NOTE.—The above description refers to the ten turbines in wheel pit No. 1. The illustrations are those of the wheels called units 1, 2, and 3 which were installed in 1894 and 1895. Units 4 to 10, inclusive, installed in 1898-1900, are of the same type except that both the penstock and wheel case have cast-iron ribs on their sides which rest on massive castings built into the masonry of the side walls. This arrangement dispenses with the supporting girders shown in Fig. 126 and gives much greater rigidity to both penstocks and wheels. [Illustration: FIG. 128.—ENLARGED VERTICAL SECTION OF LOWER WHEEL, SHOWING GATES.] The weight of the dynamo, shaft, and turbine is balanced, when the wheels are in motion, by the upward pressure of the water in the wheel case on a piston placed above the upper wheel. The upper disc containing the guides is, for this purpose, perforated, so that the water pressure can be equalized. WATER PRESSURE ENGINES. _Water pressure engines are machines with a cylinder and piston or ram, in principle identical with the corresponding part of a steam engine_; the water is alternately admitted to and discharged from the cylinder, causing a reciprocating action of the piston or ram. It is admitted at a high pressure and after doing its work on the piston is discharged. The water in some of these machines acquires a high velocity; _the useful work is due to the difference in the pressure of admission and discharge_, whether that pressure is due to the weight of a column of water of more or less considerable height, or is artificially produced. When an incompressible fluid such as water, is used to actuate piston engines, two special difficulties arise. One is that the lost work in friction is very great, if the water attains a considerable velocity; another is that there is over-straining action on the machinery. The violent straining action due to the more or less sudden arrest of the motion of water in machinery is termed _hydraulic shock_. For these reasons the maximum velocity of flow of water in reciprocating hydraulic machines should generally not exceed 5 to 10 feet per second. Under high pressure, where there is less object in saving and it is very important to keep the dimensions of the machinery small, Mr. Anderson gives 24 feet per second as the limit of velocity. In large water-pressure engines used for pumping mines the average piston speed does not exceed 1/2 to 2 feet per second. The suitability of water for _the transmission of power_ has been fully recognized in recent years; the facility with which water under pressure is capable of being utilized, and the advantages that attend its use in motors have resulted in many practical difficulties being overcome, which were at first considered insurmountable.