769 



WATER-WHEELS. 



WATERING. 



770 



one of the greatest improvements effected of late years in the con- 

 struction of overshot water-wheels consisted in the introduction by 

 Mr. Fairbairn of the ventilating buckets. The diameter of the wheel, 

 of course, should be made as nearly as possible equal to the height of 

 the fall, and the velocity of the wheel kept to about the rate of 3 feet 

 per second, measured on the outer edge. A greater velocity would pro- 

 duce a centrifugal force in the water contained in the buckets, which 

 would to some extent diminish the useful effect of the wheel. The 

 best overshot-wheels, in which the water is conducted upon the wheel 

 carefully, and the tail-bay of the channel built so aa not to interfere 

 with the escape of the water, yield an effective result of 075 P H ; and 

 they are, therefore, constantly used when a sufficient fall can be 

 obtained for their establishment. In some mining districts overshot- 

 wheels of as little as 8 feet in diameter are used ; but they do not yield 

 even so good results as the best breast-wheels in close races. A fall of 

 16 foet seems, in fact, to be about the minimum fall able to produce 

 the full amount of dynamical force an overshot-wheel is able to yield. 

 As to the backshot-wheds, they do not involve any other principle than 

 the overshot-wheels ; nor, indeed, can they be considered to offer any 

 advantage over the latter, unless in some peculiar cases, wherein the 

 motion of the macliinery may require the revolution of the wheel to 

 take place in one particular direction ; and wherein the water is 

 required to be brought upon the back part of the wheel. Sometimes, 

 it may also be added, there may be an advantage gained by the use of 

 the baekshot-wheel, if the tail-bay should be exposed to be flooded, or 

 to receive backwater ; because in such a case the revolution of the 

 wheel would still be in the direction of the natural current, and the 

 backwater would not oppose the passage of the buckets so seriously as 

 would be the case with a true overshot-wheel. 



The brcatt-uheeh are those in which the water strikes the wheel on 

 the upside, at a point intermediate between the vertical and the hori- 

 zontal lines passing through its centre ; and they are made with 

 buckets of analogous forms to those of overshot-wheels, in order to 

 retain the load on the side receiving the water as long aa possible, and 

 having the same provisions for relieving the air, and for obviating the 

 effects of the centrifug.il force of the water. In small wheels the 

 water strikes the buckets at an angle of about 40 from the vertical 

 line ; in large ones the angle may be 30, although some constructors 

 make it as much as 52 for wheels of 20 or 30 feet in diameter. 

 Evidently, however, the efficiency of a breast-wheel must depend upon 

 the length of time it can retain the water on the loaded side, and the 

 higher the point of impact, the greater must be the effect ; in other 

 words, a wheel receiving the water only a little above the centre line, 

 the loaded arc becomes so small as to produce very trifling results. If 

 then the fall should not exceed 8 feet, it would be preferable to adopt 

 an undershot-wheel working in a close race, rather than a breast-wheel ; 

 because it would allow the application of a larger first-motion wheel. 

 With a good fall, and with well proportioned buckets, the useful effect 

 of the breast-wheel is often as high as 070 PH. 



In some parts of southern Europe, where skilled labour in not 

 easily obtained, and the work required to be performed is of 

 a very simple nature, a number of water-mills exist in which the 

 wheel, instead of being placed vertically, is placed horizontally, and 

 the mill-stones for corn or oil grinding are keyed at once upon the 

 vertical shaft of the mill. Some of these machines have a series of 

 blades so placed upon the shaft, and with respect to the vein of water 

 striking them, (and it is generally the case that the water is conducted 

 for this purpose in a pipe,) that the shock takes effect normally to the 

 plane of the blades. The corn-mills of the Saracens were made in 

 this manner, and the peasants of Galicia retain them to the present 

 ilay ; but the efficiency of these rude machines depends entirely upon 

 the height of the fall disposable, and the useful result obtained is 

 rarely as much as 0-15 PH; so that these mills are never seen in 

 highly civilised districts. On the banks of the Garonne, Tame, Lot, 

 &c., numerous mills, such as those represented in fiy. 6, are to be seen. 



Fig. 6. 



I in its course round the cylinder, a centrifugal force which acts against 

 the curved blades not only by its impulse, but by ita weight. There 

 are many localities where wheels of this description would be of value ; 



j but even when the wheels are made of the best form, the useful effect 

 they produce does not exceed 0'25 PH, and it is very rarely that they 

 attain more than 0'16 PH. 



The reaction-wheels are those in which the water is allowed to escape 

 through apertures upon a vertical shaft, iu such a manner as to 

 develope a force iu the reverse direction of the outflow, which is cap- 

 able of being converted into a movement of rotation. Barker's mill, 

 those designed by de la Cour, and by d'Ector, were based upon this 

 principle ; but the useful results they have produced have not been 

 such as to lead to their practical application. They are in fact little 



j better than philosophical toys. 



