288 THE REACTION WHEEL; BARKER'S MILL. 



periments to reach 0.68. It is better to take it at 0.6 in 

 estimating the power of the wheel, so as to allow some 

 margin. 



158. The Reaction Wheel; Barker's Mill.- 



Fig. 85 shows a simple reaction wheel. ACB is a tube? 

 capable of revolving about its axis, which 

 is vertical, and having a horizontal tube 

 DBE connected with it. Water is sup- 

 plied at C, which descends through the C^f 

 vertical tube, and issues through the ori- 

 fices D and E at the extremities of the 

 horizontal tube, so placed that the direc- 

 tion of motion of the water is tangential 

 to the circle described by the orifices. ~Jr ig 85 

 The efflux is in opposite directions from 

 the two orifices ; as the M r ater flows through BD, the press- 

 ures on the sides balance each other except at D, where 

 there is an uncompen sated pressure on the side opposite the 

 orifice ; the effect of this pressure or reaction is to cause 

 motion in a direction opposite to that of the jet. The same 

 effect is produced by the water issuing at E, and a continued 

 rotation of the machine is thus produced by the reaction of 

 the jet in each arm. 



Let h be the available fall, measured from the level of the 

 water in the vertical pipe to the centres of the orifices, v 

 the velocity of discharge through the jets, and V the veloc- 

 ity of the orifices in their circular path. When the machine 

 is at rest, the water issues from the orifices with the velocity 

 \/%gh (neglecting friction). But when the machine ro- 

 tates, we have for the velocity of discharge through the 

 orifices, from (1) of Art. 89, 



v = V V* + 2gh. (I) 



While the water passes through the orifices with the ve- 

 locity v, the orifices themselves are moving in the opposite 



