some respect in contrast with each other. In taking a survey of the physical aspects of the world, we can not limit attention to the mere mass, or to questions bearing on its structure. Observation must now be turned on the perpetual change going on in form, arrangement, and distribution of materials. There is need for a science of all this, in accordance with which the perpetual round of change may be reduced to order by reference to causality and the laws of its operation. Thus we are introduced to our ordinary conceptions concerning position, that is, the situation or place of objects, or of masses of material, or even of worlds, to each other; motion, or change of position, modifying or altering the relations of objects; and force, that is the relative amount or proportion of energy at work for the accomplishment of such changes as those already mentioned. In these connections we are introduced to recently formulated doctrines of energy, reached in searching for "the cause which alters or tends to alter a body's natural state of rest," as this problem was indicated in Newton's first law of motion. In the earlier stages of inquiry, attention was directed mainly on the active forces

of nature, as these are recognized in operation, admitting of calculation as to rate of movement on the one hand, and relative amount of force on the other. Computations of this kind were necessarily involved in research connected with the movements of the heavenly bodies. When astronomical theory had been matured and a truly scientific understanding of the solar system had been reached, physical science had next to deal with the more general problem raised by the contemplated forces of nature, having a reference at once wider and more minute. It is not possible here to do more than give a very summary view of the doctrine of energy, its mutations, dissipation, and conservation, as developed through study of the laws of gravitation, electricity, light, heat, etc., and now generally accepted. An outline will, however, suffice for an understanding of the general conclusions.

Energy is the term now employed to designate every form of power belonging to the physical world capable of doing work, and of being estimated according to the comparative amount of work it can perform. The whole phenomena of motion thus belong to this department of inquiry. The first distinction

here has been described as the difference be tween energy of position and energy of motion.* Both of these must be taken into account in order to have a full view of the facts. ENERGY OF POSITION, is illustrated by a water-head, or reservoir, where an accumulation of water is laid up in store, ready to be drawn upon for motive power when machinery set up in a position somewhat lower is to be brought into action and made to do the work for which it has been constructed. ENERGY OF MOTION is seen when the storehouse of water is opened and the rush of the current sweeps along the prepared channel descending upon the great wheel, which sets in motion the whole machinery. In such a case as this, the amount of work done by the revolutions of the great wheel is an exact measure of the amount of water which has passed to a lower level. Or let us suppose there is only a limited supply in the water-head, and that there are no feeders, but that the streamlets and springs from which it is supplied, have ceased to flow, and let us suppose that the mill comes to a stand because of failure of motive power, the amount of work done up to that moment is the measure of the energy stored in the water* See Appendix III.

head before the sluice was opened. This illustration indicates the mode of calculation to be applied to energy in all its forms, including the great forces of nature, before which human power is as nothing. Taking thus the correlatives position and motion, we may regard the former as preparation for the latter, for, as Professor Balfour Stewart has said, "It is the fate of all kinds of energy of position to be ultimately converted into energy of motion."* On this account, energy of motion most naturally exemplifies what we understand by energy; but on the other hand what has been called the energy of position must be regarded as a power distinctly calculable. If a stone be thrown into the air, the energy expended in propelling it to a certain height, is the exact measure of the energy expended in its descent. There is no need for entering here upon the calculations of the relation between energy and velocity, showing the exact proportion of the one to the other, or the ratio of increase according to velocity, which is expressed in the formula "that the energy varies as the square of the velocity," giving us an exact measure of force.

• The Conservation of Energy, by Balfour Stewart, p. 26.

Aided by the conceptions of position and motion, we take the next step when we advance to transmutation or conversion of energy. What is made visible to us by motion is the transference of energy from one object or portion of matter to another. And this is the sole explanation of what occurs. There is no such thing known to physical science as the origin or creation of energy; all that is recognized is the disappearance of energy from one position and its appearance in another. If work has been done, energy was somewhere stored capable of doing it; a transmutation has taken place; and the work accomplished is the record of the process. In recognition of this, every machine is merely a more or less skilful contrivance for transmuting energy into a form more convenient or suitable for human purposes. The intelligence of man simply recognizes the law of transmutation, and deals with the problems which arise connected with the mechanical arrangements facilitating the process.

We next advance a further step, only to embrace another phase of the same truth,the complement of what has been already stated, that is, the conservation of energy.