At each joint in the truss the stresses in the members meeting there constitute, with the external forces or loads acting at the same point, a system in equilibrium represented by the two equations (35) and (36). If there are _m_ such joints in the entire structure, there will be 2_m_ such equations by which the same number of unknown quantities may be found. Since equilibrium exists at every joint in the truss, the entire truss will be in equilibrium, and that is equivalent to the equilibrium of all the external forces acting on it. This latter condition is expressed by the three equations (35), (36), and (37), and they are essentially included in the number 2_m_. Hence there will remain but 2_m_ - 3 equations available for the determination of unknown stresses or external forces. If, therefore, all the external forces (loads and reactions) are known, the 2_m_ - 3 equations of static equilibrium can be applied to the determination of stresses in the bars of the truss or other structure. It follows, therefore, that the greatest number of bars that a statically determinate truss can have is _n_ = 2_m_ - 3. (38) In Fig. 19 there are twelve joints and twenty-one members, omitting counter web members and the verticals _ab_ and _fl_, which are, statically speaking, either superfluous or not really bars of the truss. Hence _m_ = 12 and 2_m_ - 3 = 21. (39) Again, in Fig. 21 there are fifteen joints. Hence _m_ = 15, 2_m_ - 3 = 27, and there are twenty-seven bars or members of the truss. The number of joints and bars in actual, statically determinate trusses, therefore, confirm the results. =108. Continuous Beams and Trusses—Theorem of Three Moments.=—These considerations find direct application to what are known as ”continuous beams,” i.e., beams (or trusses) which reach continuously over two or more spans, as shown in Fig. 28. [Illustration: FIG. 28.]