[Illustration: FIG. 33.—Concrete Dust Chamber at the Guillermo Smelting Works, Palomares, Spain. (End elevation.)] _Foundations for Dynamos and other Electrical Machinery._—Dry concrete is a poor conductor of electricity, but when wet it becomes a fairly good conductor. Therefore, if it be necessary to insulate the electrical apparatus, the concrete should be covered with a layer of asphalt. [Illustration: FIG. 34.—Concrete Dust Chamber designed for smelter at Murray Mine, Sudbury, Ontario, Can. There are eight 9 ft. sections in the plan.] _Chimney Bases._—Fig. 36 shows the base for the 90 ft. brick-stack at Don Guillermo. The resemblance to masonry is given by nailing strips of wood on the inside of the crib. [Illustration: FIG. 35.—Concrete Dust Chamber designed for smelter at Murray Mine, Sudbury, Ontario, Can. (End elevation.)] _Retaining-Walls._—Figs. 37, 38, and 39 show three different styles of retaining-walls, according to location. These walls are shown in section only, and show the placing of the iron reenforcements. Retaining-walls are best built in panels (each panel being a day’s work), for the reason that horizontal joints in the concrete are thereby avoided. The alternate panels should be built first and the intermediate spaces filled in afterward. Should there be water behind the wall it is best to insert a few small pipes through the wall, in order to carry it off; this precaution is particularly important in places where the natural surface of the ground meets the wall, as shown in Figs. 37 and 38. If a wooden building is to be erected on the retaining-wall, it is best to bury a few 0.75 in. bolts vertically in the top of the wall, by which a wooden coping may be secured (see Figs. 37, 38, and 39), which forms a good commencement for the carpenter work. [Illustration: FIG. 36.—Concrete Base for a 90 ft. Chimney at the Guillermo Smelting Works, Palomares, Spain.]