In contrast to such high heads, we have the low-head power plants which are employed where a large volume of water is available. The most notable installation of this type, and the largest in the world, is that at Keokuk, Iowa, where a dam has been thrown across the Mississippi River. For many years it was thought impossible to make any use of the vast volume of water that flows through this great river. But above Keokuk there used to be a rapid extending back about twelve miles. By building a dam across the river just below the rapid it was possible to obtain a working head of about thirty-two feet, and with the enormous volume of water available this provided sufficient energy to make the development worth while. In marked contrast to the installation at Big Creek, it is volume rather than velocity that is employed, and hence turbines rather than Pelton wheels are used. More water goes through a single turbine than is used in the whole of the city of New York with all its elaborate aqueduct system. Enormous turbines are used, fifteen feet in diameter, and when the installation is complete there will be thirty units, each yielding 10,000 horsepower, or a total of 300,000 horsepower. A turbine, it may be explained, differs from the ordinary water wheel in the fact that the water runs through the wheel instead of around it (Figure 32). The water may enter at the center and then flow out at the periphery, or it may enter at the periphery and then be discharged from the center of the wheel, or it may run axially through the wheel. In a Pelton wheel there is a single jet which strikes but one pair of buckets at a time, but in a turbine there are many jets distributed all around the circumference of the wheel. The water is divided into a series of jets by being forced through a stationary set of curved vanes. The blades of the rotor or revolving part of the turbine are oppositely curved. If the rotor were immovable the jets would have to change their direction in passing through the rotor, but as the rotor is free to turn, the jets react against these blades and set the wheel to revolving. The turbine may be designed to run either on a horizontal axis or on a vertical one. [Illustration: FIG. 32.--TURBINE WHEELS; INFLOW TYPE SHOWN ON THE LEFT AND OUTFLOW TYPE ON THE RIGHT] The turbines used at the Keokuk plant are of the inflow type. The rotor is mounted on a vertical shaft in a scroll-shaped concrete chamber, something like a snail shell. Water pouring into this chamber is thus given a swirling motion in the direction of rotation of the wheel. As it flows into the wheel it passes first through a ring of fixed vanes, which divide it into the jets.