It may finally be stated that in the present state of the arts of con- 

 struction, the undershot-wheels with straight floats working in a close 



| race are those which are the most adapted to falls of between 4 feet, 

 and 8 feet 6 inches ; the undershot- wheels with curved floats are the 

 most advantageous when the fall does not exceed 5 feet. Bucket- 

 wheels are desirable when the fall exceeds 10 feet, and is less than 

 16 feet; and when the supply is variable, it is preferable to use the 

 breast-wheel rather than the overshot one : the breast-wheels at all 

 times suffer less from backwater than any other form of wheel. Over- 

 shot-wheels are generally used when the Ml ranges between 16 and 

 40 feet ; above the latter fall there are decided advantages in the use 

 of the turbine, which certainly has a remarkable power of adaptation 

 to varying conditions. M. Fourneyron executed one turbine with a 

 head of 9 inches, and another with a head of not less than 354 feet. 

 Mr. G. Rennie states that the coefficients of the useful effects of the 

 various descriptions of wheels are found in practice to differ slightly 

 from those quoted above ; and that they cannot with safety be 



; taken at more than, in overshot-wheels, from 0'60 to O'SO ; in 



j breast-wheels from 0-45 to 0' 50 ; and in undershot-wheels from 0'27 

 to 0-30. 



(Smeaton, Ort the Power of Milli ; Banks, Treatise on Mills; 

 Buchanan, Practical Trcatite on Mill Work; papers by Fairbairn, 

 Glynn, &c. ; d'Aubuisson, Train d' Jfydraulique ; Morin, Lccons de 

 Mtcaniyue Pratique; Belidor, Euler, Navier, Bossut, Coriolis, &c.; 

 Repertory of Arts ; Transactions of the Franklin Institution, &c., &c.) 



WATERING, in Horticulture, the process of applying water arti- 

 ficially to plants. Water in a greater or less quantity is necessary to 

 the existence of the whole vegetable kingdom : not only do the ele- 

 ments of water enter into the composition of the tissues of plants, but 

 by its agency the various saline ingredients, as well as certain gases 

 that enter into the composition of vegetable tissues, are carried into 

 the plant. Water also exerts an influence on the temperature of the 

 soil and of the plants to which it is applied. It is on these accounts 

 that the application of water to plants is an important process in horti- 

 culture, more especially in the hothouse and greenhouse. During 

 winter plants require little moisture, as the processes of life are at that 

 period very inactive, but at the same time a small quantity is required 

 in order to meet the demands of approaching activity. If plants are 

 supplied with too much water during winter, their tissues become 

 distended, and the whole plant is enfeebled. The largest supply of 

 water is required when plants are growing rapidly and at the season 

 when they are putting forth their leaves. When plants have ceased 



The wheels arc usually 3 feet ^ inches in diameter, and 10 inches deep 

 placed at the bottom of a cylinder about 7 feet 6 inches deep, and 

 3 feet 4 1 inches in diameter. A channel gradually diminishing in 

 wiilth, as shown, admits water at the side of the cylinder above the 

 wheel an'! tho water escaping from the sluice with violence acquires, 

 ABTS AND SCI. DIV. VOL. VIII. 



should be supplied with abundance of water. This is done with 

 -spinach, lettuce, and other oleraceeus plants, and by this means their 

 tissues are rendered more tender, and their peculiar secretions, which 

 are often disagreeable, are diluted. The same thing is done in the 

 cultivation of the strawberry, where the object is to render the fruit 

 as large as possible. In this case however the large size of the fruit is 

 obtained at the expense of its flavour. Even plants bearing succulent 

 fruits, as the melon, &c., may be over-watered, and the flavour of their 

 fruit destroyed. In supplying water to all plants due regard should 

 be had to temperature, as, ccetcris paribus, plants require more water in 

 proportion as the temperature is higher. 



Although the supply of water artificially to plants cultivated in 

 houses is obviously necessary, there is some doubt as to whether it is 

 required by plants growing in the open ah-, where they are exposed to 

 natural supplies. Professor Lindley, in his ' Theory of Horticulture,' 

 says, " It is indeed doubtful whether watering plants in the open air is 

 not often more productive of disadvantage than of real service to 

 plants." At the same time the practice is at present very general, and 

 there are some advantages hi it, independent of supplying plants with 

 water. It is frequently very effectual for removing insects from the 

 leaves of plants, and also for removing dust and dirt in exposed situa- 

 tions. Mildew is also prevented in annuals by abundant watering. 

 The fungi which produce or are found on mildewed peas, and those 

 which destroy the spinach and onion, may be removed by abundant 

 watering. Where the leaves of plants are watered, this should never 

 be done whilst the sun is shining upon them, as this increases the 

 evaporation. The morning and evening are the best times for watering 

 plants ; but where it is necessary to do this in the middle of the day, 

 the roots alone should be watered. After transplanting, whether of 



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