HYDRAULICS. 



main empty. The consequence of this 

 will be that the wheel will revolve in 

 the direction of the letters p o n, and if 

 the buckets q q are filled as they get 

 to the top, and those at o n emptied by 

 some contrivance when they reach the 

 bottom, the motion of such a wheel 

 will be continuous ; and it will revolve 

 with a force and velocity dependent on 

 the weight of the buckets of water, 

 their distance from the centre, and the 

 velocity with which they are filled : in 

 fact, such an arrangement is a very 

 close approximation to what is called 

 the over-shot water-wheel, which will 

 be presently described. 



Water-wheels have three denomina- 

 tions, depending on their own particu- 

 lar construction, on the manner in which 

 they are set or used, and on the manner 

 in which the water is made to act upon 

 them ; but all water-wheels consist in 

 common of a hollow cylinder or drum 

 revolving on a central 'axle or spindle 

 from which the power to be used is com- 

 municated, while their exterior surface 

 is covered with vanes, float-boards, or 

 cavities, upon which the water is to act. 

 Thusy?^. 17 is a side view of an under- 

 shot, tide, or stream wheel, which was 



fig. 17. 



the most common and is by far the 

 oldest construction in use. As this kind 

 of wheel requires no other fall in the 

 water than that which is necessary to 

 produce a rapid progressive motion in 

 it, and as it acts chiefly by the momen- 

 tum of the water, its positive weight 

 being scarcely called at all into action, 

 it is only fit to be used when there is 

 a profusion of water always in motion. 

 It has however the advantage of being 

 the cheapest of all water-wheels ; and as 

 it does not require a very considerable 

 fall of water, it is more applicable to 

 rivers in their natural state than any 

 other form. It likewise works equally 

 -well whether the water acts upon the 



one or the other side of its float-boards, 

 which renders it particularly applicable 

 to tide-rivers, where the current is some- 

 times running in one direction, and at 

 others in an opposite course. There are 

 however some practical disadvantages 

 attendant upon this form of wheel, parti- 

 cularly when made of small diameter : 

 for if the float -boards stand radiantly 

 round it, or pointing to the centre, as a 

 b c d e, although the central floats bed 

 stand in good positions to produce the 

 greatest effect, being all nearly at right 

 angles to the direction in which the 

 water moves, yet the float a enters the 

 water so obliquely as to meet with great 

 resistance to its passage, at the same time 

 that the retiring float e leaves the water 

 under circumstances that are equally 

 disadvantageous, in being obliged to 

 plough or throw up a portion of water 

 before it, which tends very materially to 

 retard the motion and impede the power. 

 This appears to be partly obviated by 

 giving the float-boards a different figure, 

 or placing them so as not to point to 

 the centre of the wheel, as shown atyf^. 

 18. In this case, the ascending float- 

 board/is nearly at right angles to the 

 water at the time of leaving it, and rises 

 almost perpendicularly out of it, being 

 thus placed in a much more beneficial 

 position than in the last figure. But, 

 although the retiring float is thus im- 

 proved, the entering one g is much more 

 disadvantageous^ placed, for now it 

 will come down almost parallel to the 



fig. 18. 



surface of the water, and thus the ad- 

 vantage that is gained at one side of the 

 wheel is lost at the other. It does how- 

 ever appear that there is a small practi- 

 cal advantage in giving the float-boards 

 a slight inclination from the centre, but 

 it must not be earned by any means so 

 far as shown in the above figure. 



As action and re -action are always 

 equal, but in contrary directions, of 



