wheel is kept face to the wind by a rudder in small mills; in large mills a subsidiary fan and gear are used. Fig. 2 shows a large mill of this kind, erected in a similar manner to a tower mill. The tower is a framework of iron, and carries a revolving cap, on which the wind shaft is fixed. Behind is the subsidiary fan with its gearing acting on a toothed wheel fi.xed to the cap. It is important that a wind-mill should control itself so that it works efficiently in moderately strong winds and at the same time runs in very light winds, which are much more prevalent. It should also, by reefing or otherwise, secure safety in storms. Table I. gives the mean velocity of the wind in miles per hour for an inland station, Kew, and a very exposed station, Scilly, for each m.onth during the period 1890-1899. The pressure of the wind on a plane normal to its direction, composed partly of an excess front pressure and negative back pressure, is given by the relation p = 0-003 t*, where p is in pounds Fig. 2 .-Warner's Annular f" '^."f'5-/°°' t'^'lZ CM !S';„^.^;m the velocity of the SailWmdm.ll. wind in miles per hour. It varies a little with the form and size of the surface, but for the present purpose this variation may be disregarded. (See experiments by Dr Stanton at the National Physical Laboratory, Proc. Jnst. Civ. Eng. 156, p. 78 .) ^ For velocities of 5, 10 and 20 m. per hour the pressures on a plane normal to the wind would be about 0-075, 0'3 and 1-2 lb per sq. ft. respectively, and these may be taken to be ordinary working velocities for windmills. In storms the pressures are much greater, and must be reckoned with in considering the stability of the mill. A favourable wind velocity for windmills is 15 m. per hour. Table I. Kew Scilly .