The draft tube _I_ is of conical shape as represented at _J. J._ to reduce the friction of discharge water which after performing its work in the wheel escapes at the bottom of the draft tube. _This tube must always project into the tail water at least two or three inches._ The gates _H.H.H._ are pivoted at the center so that they are balanced and are opened and closed with the least possible friction. These gates are operated by rods _L. L._, connecting with a rack and pinion which are manipulated by the operator as occasion requires, by an extension shaft from the coupling _K_, having a hand-wheel on top. [Illustration: FIG. 121.] Power is transmitted from the wheel shaft _F_ to the gears and pulleys connected with the coupling above; the manner of setting turbine wheels in openstocks is shown a few pages further on. From the construction of gates and guides upon turbine wheels one may readily see the absolute necessity of carefully guarding the flume against the admission of sticks and other solid materials that might wreck the wheel or jam the gates so that they could not be operated; this is best accomplished by placing a water rack in front of the head gate at the entrance to the flume. These water racks are best made of flat bars of wrought iron placed edgewise in a vertical position, or what is better, let the top incline say one foot or two (depending upon the size of flume) towards the head gates. When placed in an inclined position it is very much easier to clean the rack from drift wood and the like than when placed in a vertical position as by means of a hoe or scraper these obstructions may be hauled up over the top of the inclined rack. The racks should be made very strong and substantial to guard against being broken by ice in the winter, for should the rack give way at any inopportune time the admission of sticks and other rubbish might wreck the wheel. The “runner” which is the revolving part, as shown in Fig. 121, is composed of two separate and distinct types of wheels, and has two diameters, as shown. Each wheel or set of buckets receives its separate quantity of water from one and the same set of guides but each set acts only once and independently upon the water used, hence the water does not act twice upon the combined wheel as might be supposed, as in the compound steam engine. The upper wheel _G_ receives the water as shown by the arrows at _A_, and has _a central and downward discharge_, while the lower wheel _C_ receives the water as shown by the arrows at _B_ and has an _inward, downward and outward_ discharge as shown by the arrows at _D_.