164 Mr J. G. Stuart on the Turbine JFater-WTieel 



2. That in all ordinary circumstances, the Turbine will 

 make available — transmit a net useful effect, to use Morin's 

 words — from 70 to 78 per cent, of the theoretical power. 



3. That Turbines can be driven at speeds varying consider- 

 ably from that which theoretically is their best speed, with- 

 out seriously deteriorating from their practical efficiency. 

 One great practical excellence of the turbine is the high speed 

 at which it revolves. In the ordinary wheels, especially the 

 overshot, which is the best of them, we cannot have great 

 economy of power, without a very slow motion, and hence 

 much intermediate mechanism is necessary to bring the 

 motion up to the speed required for general use ; but in the 

 turbine, the greatest economy is accompanied by a rapid mo- 

 tion, and hence the connected machinery may be rendered 

 much less complex. The one which I have erected develops 

 its power best when at the speed of 48 turns per minute ; but 

 in consequence of the defect mentioned in the intake of the 

 water, I have not regularly wrought it above 42 turns. Still, 

 even at this speed, it is five times faster than my former 

 breast-wheels ; and, as my main shafting is about 200, much 

 intermediate raising of the motion is avoided. Turbines have 

 the farther advantage that the speed may be varied, as suit- 

 able, without materially affecting the economy of power. 



4. That they may be wrought under water, that is, in back- 

 water, without materially affecting their useful effect. This 

 is found to be experimentally true, and may, indeed, appear 

 from the consideration that the power is in the difference be- 

 tween the front and back columns of water, and that it is 

 comparatively immaterial whether the discharge into that 

 back column be on the surface or at the bottom. In erect- 

 ing a turbine, then, the wheel ought to be placed so as to be 

 rather under the back-water when full going ; and thus the 

 greatest possible fall is secured. 



5. That they can receive very variable quantities of water, 

 without the per-centage of useful effect being materially al- 

 tered. If a turbine be working with a force of 10-horse 

 power, and its supply of water be suddenly doubled, it be- 

 comes of 20-horse power. If the supply be reduced to one- 

 half, it still works to 5-horse power ; while such sudden and 

 extensive changes would altogether disarrange water-wheels 



