do not move at all, whereas in the second (of sliding motion) the particles about
move, rubbing over the floor, which thus resists the motion by friction. A thrust, thus recorded by springs or other device, in the case where the wall moves horizontally, would give an undervaluation at the lower part of
and consequently the computed center of pressure on
would be too high. On that account, only the experiments on rotating boards will be considered in this paper.
The theory of the sliding wedge, however, is justified, because no motion of either kind is actually supposed. The wedge,
