CATALOG OF THE MECHANICAL COLLECTIONS 15 



the edges, with the result that only a small portion of the energy 

 of the stream was used. The improvement of confining the channel 

 of the stream so that all the flow was caused to pass within the vanes 

 of the wheel was probably first made about the fourth century. 

 After this, little change was made in the form of the undersnot wheel 

 until 1824, when M. Poncelet of France introduced the wheel now 

 known by his name. The Poncelet wheel had backwardly curved 

 vanes designed to receive the water without shock or disturbance and 

 to discharge it promptly with little final velocity or residual energy 

 in the water. The best of these wheels had an efficiency of about 75 

 percent, as compared to the 30 percent efficiency of the simple 

 undershot wheel. 



When the overshot wheel, which takes the water at the top rather 

 than at the bottom and can utilize the weight of the water as well as 

 the energy of the current, was first used is not known. It is possible 

 that the Romans who brought water to their mills through aqueducts 

 may have used the overshot wheel, but it has not been identified before 

 the fourteenth century, and the undershot wheel continued in most 

 general use to the sixteenth century. The overshot wheel has since 

 been the most widely used water wheel. Its efficiency, when well con- 

 structed and properly used, is equal to that of the best turbine, and 

 it has the added advantage that it maintains its efficiency when the 

 water supply is less than the normal designed rate. It is capable of 

 an efficiency of about 90 percent. 



Between the undershot and overshot wheel in principle and effi- 

 ciency is the breast wheel, which turns inward to the fall and onto 

 the periphery of which water is laid at any height up to the height 

 of the axle of the wheel. The breast wheel uses the current of the 

 stream as in the undershot wheel and the weight of the water to a 

 lesser extent than the overshot wheel. It is able to employ the weight 

 of the water where the vertical fall is less than the diameter of the 

 wheel, as is necessary for the overshot wheel. 



Turhiries. — The hydraulic turbine differs from water wheels in that 

 guide vanes or nozzles direct the water into the rotating wheel, the 

 vanes of which change the magnitude and direction of the velocity of 

 the water, the force exerted to turn the rotor being equal to the force 

 required to change the velocity of the water. Most turbines are now 

 built with horizontal rotors upon vertical shafts, and because of this 

 the early Greek or Norse mills (mentioned above) are often called 

 the first hydraulic turbines. This early form of water wheel, how- 

 ever, was generally abandoned with the perfection of the water wheel, 

 and the development of the turbine is directly traced to the simple re- 

 action turbine proposed by Dr. Barker about 1743. This consisted 

 essentially of a wide vertical tube closed at the bottom and free to 

 turn on a bearing at its base with two straight horizontal tubes closed 



