MILLS, KILNS, MALT-HOUSES, ETC. 571 feet wide, two rings will suniiort the float-boards ; but the rings should never be more than 5 feet asunder, or tue float-boards may bend. Eaeh ring is tbnncd with its separate set of arms, so that every one derives its strength from the axis. 'When a wheel is of very great breadth, much additional strength may be gained by bracing it obliquely. The float-boards, d d, are twenty-four in number : they are formed of wood, and are nailed to pieces of wood called starts, which are fixed into mortises in the rings, and project outwards for that purpose. The velocity of the float-boards should be about equal to half the velocity of the stream ; not more than one half of the float should ever be below the water, and from three to five should be immersed at once, according to the size of the wheel. When the stream is very rapid, the float-boards should be inclined towards the rim, so that the water may heap upon them, and act by weight as well as impulse. When the velocity of the stream is II feet per second or upwards, the inclination should not be less than 30 degrees ; but as the velocity lessens so should the inclination diminish. When the velocity is only 4 feet per second or less, the floats should not be inclined at all, but shoidd point to the centre of the wheel. To prevent backwater, the floats should be made to rise from the water as pei-pendicularly as possible. The circular sweep of masonry, e, is to prevent the escape of the water ; the float-boards approach it as nearly as possible without touching. Beyond this sweep should be a step, or fall, /, of not much less than 9 inches, having a slope of about 45 degrees ; in order that the tail-water may run off quickly, and not retard the motion of the wheel : beyond this step, the bottom of the tail-water channel should be paved for about 50 feet, having a declivity of an inch in every 6 feet : beyond this the bottom should slope about 4 inches the first 200 yards, 3 inches the second 200 yards, and decreasing gradually to the usual fall of the river. In places liable to floods, this fall must be increased so as to prevent the water running back upon the wheel. The tail-water course must be wider than the wheel. The slope g must be paved. The pen-stock is marked h. The diameter of the wheel should be the largest of which cir- cumstances win admit ; and, as it is of great importance that none of the water sLoidd escape without contributing to turn the wheel, either below the float-boards or at tlje sides, the breadth of the float-boards should be greater than that of the sheet of water that strikes them. 1 242. " Breast Jflteels are very commonly called also undershot n-heels, because the water runs beneath the wheel ; but they differ essentially, since the principal power is derived fi-ora the weight of the water, and not from its iinpulse. A breast wheel partakes of the nature of both over and under shot n-heels, and is constructed as repre- sented in fig. 1102. The lower part of the wheel is surrounded by a sweep of masonry, which is made concentric with it ; the float-boards are exactly adapted to this masonry, so as to pass as near as possible to it without touching, and the side walls are, in the like manner, adapted to the ends of the float-boards ; the intention being, that as little water as possible shall be able to pass by the float-boards, without causing them to move before it. The water is poured upon the wheel over the top of the breasting at a; the efflus from the mill-dam, b, being regulated by the sluice or shuttle, c, which is placed in the direction of a tangent to the wheel, and is provided with a rack and pinion, (/; by which
