46 SCIENCE PRIMERS. [MATERIAL 



above the tap. Just as the energy of the horizontal 

 stream diminished as the level of the water became 

 lower, so d jes the energy of the vertical stream 

 diminish. Hence, as the vat empties, the jet be- 

 comes shorter and shorter, until at last it sinks down 

 to nothing. 



The energy of moving water makes it, under some 

 circumstances, one of the most destructive of natural 

 agents ; and, under others, one of the most useful 

 servants of man. A stream is water falling down hill 

 with a velocity depending upon the inclination of its bed. 

 As it falls it acquires momentum and, hence, energy; 

 and thus a mountain stream, suddenly swollen by rain 

 or melting snow, will tear away masses of rock and 

 sweep everything before it'. Nothing can look softer or 

 more harmless than a calm sea, but if the wind sweep- 

 ing over its surface puts the water in motion, it strikes 

 upon the shore with terrific force ; and its energy is ex- 

 pended in throwing up great waves, which lift vast 

 blocks or drive masses of shingle up the beach. 



In all kinds of watermills it is the energy of more 

 or less rapidly falling water which is turned to account. 

 The water is made to flow against buckets or floats 

 attached to the circumference of a wheel. Each bucket 

 or float is therefore an obstacle to which the water 

 transfers some of its own motion ; it moves away and 

 thus makes the wheel to which it is fastened turn. 

 But the turning of the wheel brings a new obstacle in 

 the way of the stream. This is treated in the same 

 fashion, and the wheel turns still further, thus intro- 

 ducing another obstacle in the way of the stream upon 

 which the same effect is produced. Thus each float, 

 or bucket, is a means by which some of the momentum 



