CIRCULATION OF THE BLOOD. 517 



hydraulics or hydrodynamics will greatly facilitate the explanation of the 

 circulation. 



Every fluid particle under the action of the law of gravitation falls 

 like a solid body to the earth. When, however, a large mass of fluid is 

 freely acted on by gravity the slight cohesion exerted by the molecules 

 of liquid on each otlier leads to their separation one from the other. 



Every fluid, therefore, in falling tends to separate into drops. This 

 tendency to break up into drops may be prevented either by delaying 

 the flow of the liquid, as by causing it to descend an inclined plane, or 

 permitting the liquid to flow within a vessel in which the tendency of 

 the particles to separate will lead to the production of a vacuum, and 

 atmospheric pressure will, consequently, serve to strengthen the cohesive 

 forces. Consequently, in a stream of liquid falling into a tube each par- 

 ticle is not only acted on by gravity, but also by the pressure of the mass 

 of fluid behind it. If, therefore, an aperture be made in the bottom of any 

 vessel, any particle of liquid on the surface of the fluid contained in that 

 vessel, if we could imagine that it would fall freely without reference to 

 the particles below, would have a velocity on reaching the orifice equal 

 to that of any otlier body falling through the distance between the level 

 of the liquid and the orifice. If the liquid in such a vessel be maintained 

 at the same level, the particles will follow one another with the same 

 velocity and will issue in the form of a stream; while, from the principle 

 of transmission of pressures in liquids equally in all directions, a liquid 

 would issue from an orifice in the side with the same velocity as from an 

 aperture in the bottom of the vessel, provided the depth were the same. 



The velocity of efflux, therefore, as formulated by Torricelli, is the 

 velocity which a freely falling body would have on reaching the orifice 

 after having started from a state of rest at the surface. It is expressed 

 by the formula — 



V = V2gh, in which g = 32.16 ft. 



It further follows that while the velocity of efflux depends on the 

 depth of the orifice below the surface and not on the nature of the liquid, 

 the velocities of the efflux, from the laws of falling bodies, are directly 

 proportional to the square roots of the depths of the orifices, while the 

 quantity of fluid which issues from the orifices of different areas is very 

 nearly proportional to the size of the orifice, provided the level remains 

 constant. It is evident, however, that the mass of liquid in such an ex- 

 periment at the side of the column vertically over the orifice of exit offers 

 by friction more or less resistance to the line of movement. 



And while the molecules vertically over the centre of the orifice 

 pass directly down and out by the orifice, the molecules of fluid at the 

 side of this moving column not only offer resistance to this downward 



