APPLIED MECHANICS. [WATER-POWKR-OVBRSHOT-WHEEM. 



be fitted with apparatus by means of which the direction 

 of movement may b* reversed; or, where the mill is 

 floated on a barge, the swinging of the barge by the tide 

 effect* the required change of position to suit the change 

 111 direction of the current. 



If it were practicable to make use of the tidal stream 

 in nek a rivi-r as the Thames, without interfering with 

 its navigation, the power derived from it would be 

 IH.US. If we suppose the breadth, 1,200 feet, occu- 

 pied by tidal mills side by side, with floats immersed 



3 feet deep, the total float-surface would be 3,600 square 

 feet in one section of the river. The velocity of current 

 we may take, on the average, as 3 miles per hour, nearly 



4 k feet per second ; and the power, according to our rule, 

 would be 



3600 x 4j x 4| x 4J 

 8800 



"S6 horse-power. 



Were such mills repeated at intervals of 220 feet along 

 a mile of the river, there would be 24 of them, and the 

 total power would be 86 X 24=2064 horse-power in a 

 iiiilo of the river's length. It is not, of course, presumed 

 that such an arrangement is feasible : it is only offered as 

 an illustration of the great mechanical power that mi<*bt 

 be derived from the natural movements of the water in 

 tidal estuaries. In some rivers, such as the Rhine and 

 the Seine, barges are moored carrying tidal mills of this 

 kind. In such streams the level does not greatly vary, 

 and the current sets continuously in one direction, so that 

 the power is applied with constancy and facility. 



2. The Ovenhat Water-wheel (Fig. 117) has its circum- 

 Ffc. 117. 



forenoe divided by partitions into numerous compart- 

 ments, or buckets, capable of containing water The 

 spout conveying the water to the wheel either passes over 

 Fig. 118. 



discharge the water into the buckets a little beyond the 

 summit of the wheel As the wheel revolves, each suc- 

 cessive bucket is brought under the spout and becomes 

 filled with water, the weight of which, acting on one side 

 of the wheel, is the moving force. The buckets, as they 

 descend, become gradually emptied, and return up tin- 

 unloaded side of the wheel, to be again filled, and 

 descend. 



Such wheels are only applicable where there is a con- 

 siderable fall of water ; for the height of the head, above 

 the stream as it flows away from the wheel, techn, 

 called the tail-water, must be equal to the diamet 

 the wheel, or nearly so. In overehot-wheels the u-Wity 

 of the water is no element of power, except in so far as 

 the quantity conveyed by the spout to the wheel depends 

 upon the velocity with which it flows. If the velocity of 

 discharge be considerable, a positive disadvantage results 

 from the too rapid dash of water into the buckets 

 causing it to overflow, while the bucket remains only 

 partially filled. It is easy to see that the quantity of 

 water issuing from the spout during the time which a 

 bucket occupies in passing under it, should barely exceed 

 that which the bucket will hold ; if it fall short of that 

 quantity, the bucket is only partially filled in its passage 

 and if it much exceed tliat quantity, the force of its 

 flow causes it to dash over and become wasted without 

 effectually filling the bucket. The diameter of the wheel 

 being limited by the height of the fall, when it is desir- 

 able to take advantage of a large quantity of water dis- 

 charged from the spout, the breadth of the wheel must 

 be increased, and the water in the spout caused to s] 

 itself out to a wide sheet, so as nearly to cover the whole 

 breadth of the wheel The sheet of water should always 

 be a few inches narrower than the face of the wheel, to 

 save the water from dashing ineffectively over the edges 

 (Fig. 119). Another point of great importance in the 

 construction of the buckets, is to leave a passage for air 

 at the inner upper angle of each bucket, a, a, a (Fig. 

 .19), otherwise the bucket can become only partially 

 filled, in consequence of the elasticity of the air confined 

 in it compelling the supply-water to dash over the edges 

 instead of filling the bucket. 



In constructing an overshot-wheel it is necessary to 

 give consideration to the following points : 



1 . The point of the circumference at which the spout 

 should discharge so as best to fill the buckets. 



2. The best form of bucket for receiving the water 

 and for retaining it, through a considerable part of its 

 descent. 



3. The best speed at which the circumference of the 

 wheel should travel so as to obtain the greatest effect 

 from the moving load of water which its buckets contain. 



If we suppose that a fall of water about twenty-two 

 feet in height is to act upon an overshot-wheel (Fig 120) 

 Fig. 119. 



-it, or ha. . check at it. end (Fig. 118), , to | we may make the wheel about twenty-four feet in dia 



