, to cause friction at the back of the break of sufficient intensity to prevent the semi-cylinder from descending, just as a book can be held against a vertical wall by a horizontal push.
To illustrate the principle, it will suffice to replace the semi-cylinder by the circumscribing parallelopiped,
, and suppose it to be held up by the friction on the back face, with possibly cohesion acting on the three interior vertical faces. Thus, let
, and
; then the friction on the back face is
