The document discusses balancing of reciprocating masses in engines. It describes: 1. The various forces acting on reciprocating parts and how the inertia force is balanced by an opposing force on the crankshaft, leaving an unbalanced force. 2. Methods to partially balance the primary unbalanced force using a balancing mass on the crank, which changes the direction of the maximum unbalanced force. 3. How balancing is applied to two-cylinder locomotives, reducing variation in tractive force, swaying couple, and hammer blow.