PART II.--EQUILIBRIUM OF FORCES APPLIED TO MATERIAL SYSTEMS. LESSON 21. _Résumé of the notions acquired upon the subject of forces, and their effects on material points._ Principle of inertia, notion of force, of its direction, of its intensity. Principle of the equality of action and reaction. What is meant by the force of inertia? Principle of the independence and composition of the effects of forces. Forces proportional to the acceleration which they produce on the same body. Composition of forces. Relation between the accelerating force, the pressure, and the mass. Definition of the work done by a force. The work done by the resultant is equal to the sum of the works done by the components. Moment of a force in relation to an axis deduced from the consideration of the work of the force applied to a point turning about a fixed line. The moment of the resultant of several forces applied to a point is equal to the sum of the moments of the components. Corresponding propositions of geometry. LESSONS 22-25. _Succinct Notions upon the Constitution of Solid Bodies._ Every body or system of bodies may be regarded as a combination of material points isolated or at a distance, subject to equal and opposite mutual actions. Interior and exterior forces. Example of two molecules subject to their reciprocal actions alternately, attractive and repulsive, when the forces applied draw them out of their position of natural equilibrium. Different degrees of natural solidity, stability, or elasticity; they can only be appreciated by experience. _Equilibrium of any Systems whatever of Material Points._ General theorem of the virtual work of forces applied to any system whatever of material points. It is applicable to every finite portion of the system, provided regard be had to the actions exercised by the molecules exterior to the part under consideration. Determination of the sum of the virtual works of the equal and reciprocal actions of two material points. Demonstration of the six general equations of equilibrium of any system whatever. They comprise implicitly _every_ equation deduced from a virtual movement compatible with the pre-supposed solidification of the system. Theorem on the virtual work in the case of systems where one supposes ideal connections, such as the invariability of the distance of certain points of the system from one another, and the condition that certain of them are to remain upon curves either fixed or moving without friction. _Equilibrium of Solid Bodies._ The six general equations of equilibrium are sufficient as conditions of the equilibrium of a solid body. Theory of moments and couples. APPLICATIONS. LESSONS 26-29. _Equilibrium of Heavy Systems._