BARKER'S MILL (Fr. roue a reaction, Ger. Segner'slcasserrad), a water-wheel invented by Dr. Barker towards the end of the 17th century. It is represented in its simplest or typical form in cut. A is a wide metal pipe, resting at its lower end by the steel spindle T, ou a metal block B, and kept in a vertical position by the spindle S. at its upper end, which passes through the frame of the machine, so that it can easily revolve round its axis. Near its lower end, two smaller pipes or arms, C, C. are inserted, which project horizontally from it, and these have each, at the outer extremity, a hole cut vertically in them, opening towards opposite sides. The water is supplied by the pipe P, which opens over a funnel-like widening on the upper part of A, and the quantity is so regulated that while the pipe A is kept nearly full, no more is admitted than issues from the lower orifices. The reaction caused by the water gushing from the arms, forces them back wards, and gives to the whole machine a rotatory motion. This reaction is much the same as is seen iu the recoil of a gun when fired, or in the pushing back of a small boat by the foot on stepping ashore.
It may be also thus explained: Suppose that the arms were closed all round, the water would press against the sides with a force proportional to the height of the water in the pipe A, and the pressure against any particular surface of the side would produce no motion of the arm, because an equal pressure is exerted in a contrary direction by a corresponding surface opposite to it. Now, if one of these surfaces be cut out, the pressure against the other being uncounteracted, forces the arm in the opposite direction to that of the side in which the whole is made. This being done to both arms on opposite sides, two equal pressures are produced, which conspire in generating the same motion of rotation. As soon as motion ensues, centrifugal force comes into play, which, throwing the water out towards the ends of the arms, increases the rapidity of its dis charge, and also its reacting power. When the wheel is in action, the water thus acts under the influence of two forces—one being the pressure of the column in A, and the other the centrifugal force generated by the rotation of the wheel itself. The motion of the wheel is transmitted by the spur-wheel fixed to the spindle S, to the machinery which is to be driven by it, or, in the case of a the spindle passes directly through the lower millstone, and is firmly fixed into the upper one.
The power is manifestly increased by heightening the water-column, or by lengthening the arms—the former increasing the pressure of the water, and the latter increasing the leverage at which this pressure acts. In the mill shown in the figure, the column in A cannot be advantageously heightened, for the higher it rises, the greater must be the weight which the conical spindle, T, has to sustain, and the greater, consequently, becomes the friction. It is from this circumstance that such mills are found, in practice, to yield but a small mechanical effect—the friction consuming too large a proportion of the work of the wheel. Hence, in the reaction wheels now in use, the original B. M. has been so modified as to allow of the water being conducted from the reservoir below the arms instead of above. This is effected by making the vertical pipe revolve below in a stuffing-box at its junction with the conduit, and above, by a pivot moving in the fixed frame. By this arrangement, the friction attending the rotation is reduced to a minimum, for not only is the weight of the water placed out of account, but also a large proportion of the weight of the wheel itself, which is borne by the upward pressure of the water. The mechanical performance of such wheels is said to be highly satisfactory, producing, with a limited supply of water falling from a considerable height, a useful effect, hardly to be obtained by any other contrivance. The power of these machines may be also increased by using curved instead of straight arms. With straight arms a considerable loss of force is incurred by the sudden change of the direction of the current when it leaves the arm, which does not take place to the same extent with curved arms, where this change is effected gradually. In Whitelaw's mill (hence called the Scottish turbine), the form of B. 3f. generally met with in Scotland, there are three instead of two curved arms of this description. Considerable difference of opinion still exists as to the merits of B. M., some considering it as the most perfect way of applying water-power, and others putting it in the same rank as an undershot wheel, with the same water-supply. Of late years, it has been more extensively employed than formerly, both in this country and on the continent. See 'WATER-POWER.