The most notable among the experiments which have been made upon windwheels are those made about the middle of the 18th cen tury, upon windwheels of the European type, by John Smeaton, and those made in lga upon windwheels of the American type by Thomas 0. Perry. In these experiments the following im portant facts were, among others, established: (1) The maximum power which may be ob tained from a given windwheel varies directly as the cube of the wind velocity. (2) The sail speed of a windwheel, when developing its max imum power, varies directly as the wind ve locity. (3) The load upon a windwheel, when developing its maximum power, varies directly as the square of the wind velocity. 011 The capacity of a windwheel varies directly as the square of its diameter. (5) The number of obtained from a windmill, in winds of vary ing velocity, the load must vary directly as the square of the wind velocity. In practice the load is seldom, if ever, made to vary in this manner. The most common form of installa tion is that in which a pumping windmill is directly connected to a single acting pump oper ating under a constant head. In such cases the average load during a complete cycle of the pump is practically constant regardless of the wind velocity. Moreover, the load varies, in
general, from about zero during one stroke to a maximum at about the middle of the other stroke. This periodic variation of load is also a source of loss, especially when the windwhecl is rotating slowly, and the amount of kinetic energy stored in it is small. In the case of power windmills the load depends upon the Lharacter of the work. Some grain-grinders for use in connection with power windmills are constructed so that the grain is fed to them by centrifugal action, and in these instances the load upon the windmill is made to increase as the speed increases.
Consult Barbour, 'Wells and \Vindmills in Nebraska' (Washington 1899); Murphy, E. C., 'The Windmill: Its Efficiency and Economic Use' (in United States Geological Survey, Water Supply and Irrigation Papers, Nos. 20, 29, 41, 42. Washington 1899-1901); Powell, F. E., 'Windmills and Wind Motors: How to Build and Run Them' (New York 1910); Dyer, R. M., in Machinery (August 1907); Wolff, A. R., 'The Windmill as a Prime Mover' (New York 18S5) also manufacturers' catalogues and in works on farm equipment and farm mo tors, machinery, etc.