194 PHYSIOLOGICAL PHYSICS. [Chap. xix. 



fluid to one vessel, then a movement will take place 

 through the communicating tube for the re-establish- 

 ment of equilibrium. 



The rule that the free surfaces must be in the same 

 horizontal line only applies when it is the same 

 liquid that fills all the communicating vessels. If 

 liquids of different densities are poured into the 

 vessels, then, provided they do not mix, the heights 

 of the different fluids above the surface of contact 

 will be in the reverse ratio of their densities. Increased 

 density means increased pressure, and consequently a 

 column of the denser liquid of less size will exert the 

 same pressure as a higher column of the less dense 

 liquid. This gives a means of calculating the density 

 of a liquid. The height of one liquid multiplied into 

 its density will be equal to the height of the other 

 liquid multiplied into its density. Let h stand for 

 the height in inches of the column of one liquid, and 

 h' the height in inches of the other, and let d and d' 

 represent the two densities ; then 



v -t i I T/ ^ fv J\ v 



h x d = h x d or d = 



h 



In the circulation of the blood the sig- 

 nificance of the principle of the last paragraph has 

 been pointed out. The system of blood-vessels in an 

 animal is a system of communicating vessels con- 

 taining a single fluid. The tendency is, therefore, 

 for that fluid so to distribute itself throughout the 

 vessels as to bring about a condition of equilibrium. 

 But opposed to this equalising tendency is the inter- 

 mittent action of the heart, which disturbs the equi- 

 librium at one end of the series of vessels, as it were. 

 To meet this disturbance, a flow takes place again in 

 the direction of restoration, and so on. As a result 

 of this constant effort after equilibrium, and periodic 



