BARKER'S MILL 



471 



corresponding length of the supply pipe line in the latter scheme. In 

 fact, in all turbine schemes, it may be taken as a general principle that 

 the cost per unit length of tail-race is greater than that of head-race or 

 supply pipe. Also the necessity for a long and heavy vertical shaft 

 increases the first cost of the tur- 

 bine, renders it less accessible, 

 and involves the use of expen- 

 sive and complicated bearings 

 for dealing with the end thrust 

 thus set up. 1 



AET. 130. PKESSURE OR RE- 

 ACTION TURBINES. 



All pressure turbines of the 

 radial outward flow type may 

 be considered as modifications 

 of the old Scotch turbine Re- 

 action wheel, Barker's mill, or 

 Segner's turbine, as it is vari- 

 ously called. In this turbine 

 (Figs. 214 and 215), water is 

 admitted through a vertical 

 supply pipe, flows outwards 

 through straight or curved hori- 

 zontal arms, and escapes 

 through orifices so placed in 

 these arms as to give a series 

 of horizontal jets perpendicular 

 to the diameters containing the 

 orifices. The reaction of the 

 jets then produces rotation of 



the wheel. This is, however, only made in small p*zes and is not of 

 practical importance. 



Theory of the Barker's Mill. 



Let v = velocity of water issuing from each nozzle relatively to the 

 nozzle, in feet per second. 



Let u = velocity of nozzle relatively to the ground. 

 Then v u = absolute velocity of discharge. 



1 For a description of such bearings see Arts. 133 and 135, and Figs. 225 and 234D. 



FIG. 215. Barker's Mill. 



