HYDRAULICS. 



25 



water was permitted to flow too rapidly, 

 it would splash out of the buckets near 

 k instead of filling them, and would run 

 down over the surface of the wheel 

 without producing its proper effect. 

 To prevent this the water is seldom per- 

 mitted to run upon the wheel in a stream 

 of more than from half an inch to an 

 inch in thickness, and when well regu- 

 lated there is scarcely a drop of water 

 ineffectually wasted. The over- shot 

 wheel therefore acts by the gravity or 

 weight of the water contained in the 

 buckets for nearly one-third of its cir- 

 cumference, and from the experiments 

 of Mr. Smeaton, which were made with 

 great accuracy, it appears that the di- 

 mensions, quantity of water, and height 

 of fall being the same, the over-shot 



wheel will produce double the effect of 

 the under-shot. 



The Breast Wheel is by far the most 

 common ; and may be considered as a 

 mean between the two varieties before 

 mentioned. In this, the water, instead 

 of passing over the top of the wheel, or 

 entirely beneath it, is delivered about 

 half way up it, or rather below the level 

 of the axis, and the race or brickwork 

 upon which the water descends is built 

 in a circular form, having the same 

 common centre with the wheel itself, 

 so as to make it parallel to the exterior 

 edges of the flat-boards or extreme cir- 

 cumference of the wheel. This con- 

 struction is shown at fig. 20, where 

 o p q is a side-view of a wheel, formed 

 with float -boards in the same manner as 



the under- shot wheel ; but instead of the 

 water acting upon its lower part q, it is 

 introduced upon it at p, by the sluice or 

 penstock r, which, by rising or falling, 

 permits a greater or less quantity of 

 water to act on the wheel : and as the 

 float-boards are made to fit as accu- 

 rately as possible without contact, into 

 the circular hollow s t of brickwork, no 

 water can escape past the wheel with- 

 out producing its proportionate effect. 



Mr. Smeaton makes no observations 

 on the nature of Breast Wheels in his 

 valuable Papers,* except to state that 

 all wheels by which the water is pre- 

 vented from descending, unless the 

 wheel moves therewith, are to be consi- 

 dered of the nature of over-shot wheels, 

 having power in proportion to the per- 

 pendicular height from which the water 

 descends ; while all those that receive 

 the impulse or shock of the water, 

 whether in an horizontal, perpendicular 



* Experimental Inquiry concerning the natural 

 powers of the Wind and Water to turn Mills and 

 other Machines, by John Smeaton, F.R.S., 1 vol. 

 8vo. London, 1796. 



or oblique direction, are to be considered 

 as under-shots. The breast wheel is nearly 

 allied to the over- shot : for notwithstand- 

 ing it has only float-boards instead of 

 buckets, yet as the mill course is made 

 concentric to the outside of the wheel, 

 and is not only there, but at the two 

 sides, made as close as convenient, so 

 as to prevent the escape of water as 

 effectually as possible, the spaces be- 

 tween one float-board and another be- 

 come buckets for the time being and 

 retain the water, and thus the breast 

 wheel is not only impelled by the weight 

 of water, but by its impetus or momen- 

 tum also, for the water is so confined 

 as to be incapable of splashing or being 

 lost, and consequently its moving force 

 may be exerted to great advantage. 

 Notwithstanding this apparent superio- 

 rity, still the breast wheel is, in effect, 

 vastly inferior to the over-shot wheel, 

 not only on account of the smaller 

 height at which the water is supplied, 

 but from the waste with which it must 

 always be attended, even under circum- 

 stances of the most- perfect workman- 



