OBJECTS.] INTRODUCTORY. 43 



until finally, it acquires no appreciable horizontal 

 motion at all, and so falls vertically downwards from 

 the mouth of the tap. 



29. The Energy of Moving Water. 



If a short pipe bent at right-angles like the letter 

 |_ is fitted by one arm on to the end of the tap, 

 while the other is turned vertically upwards, and the 

 vat is full as before; when the tap is turned, the 

 water will shoot up into the air, and after rising for a 

 certain distance will stop, and then fall. In fact we 

 shall have a fountain. 



Observe the difference between the vertical jet of 

 water and the horizontal jet. If we leave the resist- 

 ance of the air out of consideration, the water in the 

 horizontal jet has no obstacle to overcome ; and it might 

 go on for ever, if its weight did not gradually cause 

 its path to become more and more bent towards the 

 earth, against which it eventually strikes. 



When the jet is vertical the case is altered. The 

 water thrown up vertically constantly tends to fall 

 down vertically, as any other heavy body would do, 

 and its momentum has to overcome the obstacle of 

 its gravity. Any given portion of the water is, in fact, 

 acted upon by two opposite tendencies, momentum 

 urging it up, and gravity pulling it down. Now if two 

 equal tendencies exactly oppose one another, the body 

 upon which they act does not move at all ; while, if 

 one is stronger than the other, the body moves in the 

 direction of the stronger. 



Thus a portion of water which has just left the 

 spout shoots up, because the velocity with which it is 

 impelled upwards is sufficient to carry it through a 



