156 



WELLS'S NATURAL PHILOSOPHY. 



■weight, cr pressure, of the water applied to their circumferences. These 

 wheels are mounted upon shafts, or axles, which are in turn connected with 

 the machinery to which motion is to be imparted. 



intoho^rmany ^^^- Tlie water-wheels at prcsciit most'geii- 

 watpr-wheers eKilly usGcl may be divided into four classes — 

 divided? ^i^Q Undershot, the Overshot, the Breast 



Wheel, and the Tourbine Wheel. 



D scribe the 

 C retraction of 

 t 1 Undersliofc 

 Wheel. 



352. The Undershot FiG. 145. 

 "Wheel consists of a wheel, 

 on the circumference of 

 which are fixed a number 



of flat boards called '■'■ flvat-boards" at equal 

 distances from each other. It is placed in 

 Buch a position that its lower floats are im- 

 mersed in a running stream, and is set in 

 motion by the impact of the water on the 

 boards as they successively dip into it. A 

 wheel of this kind will revolve in any 

 stream which fiirnishes a current of suffi- 

 cient power. Fig. 145 represents the construction of the undershot wheel. 



This form of wheel is usually placed in a "race-way," or narrow passage, in 

 such a manner as to receive the fuU force of a current issuing from the bottom 

 of a dam, and striking against the float-boards. And it is important to re- 

 member, that the moving power is the same, whether water foils downward 

 from the top of a dam to a lower level, or whether it issues from an opening 

 made directly at the lower level. This will be obvious, if it is considered 

 that the force with which water issues fi-om an openinf? made at any point in 

 the dam will be equal to that which it would acquire in falling from the sur- 

 face or level of the water in the dam down to the same point. 



The undershot wheel is a most disadvantageous method of 

 applying the power of water, not more than 25 per cent, of 

 the moving power of the water being rendered avaUable 

 by it. 



353. In the Overshot 

 "Wheel, the water is received 

 into cavities or cells, called 



"buckets," formed in the circumference of the 

 wheel, and so shaped as to retain as much of 

 the water as possible, until they arrive at the 

 lowest part of the wheel, where they empty 

 themselves. The buckets then ascend empty 

 on the other side of the wheel to be filled as ^^^ 

 before. The wheel is moved by the weight of 

 tb.o water contained in the buckets on the descending side. Fig. 146 repre- 

 sents an overshot wheel 



What propor- 

 tion of power is 

 lost by the un- 

 dershot wheel ? 



Describe the 

 construction of 

 the Overshot 

 Wheel. 



Fig. 146. 



