WATEB-POWER ITTRBINES. ] 



APPLIES MECHANICS. 



835 



for it is the property of fluids to communicate pressure 

 equally in all directions.* Under these circumstances, 

 there is no tendency to produce motion in any direction ; 

 but if the holes A A be opened, then, while the pressure 

 on one side of the tube B remains the same as before, 

 that on the other is lessened by as much as its surface is 

 diminished. If we suppose each hole to have an area of 

 one square inch, then each side of the tube B sustains a 

 pressure on one square inch more than the other side ; 

 in other words, there is a pressure on B exceeding that 

 on A by that due to the area of the hole in A. This 

 excess of pressure causes motion in the direction in 

 which it acts that is, opposite to the flow of the water 

 issuing from the holes ; and the force of the movement 

 depends upon the amount of unbalanced area in each 

 arm, and the intensity of pressure upon it. 



In the simple Barker's mill there is considerable loss 

 of power from impediments to the flow of the water. 

 The water descending the tube with considerable speed, 

 is suddenly arrested at O, and spread out laterally ; 

 losing by this angular bend a considerable part of its 

 velocity. For the same reason it again loses speed in 

 issuing from the holes A A ; and, farther, a considerable 

 part of the power is expended in giving the water a cir- 

 cular motion as it passes along the arms. A certain 

 weight of water, as for instance 2 Ibs., having descended 

 the tube to O, has merely vertical motion ; half of it 

 (1 Ib.) has suddenly to be turned at right angles along 

 each horizontal branch, and is immediately put into cir- 

 cular motion with the arm in its revolution, as well as 

 direct motion along the arm. The farther it flows along 

 the tube, the more rapid is its circular motion ; for if 

 we take any points 1J, D 2 along the tube, and trace 

 circles through them, we observe that the circumferences 

 of these circles increase as their radii ; and as each of 

 the circumferences is passed over in the same period, 

 the time of a revolution, the circular velocity of the 

 water at each point increase* in like proportion. In 

 order to obviate these defects, the form of the tube and 

 its arms is modified. Thus, the arms turn from the 

 vertical to the horizontal direction by a gentle curvature, 

 as seen in the section (Fig. 130), gradually changing the 

 vertical movement of the water into a horizontal move- 

 Fig. ISO. 



witli little Ions of force. A?id again, the arms, 



Bee nntt t Hydrostatics, p. 751. 



of 



which there may be any convenient number, bend also 

 horizontally, as seen on the plan ; so that while they 

 revolve, the water contained in them is really moving 

 almost in a straight line, instead of being swept round 

 in a circle. 



As the mouths of the arms must be made of such an 

 area as to permit the issue of the water at the proper 

 velocity due to its fall, if they are made too small, less 

 water passes them than can be supplied, and the machine 

 is not so powerful as it might be with the given supply 

 of water. If, on the other hand, the mouths are inado 

 too large, the velocity of the issuing water is diminished ; 

 and the pressure on the opposite sides of the arms tend- 

 ing to drive them round, is diminished with it. The 

 area of the mouths being decided according to the quan- 

 tity of water, and its velocity from vertical fall, the arms 

 are made to taper gradually to that area, so that the 

 velocity of the water may gradually increase to suit the 

 gradually diminished area of its channel, as it would 

 naturally do during its vertical descent. Due considera- 

 tion having been given to these points, as well as to the 

 best mechanical arrangements for strength, durability, 

 and economy of execution, the machine becomes a tur- 

 bine, practically applicable in many cases with great ad- 

 vantage. M. Fourneyron in France, and Messrs. White- 

 law and Stirrat in Scotland, have executed many of 

 these machines, and made interesting experiments on 

 their power and the best modes of constructing them. 

 Their simplicity and efliciency, and the small space 

 they occupy, give them an advantage over water-wheels ; 

 and it is said that they are capable of deriving from a 

 fall of water, quite as much effective power as wheels of 

 the best construction, even if the volume of water be 

 large. -Experiments conducted by Morin in France, lead 

 to the conclusion that turbines are actually more effective 

 than wheels under similar circumstances, the useful effect 

 averaging from 70 to 78 per cent, of the power of the 

 water. It has been found that even the immersion of 

 the arms to a depth of several feet in water does not 

 materially affect their action ; so that even greater height 

 than that of the fall, measured to the level of the tuil- 

 water, can be taken advantage of. 



In estimating the power that may be derived from a 

 given fall by means of a turbine, Jths of the power 

 required to raise the water up again, may be reckoned as 

 the usual effect. Thus if the volume of water be 1C cubic 

 feet per second, and the fall 15 feet, the power required to 



16 X 60 X 62 X 15 , 

 raise it would be - .... ... ... - =27 horse-power; 



and the power of the turbine to drive machinery may be 

 taken at Jths of 27, about 20 horse-power. 



It is probable that turbines will in many cases take the 

 place of water-wheels. They are as yet comparatively 

 novel, and not widely known, or looked on with preju- 

 dice ; but as improvements are gradually made in their 

 construction and adaptation, and as they become more 

 common in their application, these prejudices will doubt- 

 less give way, and no longer interfere with the extended 

 use of a simple and elegant apparatus, instead of the 

 large and cumbrous wheels now generally ^sed. For 

 regulating the motion of turbines, arrangements may be 

 made similar to those used for governing water-wheels ; 

 the quantity of water supplied to the turbine being 

 regulated according to the speed required, and the work 

 to which it is applied. The power of the turbine is found 

 to be very nearly proportional to the quantity of water 

 passing through it ; so that having found its maximum 

 power, or the greatest quantity of water that it will use, 

 we can employ or Jtha of that power by reducing the 

 supply of water to i or Jths of the maximum. 



CONTRIVANCES FOR RAISING WATER. Be- 

 fore quitting the subject of water-power, we may notice 

 some contrivances by which a volume of water is made to 

 raise a smaller volume to a greater height for purposes 

 of irrigation or the like. The simplest of these is the 

 Persian wheel. A breast or undershot-wheel of the 

 ordinary kind, has a number of buckets hung on pivots 

 to its circumference (Fig. 131). These buckets dip into 



