ILLUSTRATING CARRIAGE OF MOMENTUM 59 



a bent brass tube ABCDE (fig. 25, plan), is 

 suspended by its middle point C so as to be in a 

 horizontal plane, and if a stream of air is blown 

 through the tube by a pump P, the stream has 

 to turn two corners, B and D. As it turns each 

 corner it presses outwards against the corner, 

 and so we get two forces turning the tube round 

 in the direction of the arrow. 



C 



FIG. 25. 



We may regard the action thus. The air pos- 

 sesses momentum, and the stream of momentum is 

 shifted from the line AB into the line DE, parallel 

 to AB. We should get the same effect by a force 

 P! acting at B against the motion, and destroying 

 the momentum along AB, and by an equal force P 2 

 acting at D along DE, giving the momentum, again, 

 in the new line DE, fig. 26. 



