Cp =~(yRzQ), Cq =~(zPxR), Cr =~(xQyP), J~(22)
If the joint be provided either with projections and recesses, such as murtises and tenons, or with fastenings, such as pins or bolts, so as to resist displacement by sliding, the question of the utmost amount of the tangential resistance CQ which it is capable of exerting depends on the strength of such projections, recesses, or fastenings; and belongs to the subject of strength, and not to that of stability.
The condition of stability of friction is that the tangential component CQ of the resistance required shall not exceed the friction due to the normal component; that is, that CQ~f.CP,
X = cq - bt, y = at -cp; if we take aq-bp= - I, we have x=bt-cq, y=cp-at.
1 AQP be the reflecting circle Caustics having C as centre, P the luminous point, and PQ any incident ray, and we join CQ, it follows, by the law of the b efiection.