7. The load at a maximum that sails of a similar figure and position will overcome at a given distance from the centre of motion will be as the cube of the radius.
8. The effects of sails of similar figure and position are as the square of the ra dius. 9. The velocity of the extremities of Dutch sails, as well as of the enlarged sails in all their usual positions, when unloaded, or even loaded to a maximum, is considerably quicker than the velocity of the wind.
Horizontal windmill&—Windmills are sometimes constructed in such a manner that the planes of the sails intersect each other in the wind-shaft, in which case they are called horizontal windmills; be cause the wind-shaft being usually verti cal, the sails have a horizontal motion. The wind-shaft, however, might be placed with equal advantage in the hori zontal position.
In order that motion may be commu nicated to the machine, the impulse of the wind on the returning sail must be removed by screening it from the wind, or at least diminished by making it pre sent a less surface when returning against the wind. The first of these methods is
said to be practised in Tartary, and some provinces in Spain ; it is the simplest and probably the best. The other method requires the sail to be formed of several lisps movable on hinges, and so adjusted that on one side of the axis they present their surfaces to the wind, and when re turning on the other only their edges. Other contrivances have been proposed ; but horizontal windmills are greatly in ferior, in point of effect, to those which have vertical sails, and are accordingly seldom met with.
On account of the irregularity of the moving force, and the interruption of calm weather, machines impelled by the wind can only be used advantagew•sly for purposes which are not urgent, and where regularity is not indispensable. The chief purposes to which they are ap plied are grinding corn, expressing oil from seeds, bruising oak bark for tan ning, sawing wood, raising water, &c. Windmillswere brought into Europe from the East about the time of the Crusades ; they are not much used in this country.