1850.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



253 



OVERSHOT WATER-WHEEL. 



Sir — A reader and admirer of your excellent Journal, I have 

 lately perused with interest a paper reported in your August 

 number, for last year, read by Mr. Fairbairn, at a meeting of the 

 '•Institution of Civil Engineers," upon the subject of "Ventilating 

 A\'ater- Wheels." To the value of this improvement in hydraulic 

 machinery I am enabled to testify, having erected, in tliis neigh- 

 bourhood, as far back as 18+3, an overshot water-wheel upon that 

 principle, although with different form of bucket (a description of 

 which I forwarded to England at the time.) 



My immediate object in troubling you with the following remarks 

 is chiefly in reference to an observation of Mr. Fairbairn's, which 

 appears to me to reciuii-e explanation; to the efi'ect, "that the 

 principle of ventilated water-wheels is not so essential to high 

 falls, being more expensive." I beg leave to submit the enclosed 

 diagram, which will testify that it is possible to avoid the objection 

 of expense in such wheels — the cost of such as this certainly not 

 exceeding tliat of the most simple of the usual forms of bucket. As 

 regards "high falls," it might be remarked (were it not admitted 



as an axiom), that such is even the most economical mode of 

 application if the power is ^'■on the level;" and, as it cannot but be 

 admitted that the other advantages of the principle are applicable 

 to the overshot wheel (the s.ame necessity existing in these, as in 

 others, "to caiTy down the water as nearly as possible to the 

 vertical centre, and there allow it to escape with facility"), its 

 application to this description of %vheel is equally important, 

 especially where economy of motive power is an object. 



The above objects are obtained to a very considerable extent by 

 the angle, and form of bucket, and mode of ventilation, as shown 

 in the diagram. An overshot wheel, 26 feet diameter, has been in 

 successful operation since 18+3, realising a great economy of power, 

 with increased effect. 



My immediate object at the time of erection was to economise 

 power — the old wheel requiring an accumulation of water during 

 eight hours of the twenty-four. After much consideration, how- 

 ever, of the various improvements which suggested themselves upon 

 Snieaton's form and make of bucket, I determined to prove, by 

 confining myself strictly in the new, to the dimensions and 

 materials of the old wheel — 1 should say, as to diameter of wheel, 

 number and width of buckets, and depth of shrouding; strengthen- 

 ing the arms, however, with right-angular flanges, to obtain 

 "stiffness." At work, the wheel perfectly answered my expectation?, 

 the stream being more than sufficient to work the wheel without 

 accumulation, and more efficiently ; proving the value of the 

 principle applied to overshot wheels — at least, with proper form of 

 bucket. 



The form used being quite different to any hitherto adopted — 

 simple and effective, and applicable also to breast wheels, the mode 

 of obtaining it may not, perhaps, be without interest. U is formed 

 by a right angle drawn from the vertical radius, at the inner cir- 

 cumference of the shrouding (at foot of wheel); continued at a 

 slightly decreased angle through the outer division of the shrouding 

 (for the purpose of assisting the retention of the water to the 

 lowest point). The soleof the bucket is formed by the next following 

 radius, intersecting the line or tangent; and the inner line, by the 

 sheathing of the wheel, as high up as the openings left for the 

 exit and admission of atmospheric air; the angle of this being 

 slightly altered from the "circle" to facilitate the passage of air, 

 by its collision with the sole of the advancing bucket. 



I adopted the above angle and form of bucket as best adapted to 

 support and retain the water to the lowest possible level; and by admitting 

 air to the buckets, my object was not only to effect a rapid discharge 

 of water at that point, but also to facilitate its admission to the 

 buckets at the head of the wheel; not only to insure economy of 

 water, by preventing loss by splashing, but to obtain the greatest 

 possible effective power, by retaining the water in the wheel. 



It might be supposed that an escape, or loss of water, would 

 take place through the openings of the upper buckets; this, how- 

 ever, at a speed of five to six feet per second of the periphery, is 

 not the case, the resistance of the air, whilst escaping through the 

 openings, being sufficient to prevent leakage until the velocity of 

 the wheel has carried round the bucket to a level sufficiently low 

 to retain the charge of water. The conical form of bucket assists 

 also the admission, as it facilitates the exit of the water ; (the 

 quantity of air admitted at that point is in proportion to the 

 velocity of the wheel.) The wheel is, in fact, perfectly water-tight, 

 as it is released from backward pressure, and exceedingly retentive. 

 By these means 1 obtained 



Economy of motive power; 

 Increase of working power; 

 Freedom and regularity of motion ; 

 "Stiffness," or inflexibility of construction; 

 Without increase of Expense; 



Depth of shrouding should be avoided; extra power obtained by 

 width (or breadth) of wheel. 



I can conscientiously advocate the adoption of the principle of 

 "Ventilation" to the "Overshot Wheel ;" which application of the 

 system does not appear to have been contemplated by Mr. 

 Fairbairn, and to have been actually considered as impracticulle 

 by gentlemen present at the meeting. 



I am, &c., 



Tnos. B<- DoD^soN, yinnngir. 

 Ponta lie Area Ironworks and Dockyard, 

 Rio de Janeiro, May Isi, 1850. 



