356 SECTION MECHANICS. 



After all, water must be looked upon as a convenient form of descend- 

 ing weight. When the fall is not great it is always practicable by 

 means of water-wheels having buckets which retain the water to 

 employ, as I have said, its mere gravity as a motor, and probably it 

 is by this mode that the highest result is procured from any given 

 quantity of water falling through a given height. By the use of a back- 

 shot wheel as much as seventy-five per cent, of the total power is 

 rendered available. The twenty-five per cent, of loss arises from the 

 friction of the axle of the wheel and of the gearing transmitting the 

 force to the machine which is to utilize it ; from some of the water 

 being discharged out of the buckets before the bottom of the fall is 

 reached ; from the necessary clearance between the wheel and the 

 tail-water ; from the eddies produced in the water as it enters the 

 buckets ; and (to a small extent) from the resistance of the air. 



When the difference of level between the source of water and its 

 delivery exceeds, however, forty or fifty feet, the water-wheel becomes 

 very unwieldy and expensive and revolves so slowly that it ceases to be 

 a desirable prime-mover. Then recourse can be had to water-pressure 

 engines, engines wherein pistons move in cylinders, and, being pressed 

 alternately in opposite directions by the head of water, set up rotary 

 motion in the machine, in the same way as if the pistons were acted 

 upon by steam. In the construction of such water-engines great care 

 must be taken to have ample inlets and outlets, in order that the loss 

 incurred either by the power requisite to drive the water through 

 restricted orifices, or by surface resistance caused by a too speedy flow 

 along the various passages, may be a minimum. Care has to be taken 

 also in the arrangements of the valves that the engines, when employed 

 for rotary movement, may be able to turn their centres without pro- 

 ducing an injurious pressure upon the water within the cylinders. 

 Water engines employed for pumping, but without rotary movement, 

 are mentioned by Belidor, in his " Architecture Hydraulique," pub- 

 lished in 1739, Article 1156. In England Sir William Armstrong has 

 brought these machines to great perfection. The first of his make, 

 erected many years ago, is still working most successfully at the Allan 

 Head Lead Mines. This machine is driven by a natural head of water 

 and not from an accumulator, and is employed in the mine as a 

 winding engine. 



