A head of 27 feet would give this stream a velocity of 41.7 feet a second, and, from formula (B) we find that the capacity of the wheel should be 30 square inches. It is well to remember that the square inches of wheel capacity does not refer to the size of pipe conveying water from the head to the wheel, but merely to the actual nozzle capacity provided by the wheel itself. In small installations of low head, such as above a penstock at least six times the nozzle capacity should be used, to avoid losing effective head from friction. Thus, with a nozzle of 30 square inches, the penstock or pipe should be 180 square inches, or nearly 14 inches square inside measurement. A larger penstock would be still better. CHAPTER IV THE WATER WHEEL AND HOW TO INSTALL IT Different types of water wheels--The impulse and reaction wheels--The impulse wheel adapted to high heads and small amount of water--Pipe lines--Table of resistance in pipes--Advantages and disadvantages of the impulse wheel--Other forms of impulse wheels--The reaction turbine, suited to low heads and large quantity of water--Its advantages and limitations--Developing a water-power project: the dam; the race; the flume; the penstock; and the tailrace--Water rights for the farmer. In general, there are two types of water wheels, the _impulse_ wheel and the _reaction_ wheel. Both are called turbines, although the name belongs, more properly, to the reaction wheel alone. Impulse wheels derive their power from the _momentum_ of falling water. Reaction wheels derive their power from the _momentum and pressure_ of falling water. The old-fashioned _undershot_, _overshot_, and _breast_ wheels are familiar to all as examples of impulse wheels. Water wheels of this class revolve in the air, with the energy of the water exerted on one face of their buckets. On the other hand, reaction wheels are enclosed in water-tight cases, either of metal or of wood, and the buckets are entirely surrounded by water. The old-fashioned undershot, overshot, and breast wheels were not very efficient; they wasted about 75 per cent of the power applied to them. A modern impulse wheel, on the other hand, operates at an efficiency of 80 per cent and over. The loss is mainly through friction and leakage, and cannot be eliminated altogether. The modern reaction wheel, called the _turbine_, attains an equal efficiency. Individual conditions govern the type of wheel to be selected. _The Impulse, or Tangential Water Wheel_