Jackson — A New Method of Producing Tension in Liquids. 105 



suspended in the liquid or attached to the sides of the vessel will 

 render a static method of producing tension unworkable, while, if 

 the particular portion of the liquid subject to a tensile stress be 

 rapidly changing, and the particles not given time to evolve gas, 

 and so induce rupture, quite a considerable tension may be produced. 

 To take a specific case, is it possible to subject ordinary water as 

 drawn from a city supply main to any considerable tension with- 

 out rupture ? Such water will not stand tension for any consider- 

 able time. Will it stand it for even a very short time ? 



A convenient and simple means of producing low and pos- 

 sibly negative pressures is afforded by the well-known varia- 

 tion of pressure in a liquid flowing through a tube of varying 

 diameter, the pressure being low where the cross-section is small, 

 and high where the cross-section is large. If the liquid were 



^S2^ rirtlu „ « F -«°£^. 



'on 



Low velocity High velocity l_ 0w velocity 



High pressure Low pressure High pressure 



Re. I. 



perfectly mobile and the flow steady, the pressure at any point 

 might be deduced from the measured pressure at any other point, 

 the density of the liquid, the rate of flow, and the areas of cross- 

 section of the tube at the two points, the relation between these 

 quantities being 



oCV 1 1 \ 



But actual liquids are far from being perfectly mobile ; and the 

 motion of a liquid in a tube is not steady if the velocity exceed a 

 fixed small value, so that the effects of viscosity and turbulent 

 motion would render the above formula inapplicable. If, how- 

 ever, we assume that the current depends only on the pressure- 

 gradient and the form and size of the tube, and does not (at least 

 in the case of an almost incompressible liquid like water) depend 

 on the absolute magnitude of the pressure, we see that the pressure- 

 difference between any two points can depend only on the strength 



