242 [Recess, 



No. 2. Report kept by Leeds Philosophical Society. 

 Leeds, May 7, 1862, 5 P.M. 



I am, &c., 



THOMAS SUTCLIFFE. 



III. " On the true Theory of Pressure as applied to Elastic 

 Fluids." By R. MOON, M.A., late Fellow of Queen's Col- 

 lege, Cambridge. Communicated by Professor SYLVESTER. 

 Received June 26, 1862. 



(Abstract.) 



It is the author's object 



I. To show that, in elastic fluids in motion, or tending to move, it 

 is not generally true, or at least not accurately true, that the pres- 

 sure depends solely on the density, as is assumed in the ordinary 

 theory of the motion of elastic fluids. 



II. To show that, within certain limits and under certain circum- 

 stances, pressure may be transmitted instantaneously from one point 

 of an elastic fluid to other points situated at finite distances from the 

 first, before any change has been eifected in the density of the inter- 

 mediate fluid in a manner analogous to that in which, in the theory of 

 dynamics as applied to rigid bodies, force is assumed to be propagated 

 instantaneously from one point to another. 



III. To show that in elastic fluids in motion, or tending to move, 

 the pressure at any point in a given direction will consist of two 

 parts : one depending solely on the density, which will be equal in 

 all directions ; the other depending on the state of motion throughout 

 the fluid generally, and which will vary with the direction in which 

 the pressure is estimated. The former of these two constituents the 

 author proposes to designate the statical pressure ; the latter, the 

 instantaneous pressure. The true pressure at any point in a given 

 direction will be found by taking the sum or difference of the statical 

 and instantaneous pressures, according to circumstances. 



