196 PHYSIOLOGICAL PHYSICS. [Chap. xix. 



a position of equilibrium ; if the weight of the dis- 

 placed liquid is less than that of the body, the latter 

 will sink, the downward exceeding the upward pres- 

 sure; if the displaced liquid exceed the body in weight, 

 then the body will be forced upwards partly out of 

 the water, till the quantity of displaced water is 

 reduced to equal the weight of the body. 



Centre of gravity and inetaceiitre A 

 floating body is thus under the action of two forces ; 

 one, the force of gravity acting through the centre of 

 gravity urging the body downwards ; the other, the 

 force of the displaced water acting through what 

 would be its centre of gravity, called the centre of 

 buoyancy, urging the body upwards; and these two 

 forces are equal. When the body floats in stable 

 equilibrium the two forces are opposed, that is, the two 

 centres are in the same vertical line. If the floating 

 body, being of irregular form, move to one side, the 

 amount of the liquid displaced by it will be different, 

 and consequently the centre of buoyancy will be 

 different. If, now, a vertical line be drawn through this 

 new centre 6f buoyancy and be continued to meet what 

 was, before the new position, the vertical through the 

 centre of gravity, the point of intersection marks what 

 is called the METACENTRE. The position of this is of 

 great moment, for when the rnetacentre is above the 

 centre of gravity, the upward force of buoyancy and 

 the downward force of gravity act in a way to restore 

 the equilibrium of the body. When the metacentre 

 is below the centre of gravity, the tendency is to in- 

 crease the displacement. The former is a condition of 

 stable and the latter of unstable equilibrium. 



Fishes are capable of moving towards the surface 

 or the deep parts of water by their means of regulating 

 the quantity of water they displace, that is, the force 

 that urges them upwards. By a distension of their 

 air-bags the volume of their bodies is increased, the 



