power, nothing need be said. They are extensively used on the Pacific coast in raising water for irrigation; and they might with advantage be employed generally by farmers and others for numerous domestic purposes, such as churning, turning grindstones, &c. I think their application would be more extensive in this country but for the numerous patented and more or less complicated modifications, which tend both to increase their cost and to limit their power.

There are two great classes of windmills, distinguished by the position of the sails. The vertical mills have their wings or sails so placed as to turn in a nearly vertical plane, about an axis nearly horizontal; and the horizontal mills have their wings turning about a vertical axis. The latter are far less effective, giving for the same surface of sails and strength of wind, according to Sir David Brewster, not more than onethird or one-fourth as much power as the former. They are seldom constructed, except in situations where the necessary height cannot be given to vertical sails. I have seen some of them along the Pacific railroad, employed for raising water into tanks for locomotive supply. As might be expected, they do not give satisfaction. The secret of their comparative inefficiency is the small surface exposed to the impulse of the wind at any one moment, and the interference of the wind passing through and striking the backs of the opposite sails. The mechanical contrivances intended to obviate this difficulty by a continual adjustment of the sails are liable to derangement. At all events, the vertical mills are more suitable where great power is desired, and this is the purpose to which I desire to call particular attention.

The vertical windmills may be divided again into two classes, in the first of which the whole mill building is revolved, so that the sails may

face the wind from any quarter, while in the second only a dome revolves, carrying the sails and their axis, while the building with its machinery remains stationary. The former are the post or Bock mills, to which allusion has already been made. Their construction may be understood from the accompanying illustration, which, with the two following, has been kindly lent me by Messrs. Western & Co., publishers of the Manufacturer and Builder, a pictorial monthly magazine of New York. A strong, conical tower of heavy timbers, from ten to twenty feet high, rests upon five piers of masonry. At the apex of this framework is a pivot or column of iron, fitting into a corresponding socket or box in the floor of the mill. Upon this column the mill rests and revolves. The building is entered by means

Old-style Post Mill of Holland. of a staircase extending nearly to the ground, on the side opposite the sails, and so strongly built and braced as to furnish a convenient point for attaching the ropes or chains which anchor the mill in any desired position. A dozen posts are set firmly

in the ground about the tower, and a capstan wheel is attached to the bottom of the staircase, with two ropes or chains around its axis. These are made fast to the nearest posts, one on each side of the staircase, and the mill is thus held securely as long as desired. The revolution is effected by pushing a stout lever, which projects downward from the mill.

It should be remarked that in Holland all contrivances for turning the mills by vanes or otherwise, through the action of the wind itself, or for regulating the position of the sails by mechanical contrivances, operated by the greater or less force of the wind, have been abandoned. They are only applicable on a small scale, as a little consideration will show. The force or change in force of the wind which would suffice to operate such devices in small, toy windmills, such as are frequently used in this country, would not suffice to turn a heavy building or adjust large sails, such as are required for the development of many horsepowers.

This is especially the case with the other class of vertical mills, in vented in Holland during the 16th century, and hence known in Europe as the Holland windmills. The post-mills are convenient enough for grinding grain, since it is a matter of indifference where the grindstones are situated, or where the entrance of the mill is found; and the revolution of the whole mill on its vertical pivot is therefore not objectionable. But in driving a wheel to dis

place water, it is impossible to carry the channel around when the mill turns, and in sawing lumber it is highly inconvenient to have the floor on which the saws are working, the discharg ing doors, &c., continually changing position. Moreover, the size of sails and amount of power are limited by the weight of the structure which can be supported and revolved upon the iron pivot below. These considerations gave rise to the larger and very different mills just mentioned.

The annexed illustration represents one of these mills of moderate size, used for sawing lumber. Here the revolution of the dome is effected by levers projecting from the sides, and worked from the balcony at the top of the stationary portion. The sails are also adjusted from this balcony, their motion being stopped for the purpose by a brake, which is indicated in the figure, with a rope reaching down to the balcony.

Still larger mills are employed for raising water. The sectional view on the next page represents the system employed in Holland to drain the lakes.

H. Ex. Doc. 207-46

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 4, 710; at 1, 72°; at 740; at 4, 7730; 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.

A low wind..
A low wind..

A moderate wind.

A moderate wind.

A moderate wind.

A fresh wind...

Best wind for wind-sails.

Best wind for wind-sails..

A good breeze for sailing vessels.
A good breeze for sailing vessels.
A stiff breeze.

A stiff breeze..

State of wind.

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."