42 SCIENCE PRIMERS. [MATERIAL 



length strikes the ground ; and, in fact, the stream may 

 be regarded as so much water thrown horizontally. 



These reasons are two : firstly, as soon as the water 

 has left the tap it is an unsupported heavy body ; and, 

 as such, it begins to fall to the ground. Secondly, the 

 momentum of the water is continually being dimi- 

 nished by the resistance of the air through which it 

 passes. For, although the air which surrounds us is 

 so thin and movable a body that we ordinarily take 

 no notice of it the fact that it offers resistance to 

 bodies which move through it is easily observed ; 

 as, for example, in using a fan. The water has to 

 overcome this resistance, and its momentum is 

 proportionally diminished. 



If, when the water leaves the tap, the air and gravi- 

 tation were alike abolished, the water, keeping its 

 momentum, would travel for ever in the same 

 direction. 



As the water runs out, it will be observed that the 

 velocity of the stream becomes less and the curve 

 which it describes sharper, so that it comes to the 

 ground sooner; and finally, when the vat is nearly 

 empty, the stream falls nearly vertically downwards. 

 The reason of this is that the level of the top of 

 the water is gradually lowered; consequently, the 

 height of the column which presses on the water close 

 to the tap is gradually lessened, and therefore its 

 weight is diminished. But this weight or pressure 

 is the cause of the motion of the water, and as the 

 cause diminishes the effect of that cause must dimmish. 

 Therefore the momentum of the water is gradually 

 lessened and it is carried less and less far horizontally 

 in the time which it takes to fall to the ground ; 



