OBJECTS,] INTRODUCTORY. 41 



inserted close to the bottom, to be filled with water 

 up to 100 inches above the tap. Then supposing the 

 tap to be shut, the pressure upon its sectional area will 

 be 25,250 grains, or rather more than three pounds 

 and a half and there is the same pressure on every 

 square inch of the bottom of the vat. 



If the tap is now turned, the water nearest to it 

 being unsupported on its outer side, the pressure on 

 the inner side sets it in motion, and it flows out in a 

 stream. At first the stream shoots out violently and 

 the water is carried to a long distance. That is to 

 say, the weight of the column of water 100 inches 

 high acts as a force, or cause of motion, upon the 

 water nearest the tap, and this water is forced out 

 with a velocity depending on that force in a 

 horizontal direction. Now suppose that you take a 

 common toy cup-and-ball and bring the ball in to the 

 way of the stream of water. The stream will at once 

 strike the ball and drive it in the same direction as 

 that which it is itself taking. The power which the 

 moving water has of transferring or communicating 

 motion to a body which is at rest, but free to move 

 as the ball is, is due to its momentum, The 

 greater the mass of the stream and the more rapidly 

 it moves, the more motion will it communicate to the 

 ball, or the heavier the ball it will move. Close to 

 the mouth of the tap the direction of the stream is 

 horizontal; but it very soon begins to bend down- 

 wards, and describing a rapid curve, comes to the 

 ground. It does this for just the same reasons that 

 a stone thrown horizontally describes a curve, and at 



surface of a piece of cardboard, like the wad of a gun, just 

 large enough to go into the tube. 



