men, to have universal application in this time. II. The doctrine of the correlation, equivalence, persistence, transmutability and indestructibility of force, or the conservation of energy has had vast influence upon the thought and life of our time. It has furnished a new opening through which to behold the nature of things. It has given to men a new working hypothesis and richer views and conceptions of the universe and its author. The tremendous advancement made in the material civilization of the present is due more to this than any other scientific doctrine or principle. According to Professor Balfour Stewart, there are eight forms of energy or force. The energy of visible motion, visible energy of position, heat motion, molecular separation, atomic or chemical separation, electrical separation, electricity in motion, and radiant energy. Now taking this earth as a complete whole, containing within itself all these forms of energy, and so isolated from the rest of the universe as to receive nothing from it and to add nothing to it, then the principle of the correlation of forces asserts that the sum of all these forces is constant. “This does not assert that each is constant in itself, or any other of the forms of force enumerated, for in truth they are always changing about into each other—now some visible energy being changed into heat or electricity, and heat or electricity being changed back again into visible energy; but it only means that the sum of all the energies taken together is constant. There are eight variable quantities, and it is only asserted that their sum is constant, not by any means that they are constant themselves.” For the purpose of elucidating our principle in the realm of nature, we will consider it as it applies to some of the useful forces whose effects we can measure and whose origin we can trace and determine. There is the force of conserved fuel. Away back in the carboniferous period of the world’s history, there grew immense forests, which in succeeding ages were turned under the earth, and, in the process of the years, were changed into coal and oil and gas. These have been treasured for untold ages in the mountains and in the bowels of the earth. Now they are brought forth by the applied intelligence of man, to turn his wheel, draw his car, cook his food, propel his plow, and to light his home and his street. The force in one ton of coal is capable of accomplishing more work in a few hours than one man could in a lifetime. All this force, as well as that contained in the growing forests of to-day, originated in the sun.