234 



THE HUMAN BODY. 



that its elastic walls are slightly stretched. These will in 

 -consequence press upon the liquid inside them and the 

 amount of this pressure will be indicated by the gauges; 

 so long as the pump is at rest it will be the same everywhere 

 (and therefore equal in the gauges on B and A), since 

 liquid in a set of horizontal tubes communicating freely, 

 as these do at D, always distributes itself so that the 

 pressure upon it is everywhere the same. Let the pump 

 c now contract once, and then dilate: during the contrac- 

 tion it will empty itself into B and during the dilatation fill 

 itself from A. Consequently the pressure in B, indicated 

 by the gauge x, will rise and that in A will fall. But very 

 rapidly the liquid will redistribute itself from B to A 

 through D, until it again exists everywhere under the same 



FIG. 87. Diagram of Weber's Schema. 



pressure. Every time the pump works there will occur a 

 similar series of phenomena, and there will be a disturbance 

 of equilibrium causing a wave to flow round the tubing; 

 but there will be no steady maintenance of a pressure on 

 the side B greater than that in A. Now let the upper 

 tube D be closed so that the liquid to get from BtoA must 

 flow through the narrow lower tubes D', which oppose con- 

 siderable resistance to its passage on account of their fre- 

 quent branchings and the great internal friction in them; 

 then if the pump works frequently enough there will be 

 produced and maintained in B a pressure considerably higher 

 than that in A, which may even become negative. If, for 



