OBJECTS.] INTRODUCTORY. 39 



the cork will be driven out with as much force as 

 before, so that the water presses just as much side- 

 ways as downwards. It is easy to satisfy oneself of 

 this by inserting a long glass tube, with its lower end 

 bent at right angles and fitted with a cork, into the 

 side of the wooden pipe. The water will at once 

 rise in the tube to the same height as it has in the 

 pipe. Whence it is obvious that the pressure of the 

 water on any point of the side is exactly equal to the 

 vertical pressure at that point; for the pressure outwards 

 is exactly balanced by that of the vertical column in 

 the tube inwards. The water in a watering-pot always 

 stands at the same level in the can and in the spout. 



If a glass tube is bent into the shape of a U, and 

 water is poured into it, the water will always stand at 

 the same level in the two legs of the tube, whatever 

 the shape of the bend may be, or the relative capa- 

 cities of the two legs, or the inclination of the tube. 



And this must needs be so, for the force with which 

 the water tends to flow out of the one half of the 

 arrangement depends on the vertical height 1 of the sur- 

 face of the water above the aperture of exit ; so that 

 any column of equal vertical height must balance it. 



That a column of wate: will stand at exactly the 

 same level as any other with which it communicates, 

 may be seen still more simply by placing a glass tube, 

 open at each end, in a basin of water. However the 

 tube may be inclined or bent, whether its lower end is 



1 Vertical height is the height measured along a line drawn 

 from the surface of the water perpendicularly to the surface of the 

 earth. A plumb-line is a string to one end of which a weight 

 is attached and thus hr.ngs suspended. If the other end of the line 

 is brought opposite the surface of the water the direction of 

 the string answers to the line of vertical height. 



