test. Mr. F. P. Stearns, in describing the recent work of excavating the cut-off trench of the North Dike of the Wachusett reservoir, Boston, said it was found to be both better and cheaper to excavate a trench with slopes than with vertical sides protected by sheeting. He favored this shape in case of pile-work and for the purpose also of wedging materials together. Mr. Wm. J. McAlpine’s “Specifications for Earth Dams,” representing the best practice of 25 years ago, which are frequently cited, contain the following description of how to prepare the up-stream floor of the dam: Remove the pervious and decaying matter by breaking up the natural soil and by stepping up the sides of the ravine; also by several toothed trenches across the bottom and up the sides. One of Mr. McAlpine’s well known axioms was, “water abhors an angle.” The “stepping” and “toothed” trenches above specified need not necessarily be made with vertical planes, but should be made by means of inclined and horizontal planes. The writer’s experience and observation leads him to think that all excavations in connection with earth dams requiring a refill should be made wedge-shaped so that the pressure of the superincumbent materials in settling will wedge the material tighter and tighter together and fill every cavity. A paper by Mr. Wm. L. Strange, C. E., on “Reservoirs with high Earthen Dams in Western India,” published in the Proceedings of the Institution of Civil Engineers, Vol. 132, (1898), is one of the best contributions to the literature on this subject, known to the writer. Mr. Strange states that