Overshot Wheel Water Mills

This wheel consists of a frame of open buckets, placed round the rim of a vertical wheel, to receive the water from a spout placed over it, so that the buckets on the one side shall be always loaded, while those on the opposite side are empty. The loaded side will of course descend, and the wheel in its revolution will bring the empty buckets under the spout, to be in their turn filled with water.

Here is the outline of a wheel having 40 buckets. The
ring of board contained between the concentric circles Q D S and P A R, making the ends of the buckets, is called the shrouding, and Q P the depth of shrouding. The inner circle PAR is called the sole of the wheel, and usually consists of boards nailed to strong wooden rings of compass timber of considerable scantling, firmly united with the arms or radii, The partitions, which determine the form of the buckets, consist of three different planes or boards, ? ?, ? ?, CD, which are variously named by different artists. We have heard them called the start or shoulder, the arm, and the wrest : (probably for wrist, on account of a resemblance of the whole line to the human arm:) ? is also called the elbow.

The principal thing to be attended to in the construction of this wheel is to have the buckets of such a form as will retain the water along the greatest circumference of the wheel: and as this is a thing not easily to be accomplished, numerous contrivances have been resorted to by mill-wrights to determine the best possible form.

Mr. Robert Burns, of Cartside, in Renfrewshire, has made what appeared to be a very considerable improvement in the construction of the bucket. The principle of this improvement consisted in dividing the bucket by a partition of such a height, that the inner and outer portions of the bucket on each side were nearly of equal capacity. The bucket consisted of a start A B, an arm B C, and a wrest C D, concentric with the rim, and was divided by the partition LM, concentric with the sole and rim. If these buckets be filled one-third, they will retain the whole of the water at 18°, and the half at 11°, from the bottom. These advantages however were found to be counterbalanced by disadvantages ; and Mr. Burns did never, we believe, put the construction in practice.

The velocity of an overshot-wheel is a matter of very great nicety; and authors, both speculative and practical, have arrived at very different conclusions respecting it. M . Beiidor very strangely maintains, that there is a dertain velocity related to that obtainable by the whole fall, which will procure to an overshot-wheel the greatest performance. Desaguliers, Smeaton, Lambert, De Parcieux, and others, maintain, that there is no such relation, and that the performance of an overshot-wheel will be the greater, as it moves more slowly by an increase of its load of work.

Belidor again states, that the active power of water lying in a bucket-wheel of any diameter is equal to the impulse of the same water on the floats of an undershot-wheel, when the water issues from a sluice in the bottom of the dam. The other writers whom we have named assert, that the energy of an undershot-wheel is but one-half of that of an overshot, actuated by the same quantity of water falling from the same height.

The most generally received opinion is, that the overshot-wheel does the more work, as it moves slower ; and the following is the reasoning adduced to prove it. Suppose that a wheel has 30 buckets, and that six cubic feet of water are delivered in a second on the top of the wheel, and discharged, without any loss by the way, at a certain height from the bottom of the wheel. Let this be the case, whatever is the rate of the wheel's motion, the buckets being of a sufficient capacity to hold all the water which falls into them.

Suppose this wheel employed to raise a weight of any kind, water for instance, in a chain of 30 buckets, to the same altitude and with the same velocity. Suppose, further, that when the load on the rising side of the machine is one-half of that on the wheel, the wheel makes four revolutions in a minute, or one turn in 15 seconds. During this time 90 cubic feet of water will have flowed into the 3O buckets, and each have received three cubic feet. In that case each of the rising buckets contains li feet; and 45 cubic feet are delivered into the upper cistern during one turn of the wheel, and 180 cubic feet in one minute. Continued>>>