In a breast wheel, the water has no previous fall, and therefore does not strike the floats at the bottom of the wheels, as in the undershot wheel, with an increased velocity. The breast wheel is therefore fixed in what is called a race, formed of stone or brick work, agreeing with the curvature of the wheel, and being thus let on from its own level, acts both by its weight and momentum, or movement. This wheel is unlike the undershot wheel, being close boarded round its circumference, like an overshot or balance wheel, the undershot wheel being always open; in short, it is a sort of bucket wheel; the buckets, however, being constructed differently from those of the overshot and balance wheels. A tide-mill, as the name imports, is worked by the ebbing and flowing of the tide. Of these mills there are various kinds; first, those in which the water wheel turns one way when the tide is rising, and the other when it is falling; secondly, those in which the wheel turns the same way whether it is rising or falling; thirdly, those in which the wheel itself rises or falls as the tide flows or ebbs; and fourthly, those in which the axle of the water wheel is so fixed, that it shall neither rise nor fall; the rotary motion being still given to the wheel, whether it be partially or wholly immersed in the water. All the machines for grinding corn and seeds are mills, whatever may be their particular application. One very common form is that of an iron machine, supported on four legs, having a winch handle on one side, a fly wheel on another, a hopper at the top, and a crushing apparatus in the centre. The grain or seed is put into the hopper, the winch handle is turned, the grain becomes crushed to powder, and falls out at the bottom of the apparatus. Sometimes the mill is made chiefly of wood, but with iron wheel and crushing apparatus. One kind of wheat mill, in addition to the usual mill apparatus, has a chest which acts as a flour-dressing machine. Some mills are adapted for crushing beans rather than seeds. The crushing apparatus in mills is of two kinds, either one stone working round in contact with another, or two metallic surfaces, between which the substance is forced, but between which it cannot pass except in a fine state.