MACHINERY.] HYDROMECHANICS 531 arises in adopting the system of partial admission. But if, as is more commonly the case, the tail-water level varies, then there is danger that the turbine will be drowned in flood time, and the essential condition of the system that the wheel passages should be empty when they come in front of the open sluices will not be satisfied. If the fall is considerable, a portion of it may be sacrificed without much harm, and the wheel placed sufficiently high above the tail water to secure it from being drowned ; but with low falls this is impossible. The difficulty has been over come by a method invented by M. L. D. Girard in 1849, and termed the hydropneumatic system^ The turbine is placed below the tail-water level in a casing supplied with air by a small air-pump. It therefore always discharges freely into an atmosphere of air, the pressure of which, however, varies with the height of the tail-water level out side the casing. Inside the casing the free water surface is maintained at an invariable level just below the dis charge orifices of the wheel. 183. General Description of an Impulse Turbine or Turbine with Free Deviation. Fig. 196 shows a general sectional elevation of a
Girard turbine, in which the flow is axial. The water, admitted
