PHILOSOPHICAL SOCIETY OF WASHINGTON. 131 



compressible] and not an aggregation of other solid particles. 

 Hence they cannot be supposed to have any internal voids in which 

 their parts could be contracted or dilated. - - - We believe we 

 are able to show that it is by no means a necessary position to 

 accept this elastic property as a primitive force, but that the ap- 

 parent repulsion of these atoms and their rebound originates solely 

 from their proper motion, and for this it is sufficient simply to sup- 

 pose them to be in rotation."* He then proceeds to develop his 

 theory of mechanical elasticity from the co-operation of the projec- 

 tile motion of bodies with the internal rotations of their constituent 

 molecules ; citing in support of his assumption, the mathematical 

 researches of Poiusot.f In this important foundation of his system 

 however, the zealous physicist has built upon an entirely mis- 

 taken apprehension of true mechanical principles, and hence of 

 course upon a strauge misapprehension of the actual discussion by 

 Poinsot. This eminent mathematician who has investigated so 

 thoroughly the theory of rotatory movements has shown that in the 

 collision of inelastic bodies, endowed with rotation, the velocity of 

 deflection may in special cases exceed the velocity of incidence, in 

 other special cases may be just equal to it, and lastly in general will 

 fall short of it, being in many cases entirely destroyed. Thus a 

 rotating inelastic body has two points between the center of inertia 

 and that of percussion, Which on impact with a fixed resistance 

 in the line of their direction will produce a resilience of higher ve- 

 locity than that of collision, — of course by the conversion and ab- 

 sorption of so much of the rotary motion. There are other two 

 points from the direction of whose impact will result a velocity just 

 equal to that of the original motion of the body ; — in the one case 

 absorbing one-third of the rotary motion, in the other case absorb- 

 ing two-thirds of it. If the impact be in the line of the center of 

 inertia, the whole of the translatory motion is arrested without 

 affecting the rotary motion. [In the case of two equal inelastic 

 spheres rotating with equal and opposite velocities on parallel trans- 

 verse axes and meeting at a point on their equators, the bodies 



* L' Unita delle Forze Fisiche ; Saggio de filosofia naturale. Del P. Angelo 

 Secchi. 12mo. JRome, 1864: chap, i, sect. 6, pp. 36, 37. 



f Father Secchi's reference in a foot-note is to "Questions dynamiques 

 sur la percussion des corps: pag. 21 e 29, dell' edizione a parte, ed anche 

 il Giornale di Liouville, - - - a pag. 36." 



