2S 



HYDRAULICS, 



ship. When well constructed and close- 

 ly built in, its effect, according to Mr. 

 Smeaton, should be the same as an 

 under-shot wheel, whose head of water 

 is equal to the difference of level be- 

 tween the surface of the stream and the 

 point where it strikes the wheel, added 

 to the effect of an over-shot wheel, 

 whose height is equal to the distance 

 from the striking point, to the tail-wa- 

 ter of the mill, or that which runs to 

 waste. This is however on the pre- 

 sumption that the wheel receives the 

 impulse of the water at right angles to 

 its radii, and that every thing is con- 

 structed to the best advantage. In prac- 

 tice it is found that the breast wheel 

 just consumes about double the quan- 

 tity of water that the over-shot wheel 

 requires, to do the same quantity of 

 work, when all things are alike, that 

 is to say, the diameter and breadth of 

 the wheel, number of float- boards, &c., 

 though from theory and calculation it 

 should do rather more ; for Lambert and 

 others who have written on this subject 

 attempt to demonstrate, that the power 

 of the over- shot to that of the breast 

 wheel is as thirteen to five ; but this is 

 upon a supposition that no water escapes 

 ineffectually which is utterly impossible 

 in practice. 



In order to permit any of the above 

 wheels to work with freedom, and to the 

 greatest advantage, it is absolutely ne- 

 cessary that the tail- water, as it is called, 

 or that which is discharged from the 

 bottom of the wheel after it has pro- 

 duced its effect, should have an unin- 

 terrupted passage "to run away, for 

 whenever this is not the case, it accu- 

 mulates, and forms a resistance to the 

 float-boards, and consequently abstracts 

 considerably from the velocity and power 

 of the wheel, sometimes indeed to so 

 great an extent as to prevent its work- 

 ing altogether. One of the simplest 

 and most effectual means of removing 

 this inconvenience is by an expedient 

 not much known or practised, and which 

 consists of forming two drains or tun- 

 nels through the brickwork or masonry 

 at each side of the water-wheel, what- 

 ever may be its construction, so as to 

 permit a portion of the upper water to 

 flow down into the tail or lower stream 

 immediately in front of the wheel. The 

 water thus brought down with great im- 

 petuosity drives the tail-water before it, 

 in such a manner as to form a basin or 

 hollow place, in which the wheel can 

 work free from interruption, even if the 



natural state of the water were such as 

 might produce a tailing of from twelve 

 to eighteen inches without this assist- 

 ance. And since the tailing of mill- 

 streams only occurs in the winter sea- 

 sons, or at times when there is a profu- 

 sion of water, so the quantity that is 

 thus thrown away without operating 

 upon the wheel can be spared without 

 inconvenience. Each of the drains or 

 tunnels is furnished with a sluice-gate 

 or penstock at its upper end, by which 

 the quantity and impetus of the water 

 can be regulated at pleasure, or the 

 whole be shut off whenever water hap- 

 pens to be scarce. 



The three varieties of water-wheels 

 already noticed are the only ones gene- 

 rally admitted into practice, and they 

 do not admit of much improvement, 

 since their principles must always re- 

 main the same. The over-shot wheel 

 has, perhaps, been brought nearer to 

 perfection than any of the others, by the 

 contrivance of Peter Nouaille, Esq., 

 who, in a mill that he has near Seven 

 Oaks, in Kent, has caused the water to 

 revert back again from the top of the 

 wheel, instead of passing over it, and 

 in this way a much greater portion of 

 the circumference of the wheel is brought 

 into action than is generally the case. 

 Other improvements or variations in the 

 form and construction of water-wheels, 

 have been contrived by Mr. Besant, Mr. 

 Smart, Mr. Perkins, and others, which 

 will be found described in the Transac- 

 tions of the Society for the Encourage- 

 ment of Arts, Manufactures, and Com- 

 merce ; the object of them principally 

 being to obtain as much force as pos- 

 sible from the water, by arranging the 

 forms of the buckets or float-boards in 

 such manner that they may receive the 

 impulse of, or retain the greatest quan- 

 tity of water, which is of great import- 

 ance, particularly in the construction of 

 under- shot wheels, which act by the 

 impulse of the water alone. The over- 

 shot wheel depends entirely on the 

 weight of the water delivered into its 

 buckets, which ought therefore to be as 

 capacious as they can conveniently be 

 made, not only that they may contain 

 as much water as possible, but allow 

 ample room for the discharge of the air 

 that will be thrown into them with the 

 water, as well as for the delivery of that 

 water when done with. From the nature 

 of a water-wheel it will be evident, that 

 if it had no work to perform or resist- 

 ance to overcome, it would move with. 



