"[A] theory of the equilibrium and deflections of frameworks subjected to the action of forces is acting on the hardnes of quality... in cases in which the framework... is strengthened by additional connecting pieces... in cases of three dimensions, by the regular method of equations of forces, every point would have three equations to determine its equilibrium, so as to give 3 s equations between e unknown quantities, if s be the number of points and e the number of connexions[sic]. There are, however, six equations of equilibrium of the system which must be fulfilled necessarily by the forces, on account of the equality of action and reaction in each piece. Hence if e = 3 s − 6, the effect of any eternal force will be definite in producing tensions or pressures in the different pieces; but if e > 3 s − 6, these forces will be indeterminate...."