Page:The American Cyclopædia (1879) Volume XVI.djvu/686

662 WINDMILL takes effect in the direction at right angles to the axis, and produces the revolution. But the different parts of the sail have not the same angle in respect to the line of the wind. The velocity of revolution of each wing increases from its inner to its outer end; and mathe- matical considerations show that the inclina- tion of the sail to the wind should increase as the velocity increases, the best effect being ob- tained when at different lengths along the wing the inclinations are about those here named : at | the length of the wing from the centre, 70 ; at i, 71 ; at , 72 ; at f , 74 ; at f 77f ; at the end, 83. Other authorities give the in- clinations from 60 to 80. The result is that the surfaces of the sails are not oblique planes, but curving, or rather warped outward, in going from the centre to the extremities. Mr. Srneaton found that the velocity of the ex- tremity of the sails is often to that of the wind in a ratio greater than that of 2 to 1 ; and according to Euler, when the velocity is that of 2 to 1, the efficiency of the mecha- nism is greatest. When the tower or mill, is of timber and small, it is so fixed upon a strong column or axis entering its base, which is also sufficiently elevated, that the whole tower can be turned around so as to bring the axis of the machine in a line with the wind, by means of a long lever projecting from it below. In the case of stone and all large and heavy towers, the dome only is turned, carrying the axis and sails with it into the required position, while the vertical wheel merely travels about the pinion, and the connection is not broken. The turning of the dome to the wind is effect- ed in different ways : 1, by the employment of a toothed wheel engaging in a rack on its inner side, and turned by means of a so-called end- less cord, by a man below working a winch ; 2, by a method invented by Sir W. Cubitt, con- sisting of a set of small vanes, placed in an up- right position upon a long arm projecting in the same line with the horizontal axis, and by their revolution turning a shaft and pinion, and acting upon teeth surrounding the exterior of the dome and moving it ; 3, by the much more simple and quite aa effective means of having a single large vane extending behind the axis, and with its plane vertical, so that it, and con- sequently the axis, shall always be in the di- rection of the wind. In situations in which the great height of the vertical sails would be objectionable, the horizontal windmill is some- times used. It has six or more wings, usually of plain boards, set upright the whole height of the tower, being attached to upper and lower disks or platforms, and the whole is turned by the force of the wind about a vertical axis at its middle part. If the wings are fixed in position, they are set obliquely to the direction in which the wind will strike them. Outside of the whole is then pjaced a screen or cylin- drical arrangement of boards not intended to revolve, these boards being also set obliquely and in planes lying in opposite course to those of the wings. The result is, that from what- ever direction the wind may blow against the tower, it is always admitted by the outer boards to act on the wings most freely on that half of the side it strikes, on which the wings are turn- ing away. But with an equal area of the wings, the power of the horizontal is always much less than that of the vertical windmill. Sir David Brewster concludes that the ratio is no less than that of 1 to 8 or 4. Mr. Smeaton found that the efficiency of the sails is greater as they are broader at the extremity than near the centre, up to but not beyond a greatest breadth equal to one third the length of the wing ; that if the total area of sails exceed seven eighths of the area of the circle described by the wings in their revolution, the velocity is diminished ; that the maximum of work is obtained when the velocity of the wings as loaded with the work performed is to that they would have without load as 2 to 3; and that when the work is a maximum, the velocity of the sails still varies nearly with that of the' wind. The variations in the pressure of the wind being considerable, and sometimes sudden and ex- treme, it becomes desirable to provide for regu- lating the sails accordingly ; and a large share of the more recent inventions in connection with windmills have this for their object. The old plan is attended with much trouble and delay ; in it the canvas, by menns of a rope to each wing, can be taken in or let out, or that of each wing is made in three portions con- trolled by separate ropes; in either case, the mill must bo stopped, and a man must usually ascend the wings successively for the purpose. One of the inventions in connection with the improvement of windmills was patented in 1861 by Mr. A. Giraudat of New York. In this, all necessity of a turning dome and hori- zontal axis is simply obviated, while in fact the wind wheel can be conveniently erected above the roof of any building, its axis descending through the roof to machinery within ; and this machinery can be of almost any sort re- quiring moderate or ordinary power ; for one important application of it, the running of sewing machines, a patent was obtained in July, 1862. The wind wheel is constructed simply with four or eight horizontal arms, on which solid square or oblong sails (rather tables) are carried, and by the revolution of which the vertical axis supporting them, and the machi- nery connected with it, are directly turned. The sails swing on the arms so as to be brought down perpendicular to a wind striking them on one side, and lifted toward a horizontal po- sition, so as to prove ineffective when it strikes them on the other. For regulating the velo- city, each sail can slide in from the end of its arm to near the axis ; and it is caused to slide one way or the other by the action of a heavier and of a lighter weight on parts of a sort of endless cord attached to it, and both meanwhile upheld by means of iron links at the ends of the respective arms. If the impulse of the