on Revetment Walls. 493 



plane-mass at rest, imagine a small rectilinear element to be 

 drawn. The entire molecular force exerted on this actual 

 element might be termed the thrust ; but by the thrust I shall 

 understand the unit of thrust, corresponding to the well-known 

 conception of unit of pressure for the particular case of a fluid 

 mass. This thrust (or unit of thrust) may be imagined separated 

 into two parts, one perpendicular to the element, which may be 

 termed the pressure, the other parallel to it, which may be termed 

 face-force [for the case we shall have more especially to consider, 

 the face- force receives the name of friction, and is limited to be 

 less than the pressure multiplied by the so-called coefficient of 

 friction] . As the element acted upon turns round, the thrust 

 changes in magnitude and direction, and to the totality of the 

 thrusts going forth in all directions from a given point we may 

 give the name of stress*. We shall now be able to obtain two 

 sorts of conditions — one giving the necessary law connecting the 

 various thrusts of the same stress, the other expressing the law 

 of the variation of the pressure and facial force (together consti- 

 tuting the thrust) upon an element given in direction in passing 

 from one stress to another ; we may call these respectively the 

 equations of distribution and the equations of variation. 



Let P Q R S be any infinitely small molecule 

 bounded by lines at right angles to one another. 

 Since this is kept at rest by its own weight, by 

 the lines of pressures perpendicular to Q R and 

 P S, and the other pair perpendicular to P Q 

 and R S, and by the facial forces acting along 

 P Q, Q R, R S, S P respectively, if we call / 

 the face-force [i.e. the unit of face-force] on P S, 

 it is obvious that the corresponding quantity for Q R will differ 



statically from force measured by acceleration, by giving to the former the 

 name of pressure. But surely unnecessary confusion is introduced into 

 mechanical language when we are thereby reduced to speak of the pressure 

 of friction, and ought to enunciate the cardinal law of friction by stating 

 that the pressure of friction bears to the pressure of pressure & certain 

 limiting relation. I acknowledge an objection scarcely less valid (except 

 that it has antiquity to plead in excuse) to the use of the term accele- 

 rating force ; as we may be thereby reduced to speak of the accelerating 

 force of a retarding influence, as friction, or of an influence which does not 

 necessarily either accelerate or retard, as in the case of a centripetal pull 

 upon a body moving uniformly in a circle. I think this difficulty in lan- 

 guage may be met to some extent by giving to force, usually called accelera- 

 tive, the designation of alterative, and to force measured by weight or mo- 

 mentum that of quantitative force. There is no magic in names, however 

 well selected, but there may be a great deal of mischief arising out of a 

 confused and uncertain nomenclature. 



* Thus, stress stands in somewhat the same relation to its component 

 thrusts, as a radiant point to the luminous rays which it emits. 



