440 



HYDRAULICS AND ITS APPLICATIONS 



The first important improvement in the machine was made about 

 1870, when the flat plates were replaced by hemispherical cups 

 fixed alternately on each side of the centre line of the wheel with 



their concave sides to the jet. This was 

 known as the Cascade wheel (Fig. 195). By 

 this means the jet was deflected backwards, 



(j p * =r ~ J T !f * r ~^ m T~ an d the theoretical possibilities of the wheel 



_ ^ V.. V \ were at once doubled (see p. 378), the maxi- 



mum theoretical efficiency becoming unity. 

 The next step was to replace these cups by a 

 series of concave buckets mounted on the centre line of the wheel, fitted 

 with knife-edged ridges to split the jet and having surfaces curved so as 

 to give the jet its backward deflection as smoothly and uniformly as 

 possible. Several types of bucket have been designed with this end in 

 view and some of the more successful are illustrated in Fig. 196 (a, b, 



and c) and in Fig. 197 

 (a and b). 



While the type of 

 bucket fitted with a lip, 

 as shown in Fig. 196 

 (b) and in Fig. 197 (a), 

 is common in practice, 

 it does not satisfy the 

 conditions necessary 

 for high efficiency so 

 well as the bucket 

 which omits that por- 

 tion of the lip in the 

 line of the jet as shown 

 in Figs. 196 (a and c) 

 and 197 (b). The lip 

 and ridge of the bucket 

 deflect the jet in two 

 planes, approximately 

 at right angles, and 

 as the paths of the 

 streams thus formed cross, a certain amount of energy is dissipated by 

 their impact. Futhermore, the lip tends to deflect the jet radially inwards 

 towards the rim of the wheel, in which case some fouling of the succeeding 

 bucket is inevitable. Relative tests of buckets (Fig. 197, a and b) fitted 



FIG. 196. 



