240 Introduction to the Study of Science 



pressure in the lower sections of the pipes is 420 pounds per 

 square inch. The Big Creek power plant, California, utilizes 

 in each of the two stations a head of nearly 2000 feet. The 

 water, with a velocity of 350 feet a second, is forced through 

 nozzles six inches in diameter, and as it issues from the nozzles 

 resembles a solid rod of polished steel. The wheels, ninety-four 

 inches in diameter, arranged in pairs, revolve as fast as the 

 driving wheels of an express locomotive traveling one hundred 

 miles an hour. So accurately, however, are the buckets of the 

 wheel adjusted to the angle at which the water strikes them, 

 that there is no shock of impact. So nicely adjusted also is the 

 speed of the wheel to the velocity of the water issuing from the 

 nozzles, that it brings the water practically to a standstill as it 

 leaves the buckets. Thus the water is made to give up almost 

 all its energy. 



This point may be made clear in this manner. Suppose that 

 the buckets on the periphery of the wheel are moving in the 

 same direction and with the same speed as the water ; they 

 can therefore take from the water none of its velocity and con- 

 sequently none of its energy. If the buckets, on the contrary, 

 were kept stationary, they would throw the water back with 

 about the same velocity which it has when it strikes them. If, 

 however, the buckets move at a speed about one half that of 

 the velocity of the water, they take from the water and utilize 

 practically all its energy. For example, the velocity of a jet of 

 water is one hundred feet per second, and the speed of the 

 buckets of the wheel is fifty feet per second. The water moves 

 forward fifty feet a second faster than the bucket which it 

 enters. As it leaves the bucket, it must move backward with a 

 speed of fifty feet per second with reference to the bucket. 

 Since the bucket is moving forward at the rate of fifty feet per 

 second, the water is left practically at a standstill with refer- 

 ence to a stationary object. 



Engineers compute accurately the velocity of a jet of water 

 when they know the head or the vertical distance to the intake. 



