, in equilibrium.
A similar investigation pertains to any other supposed prism of rupture. To find the true one, a number of trial planes of rupture,
,
, ..., are assumed, and each is treated in turn as a true one (though there can be only one true one). As seen above, the resultant of the normal reaction and friction on any trial plane of rupture must be inclined below the normal to the plane at the angle,
. To lay off the directions of these resultants, from any convenient point,
, say, in the vertical through