572 COTTAGE, FARM, AND VILLA ARCHITECTURE. it can be drawn up, so as to make any rc<juired degree of opening, and admit more or less water to flow on the wheel. The framing of this wheel is all of cast iron ; and the floats are forty-two in number. The water first strikes upon the float, and urges it forward l)y imjjulse ; but when the floats descend into the sweep, they form, as it were, close buckets, each of which will contain a given quantity of water, and from which the water cannot escape except the wheel moves ; at least this is the intention, and the wlieel is fitted as close to the race as it can be with this view. The float-boards are inclined to frie rim, and there arc other boards, e e e, placed obliquely, which extend from each float- fooard to the rim of the wheel, and nearly fill the space between one float and the next. These are called rising boards, and the u.'io of them is to prevent tlie water flowing over the float-board into the interior of the wheel. The edges of these boards are not con- tinued so far as to join to the back of the next float, because that would make all the boards of the wheel close, and prevent the free escape of the air when the water entered into the spaces between the floats. In breast wlieels, the greatest constant height of water should be made use of for the fall ; as water acts with much more effect by its weight than by its impulse. Any greater height that the water in the mill-pool may attain in wet seasons can only be employed in the way of impulse, unless a shuttle be used, which would allow of the water being discharged at different levels : this is some- times done, and it is considered to be a great improvement. The velocity of the floats of a breast-wheel should be from three to four feet per second. 1 243. " An Overshot Water-wheel is simply a circular ring of open buckets, so disposed round the circumference of a vertical wheel, as to receive the water from a trough placed over it in such a manner as to have the buckets on one side of the wheel always loaded, while those on the other side are empty. The weight of water on the loaded side of the wheel will cause it to descend ; and by this motion the water runs out of the lower buckets, while the empty buckets of the rising side of the wheel, each in their turn, come under the spout, and are filled with water. Fig. 1103 is a vertical section of an overshot water-wheel, the framing of which is of cast iron ; aaaa are the buckets. As it is of consequence to have them so formed as to retain the water on the descending side as long as possible, much attention has been paid to their make : those in most general use are called elbow-buckets, because each partition is formed of two boards, which are put together with an angle or elbow ; and this is the kind represented in the figure. It is a great advantage to make the partitions of the bucket thin ; particularly the edges, which meet, and divide the stream of moving water flowing upon the wheel. If the edges are not sharp, they will splash the water about ; and they are, therefore, generally finished by iron plate, or, what is better, all the inclined parts of the partitions are made o' iron plate. The greater number of buckets, and the shallower they are, the more regularly the wheel will act. The mouth of the buckets should be of such a width as to allow the air to escape at the same time that the water flows in. To facilitate this, in some wheels, the buckets are made longer than the width of the stream of water acting on them, which allows the air to escape freely at the ends, and likewise afforos
