These two sets of buckets need to be exceedingly strong. The lower set _B_ are made of heavy flanged steel plate and are cast into their places by being placed in the sand mould, and the cast iron flows around them forming the heavy ring _C_, surrounding the outer and lower edges. This ring is a part of the diaphragm which separates the two wheels. The upper edge of the ring _C_ is beveled to form a neat joint which prevents any unnecessary loss of water. This runner is balanced and secured to a hammered iron or steel shaft _F_. It is supported usually by a step of the best specially selected hard wood thoroughly soaked in oil for months before use. The lower end of the shaft is dished out at _E_, forming a true arc of a circle—concave—while the wooden step is made spherical—convex—to fit into the end of the shaft. The step is formed in this way so that no sand can lodge between the bearing surfaces, and cut them out. The resident oil in the wood combined with the water make a most durable means of lubrication, and these steps last for many months where the water is clear. [Illustration: FIG. 122.] To get at the exact quantity of water consumed by a turbine wheel, one cannot make an accurate calculation from the openings through the wheels but the water is measured after it has passed through the wheel, as it flows away into the tail race. Any slight variation in the form of buckets or admission apertures will make an appreciable variation in the quantity of water discharged by a turbine wheel. These wheels are made either for vertical or horizontal shafts and are also made single or double. The engraving, Fig. 122, shows a _Hercules turbine within the case and gate ready to set in the penstock_. Up to the year 1876 this make of wheel tested at the flume of the Holyoke Water Works showed the highest efficiency at all stages of gate, namely 87 per cent. (page 97, Emerson’s tests). The design of case will naturally lead the reader to conclude that this wheel has, 1, an inward, 2, downward and, 3, an outward discharge which is correct. _The gate is simply a curb_ or hollow cylinder which forms a sleeve outside the case and is raised and lowered by the gearing and rack shewn in the engraving. As this sleeve rises it gradually uncovers the openings shown which admits water into the wheel.