PHYSICAL CONSIDERATION 351 



is not exerted continuously, the fluid will escape from the distant orifice of the 

 tube only when a definite quantity has been forced into its central end. Under this 

 condition, the outflow becomes intermittent, although it does not cease as yet 

 at the very moment when the pressure is discontinued. It lags behind, because 

 its inherent sluggishness causes it to escape with a rapidity which is less than that 

 of the influx. 



Flow of a Liquid Through Elastic Tubes. — If the rigid tube is dis- 

 placed by one possessing elastic walls, a condition of flow will be estab- 

 lished in time which is practically the same as that described pre- 

 viously. To begin with the walls of the tube move outward in the 

 direction of the lateral pressure exerted by the liquid, and this disten- 

 tion continues until the elastic power of the walls exactly counter- 

 balances the internal pressure. At this time, the elastic tube really 

 displays the same phenomena as those previously observed in the rigid 

 system, but naturally, only as long as the head-pressure remains con- 

 stant. If the latter is diminished, the vessel wall must first recoil to 

 adapt itself to the new conditions. 



If the head-pressure is now permitted to act intermittently, the conditions 

 of pressure and flow must be the result of the force and frequency with which 

 the primary power is applied and secondly, of the resistance which this primary 

 power encounters in the system of tubes. To begin with, let Us suppose that the 

 pressure acts at long intervals and that the resistance is slight. The latter con- 

 dition may be produced without difficulty by using a short tube of relatively large 

 diameter. In this case, the entrance of the fluid is associated with a distention 

 of the walls of this tube and a discharge from its outlet which is greatest during 

 the period of highest pressure, and becomes less and less as the driving force is 

 diminished. A flow of this kind is characterized as intermittent. If the pressure 

 is now allowed to act more frequently, or if the resistance is heightened, or 

 both, the outflow becomes smaller during the interims, but does not cease altogether. 

 The flow is then said to be remittent. By continuing to increase the force and 

 frequency of the pressure, as well as the resistance, a point will finally be reached 

 when the outflow ceases to fluctuate. It is then constant in its character. 



If a certain quantity of liquid is permitted to escape from the reservoir into the 

 elastic tube, the walls of the latter are forced apart. The influx having ceased, 

 the walls tend to come together again. This recoil is a property of all elastic 

 bodies. If the pressure is now applied more frequently, while the resistance is 

 permitted to remain the same or is increased, the mass of the liquid in the tube 

 increases steadily. This is made possible by the steady yielding of the walls of 

 the tube in an outward direction. The tube is distended. Eventually, however, 

 its elastic recoil effectively counteracts all further distention and storage of liquid. 

 It must be evident, therefore, that the quantity of fluid which is present in the 

 tube in excess of that constantly escaping through the outlet, is sufficient to main- 

 tain a certain pressure even during the intervals of time when the head pressure 

 is not being exerted. In this way, the fluid is held under a continuous pressure 

 with the result that the outflow remains practically constant. Thus, it will be 

 seen that the property of elasticity by means of which the walls of the tube en- 

 deavor to regain their original position, is of greatest importance to the agent 

 producing the pressure, because it helps to preserve normal conditions of flow 

 even during the periods when the latter is at rest. Obviously, therefore, the 

 energy developed by the generator is stored each time in the walls of the tube 

 in the form of elastic tension. It is then spent during the periods when the pri- 

 mary force is not acting. In this way, the flow is kept constant in spite of the fact 

 that a new supply of fluid is had only every now and then. 



