HYDRAULICS. 



157 



Tho overshot wheel is one of the most effective varieties of 

 ■water-wheels, and receives its name from the circumstance 

 that the water shoots over it. It requires a fall in the stream, 

 rather higher than its own diameter. "Wheels of this kind, 

 when well constructed, utilize nearly three fourths of the mov- 

 force of the water. 



354. The Breast TVheel may be considered as a variety 

 intermediate between the overshot and the undershot wheels. 

 In this, the water, instead of falling on the wheel from above, 

 or passing entirely beneath it, is deUvered just below the level 



What propor- 

 tion of tho 

 moving power 

 is utilized by 

 tlie overshot 

 irhcel ? 



ins 



Describe the 

 construction of 

 the Breast- 

 ■»Tieel. 



Fig. 147. 



of the axis. The race-way, or passage for 

 the water to descend upon the side of tho 

 wheel, is built in a circular form, to fit the 

 circumference of the wheel, and the water 

 thus inclosed acts partially by its weig'..t, 

 and partially by its impulse, or momentum. 

 Fig. 147 represents a breast-wheel, with it3 

 circular race-way. 



The breast-wheel, when well constructed, 

 will utilize about 65 per cent, of the mov- 

 ing power of the water. It is more efBcient 

 than the undershot wheel, but less than tho 

 overshot. It is therefore only used where the fall happens to be particidarly 

 adapted for it. 



355. The fourth class of water-wheels, the " Tour- ^^^- ^'^^• 



bine," or " Turbine,"' is a wheel of modem invention, 

 and is the most powerful and economical of all water- 

 engines. 



The principles of the construction and action of the 

 Tourbine wheel may be best understood by a previous 

 examination of the construction of another water- 

 engine known as "Barker's MiU." (See Fig. 148.) 



„ ., „ This consists of an upridit lube or 



Descnbe the ,. , ^ . , , • , i, 



construction of cvhnder, furnished with a smaller 

 Barker's MUl. cross-tube at the bottom, and en- 

 larged into a funnel at the top. The whole cylinder 

 is so supported upon pivots at the top and bottom, 

 that it revolves freely about a vertical axis. It is 

 evident if there are no openings in the ends of tho 

 cross-tubes, and the whole is filled with water, that 

 the entire arrangement will be simply that of a close 



vessel filled with water, without any tendency to motion. If, however, the 

 ends of the arms, or cross-tube, have openings on the sides, opposite to one 

 another, as is represented in the figure, the sides of the tube on which the 

 openings are, will be relieved from the pressure of the column of water in the 

 Upright tube by tho water flowing out, while the pressure on the sides oppo- 



