Chap, xx.] RAMIFIED TUBES. 215 



however, diminished resistance in front, diminished 

 amount of current, but a proportionately speedier 

 advance. The result of the counterbalancing is, that 

 in the end the pressure and speed of movement are 

 unaffected. 



In a ramified system of tufees a similar 

 compensating arrangement is found to exist. Here 

 certain conditions exist tending to increase friction, 

 viz. increased surface of tubes, multiplying opportu- 

 nities for cohesion, as well as angles and bends ob- 

 structing the current. These exist at the places where 

 the main trunks branch out into others. Opposing 

 this tendency is the increased calibre permitting easier 

 flow. Similarly on the reunion of the branches to form 

 a common trunk, elements of increased resistance are 

 present in the retarding influence of the current of one 

 branch upon another as they meet, and on the in- 

 fluence of the angles at the junctions. This does cause 

 a backward pressure, which is yet to some extent 

 counterbalanced by the increased speed of a dimin- 

 ished current in front, and which is finally lost in 

 the increased calibre of the branches behind. Thus 

 it appears that, over all, a ramified system of tubes 

 does not offer more resistance than a single tube, and 

 may even effect a greater discharge than the single 

 tube. 



The flow of liquids \\\ capiSlary tubes was 

 investigated by Poiseuille with great care, for it is 

 found that below a certain diameter the flow does not 

 follow the laws already laid down. The diameters of 

 the tubes used by Poiseuille were all under one milli- 

 metre. He found that with capillary tubes of equal 

 length, and with other things equal, the discharge 

 increases in proportion with the fourth power of the 

 diameter, while in other tubes it is directly as the 

 sections. For different lengths, other things being 

 equal, capillary tubes obey the same laws as others, 



