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LI II. On Texture in Media, and on the Non-existence of 

 Density in the Elemental JEther. By Gr. Johnstone 

 Stoney, a Vice-President of the Royal Dublin Society, 

 M.A., JD.Sc, F.U.S* 



N the investigations of ordinary dynamics — the dynamics 

 of secondary f motion — integrations have to be ex- 

 tended throughout the bodies with which we are dealing, or 

 over their surfaces. Now whenever we employ this operation, 

 assumptions are tacitly made which do not accord with what 

 exists in real objective nature. 



Suppose that the problem is to obtain the pressure of water 

 against a sluice ; to ascertain the amount and distribution of 

 the pressure, we integrate over the surface between the water 

 and the sluice, and in doing so assume : — 



(1) That the boundary is a surface ; and 



(2) That the elements into which we conceive this surface 



* Read before the Royal Dublin Society, February 19, 1890, and 

 reprinted by permission from a proof of the paper in the Scientific Pro- 

 ceedings of the Society. 



t In the computations of ordinary dynamics, we conceive the portion of 

 space occupied by the body with which we are dealing to be divided into 

 elements of volume (the dxdydz's), which elements of volume we regard 

 as movable. Each of these we multiply by a coefficient called the den- 

 sity, and call the product the element of mass (dm = p . dxdy dz). These 

 elements of mass we picture to ourselves as acting on one another, or as 

 being acted upon by external forces ; and from the laws of these actions 

 we endeavour to deduce the motion of the element of volume, carrying 

 its contents with it, and in some cases changing its form or volume. 



In this process we take no notice of any motions which may be going 

 on within the element of volume, except so far as that some imperfect 

 account may perhaps be indirectly taken of them when we multiply the 

 element of volume by a density. Nevertheless, in all the real cases that 

 occur in nature, there are, as a matter of fact, very active motions of 

 various kinds going on within the element of volume : motions of the 

 molecules which it contains, and still more deep-seated motions within 

 the portion of the element of volume occupied by those molecules, or in 

 the interspaces between them. 



Accordingly the motions with which we deal in our ordinary dynamical 

 investigations are merely drifting motions — the drifting about of elements 

 of volume, within each of which, as events really occur in nature, there 

 are elaborate subsidiary motions going on. Now, secondary motion is to 

 be denned as the motion which consists in the drifting about, with or 

 without changes of size and form, of elements of volume within each of 

 which there are subsidiary motions. 



If the subsidiary motions consist exclusively of mutational motions in 

 an incompressible and perfectly fluid medium, they cannot contribute to 

 the density by which the element of volume in which they occur is to be 

 multiplied. It is, however, otherwise if there are any rotational motions 

 present. 



