214 PHYSIOLOGICAL PHYSICS. [Chap. xx. 



movement increases so does the friction, i.e. the 

 resistance. The smaller the diameter of the tube, the 

 greater is the speed of the current, so that 



(4) The resistance is in inverse proportion to the 

 diameter of the tube. 



In short, the resistance is directly proportional to 

 the length of the tube, is inversely proportional to its 

 cross section, and increases ivith the speed of the 

 stream. It may be added that the resistance will also 

 increase with the force of cohesion exercised by the 

 molecules of the liquid. So that a liquid like blood, 

 with greater cohesive power, would offer greater resis- 

 tance than water. 



Heat diminishes the cohesion of a liquid, and so 

 lessens the resistance. 



What has been said applies to tubes of uniform 

 diameter, but it explains also the influence of TUBES 

 OF VARYING DIAMETER. When a sma 1 ! tube passes 

 suddenly into a tube of larger diameter there is 

 sudden increase of pressure at the surface of junction, 

 accompanied by a diminution in the speed of move- 

 ment through the wider tube. The molecules of 

 which the fluid consists cannot suddenly change the 

 swift movement into a slower one, and on account of 

 their inertia the pressure exerted by them on one 

 another develops the increased force. On the other 

 hand, the abrupt transition from a slow to a quick 

 movement, at the place where a wide tube passes into 

 a narrow one, diminishes the pressure. The effect 

 however, in a system of tubes of a series of widei 

 parts is to diminish the total resistance. 



lending of the tube causes serious retardation 

 at the place of bend, and, if great, may produce some- 

 thing of the nature of a whirl still further to arrest 

 the movement by the pressure of the molecules on 

 the inner side of the bend. The result is, that behind 

 the bend the resistance is increased. This means, 



