A horizontal windmill is a great cylindrical frame of timber, which is made to revolve about an upright centre, and its convex surface is formed of boards attached in vertical positions to the upper and lower parts of the frame. The whole is enclosed in a fixed cylinder having the same upright centre as the other; this consists of a _revolving screen_ or a number of boards, which are so disposed that in whatever direction the wind may blow, it may enter between them on one side only of a vertical plane, passing through the axis, and thus give motion to the interior cylinder. The effective power of the vertical windmill is, however, so much greater than that of the horizontal windmill, that the latter is now seldom constructed. [Illustration: Dressing Machine or Bolter. Jack.] The _dressing machine_ consists of a hollow cylinder, or frame, covered with wire cloth of different degrees of fineness, the finest being at the elevated, or upper end of the cylinder, which is inclined in the same way as the bolting mill. Within the cylinder, which may be made of pieces of wood rendered circular, and, like the ribs of an animal, placed at certain distances from each other, a reel is placed with its axle in the centre of the cylinder, which is fixed, or stationary. To the rails of this reel are attached hair brushes, which, when made to revolve, or turn round, brush against the interior cloth wire surface of the cylinder. The machine is provided with a shoe or jigger, very similar to that of the millstones, to cause a regular supply of flour or meal in the same state in which it came from the millstones through a spout from the floor above, to which it is elevated by the sack tackle, or elevator, after being ground. The meal by this means being gradually let, or fed into the cylinder, is, by the motion of the brushes of the reel, sifted or rubbed through the cloth wire with which the cylinder is covered. The finest of the flour will go through the upper end, where the finest wire cloth is placed; the next finest through the next division of the wire cloth, which is coarser, the middlings through the following division, the pollard, or sharps, through the last, and the bran, not being able to get through the wire cloth at all, on account of its coarseness, is thrown out at the end of the cylinder. The cylinder is enclosed in a large and close box, to prevent the waste of flour by its going off in dust. This box is divided into several compartments, or partitions, by means of moveable boards. Some millers have more partitions than others, and indeed they all vary the number to suit the nature or sort of flour which they are manufacturing. In a dressing machine of three divisions, the flour deposited in the first is called household, or seconds, that in the second middlings, and in the third pollard, which is not flour, but a fine description of bran. [Illustration: Revolving Screen. Jacob’s Ladder.]