Section of Holland Mill for raising water. The mill simply turns a wheel, similar to the paddle-wheel of a steamboat, and immersed three or four feet in water, which it pushes forward, not raising it more than one or two feet, but displacing immense masses in a very short time. If water has to be raised, say sixteen feet, eight or ten such mills are placed at suitable distances along a long narrow channel, each feeding the next and each doing its share in lifting the water. These are often immense structures, a hundred feet in height, and built of brick, on foundations prepared with especial precaution, by reason of the constant vibration to which they are subjected. The diameter at the bottom is about forty feet, and the walls are three or four feet thick, while at the top the diameter is some twenty feet, and the walls are thinner in proportion. The vertical shaft for the transmission of power extends through the whole height of the tower, and is placed exactly in the centre. The axis of revolution for the sails is supported by a dome, which revolves like the dome of an observatory, and the connection with the vertical shaft is made by means of bevelgearing, so that the revolution of the dome does not disturb nor in any way affect the gearing for the transmission of power. The figure shows the balcony, with the lever arrangement attached above to the cap, and provided below with a windlass. The sails have been intentionally omitted in this engraving. Indeed, they always are furled in high winds, the surface of the beams being such that they may be driven without sails if the wind is strong enough. The figure also makes it evident how the cap and upper axis can be turned round without interfering with the operation of the mill, as the main shaft, transmitting the motion, stands in the middle of the structure. The free space in such a mill is occupied by lodgings for the attendants and their families. The working corps is much like that of a sailing-vessel, consisting of commander or foreman, first and second mates or assistants, carpenter or millwright, &c. The capacity of such a mill may exceed a hundred horse-power. There are several principles and rules of experience to be observed in the construction of all large windmills. One concerns the inclination of the axis of revolution of the sails, which should not be perfectly horizontal, but inclined 100 or 150 above the horizon, observation having shown that the impulse of the wind is commonly exerted in a line descending at this angle. Another principle of construction concerns the angle of the sails. The velocity of the wind being nearly uniform over the whole sail, while the rapidity of movement is much greater at the extremities of the latter, it is evident that the inclination of the sail should increase as the velocity of revolution increases. The best results are obtained when the inclinations to the line of the wind are nearly as follows: at the length of the wing from the centre, 70°; at 1, 710; at 1, 72°; at 3,740; at 5, 7710; at the end, 83°. Some authorities prescribe a scale of variations from 600 to 80°. According to Euler's mathematical calculations, the efficiency of the machine is greatest when the velocity of the extremity of the sail is twice that of the wind. The following table presents approximately the effective total pressure of the wind per square foot upon the sails, at different velocities. From these figures the proper deductions must be made for the angle at which the wind meets the sail, and for the friction of the machinery, in order to obtain the amount of power available.* * See New American Cyclopædia, Vol. XVI, p. 468, Art. Windmills. These figures show how great a power may be obtained by suitable machinery from the motion already provided by nature in localities where other sources of power seem to be lacking. The necessities of many such regions in this country should lead to the study on the part of engineers of the neglected, and perhaps despised, "old-fashioned Dutch mills." |