Chap, xix.] METACENTRE. 197 



weight remaining the same. The upward force is, 



O O * 



therefore, augmented, the downward being unaffected. 

 So they rise. By muscular compression of the air-sac 

 the volume of the body is diminished, and they sink. 

 Thus, if a fish in a jnr of water be placed under the 

 bell of an air pump, and the air exhausted, the fish 

 will come to the surface ; rupture of the air-sac will 

 occur, and on removing the jar the animal will sink to 

 the bottom, and be unable to float. It will also be 

 unable to keep itself in a proper position, the tendency 

 being for it .to turn on its back. This, it is said, is 

 due to the fact that the equilibrium of fishes is un- 

 stable, the centre of gravity being above the centre of 

 buoyancy, the condition of instability for completely 

 immersed bodies. In animals that swim on the 

 surface of the water the conditions of instability are 

 also present, the stable condition being maintained by 

 muscular effort. Swimming on the back for them is, 

 however, a condition of stable equilibrium, for then 

 the centre of gravity is below the centre of buoyancy. 

 Specific gravity. The density, or relative mass, 

 of a body is obtained by comparing the weight of that 

 body with the weight of the same volume of a given 

 body used as a standard. The standard adopted is 

 that of water at its maximum density, viz. at a 

 temperature of 4 C. If, then, the weight of a 

 body is obtained, and is compared with the weight of 

 the same volume of water at a temperature of 4 C., 

 the specific gravity of the body is obtained. A method 

 for determining this, accordingly, is to take the body 

 and weigh it. Let its weight be represented by 10. 

 Place the body in the pan of a balance, and in the same 

 pan place a flask with a wide neck, to which is care- 

 fully fitted a ground-glass stopper. The stopper has a 

 fine tube in connection with it, the bore of the tube 

 being continued through the axis of the stopper. Let 

 the flask be accurately filled with water up to a mark 



