ARCHIMEDES' PRINCIPLE. 131 



a glass rod in water ; I, a glass tube in water ; and c, a 

 glass tube in mercury. 



(a.) This form of adhesion is known as capillary attraction be- 

 cause its phenomena are best shown in tubes as fine as a hair (Latin 

 capittus). If fine glass tubes be placed in water, the liquid will 

 rise, wet the tube, and have a concave surface. If they be placed in 

 mercury, the liquid will be depressed, will not wet the tube, and 

 will have a convex surface. The finer the tube, the greater the 

 capillary ascent or depression. 



237. Displacement of a Fluid by an Im- 

 mersed Solid. A solid immersed in a fluid will 

 displace exactly Us own bulk of the fluid. This may 

 be proved, if desirable, by plunging a heavy body of known 

 rolume, as a cubic centimeter of iron, into water contained 

 in a glass vessel graduated to cubic centimeters. The 

 water will rise just as if another cubic centimeter of water 

 had been added. Thus, the volume of any irregularly 

 shaped body may be found. 



238. Archimedes' Principle. The loss of 

 weight of a body immersed in a fluid equals the 

 weight of the fluid ivhich it displaces. 



(a.) It is a familiar fact that a person may easily raise to the sur- 

 face of the water a stone which he cannot lift any further. When 

 an arm or leg is lifted out of the water of a bath-tub, there is a 

 sudden and very perceptible increase of weight at the surface. Let 

 us try to find a reason for these familiar truths. Imagine a cube, 

 six centimeters on a side, immersed in water so 

 that four of its surfaces are vertical and its 

 upper horizontal surface twelve centimeters 

 below the surface of the water. The lateral 

 pressures which the water exerts upon any two 

 opposite vertical surfaces are clearly equal and 

 opposite. They will have no tendency to move 

 the body. But the vertical pressures upon the 

 two horizontal surfaces are not equal. The 

 lower face will be pressed upward with a force 

 represented by the weight of (6 x 6 x 18 =) FIG. 79. 



