Motion is a constant quantity; “ The sum of all the motions in the universe is always tho sojno." This sentence placed at the foot of a column in a recent issue of our paper, has attracted the attention of a correspondent, who, while admitting its truth, says he finds it " hard to solve all the perplexing problems that grow out of such an admission. For instance, suppose a terrible conflagration to take place at midnight. Thousands of persons awake from sleep and rush to the fire. Where so many are rushing, in what form would thatmotion have been manifested, if there had been no fire and the people had remained in bed V The doctrine of the perpetuity and indestructibility of mo- I tion involves the truth that all motions originate, or are increased by subtractions from other pre-existing motions, or cease, or become diminished, only by imparting motion. The difficulty in accounting for the origin of a new motion, arises chiefly from not clearly comprehending the distinction between mass motion and molecular motion. In the motion of a mass, the relative position of its geometrical center is con- stantly changed. Molecular motion may exist in a body without any relative change in the position of its geometrical center. When mass motion suddenly appears, without being immediately caused by other mass motion, it results from the immediate conversion of molecular motion. j Of all the molecular motions heat is the one most concerned in the direct production of mass motion. The case suggested by OUT correspondent, of people suddenly aroused from sleep into action, is analogous to that of a locomotive standing in a depot with steam up, and then suddenly, by the simple act of the engineer, expending the power confined in the boiler in the propulsion of itself and its load. All the motion that it and the train it draws possess after starting, existed previously in the form of heat in the furnace and boiler, and molecular motions of the coal in the tender and oxygen in the atmosphere, which, when chemical combination (combustion) takes place between these elements, are converted into heat, which in its turn is converted into mass motion. Men and animals are locomotives. Their food is the fuel whch drives them; their wills are the engineers which control them. The fuel (food), which is put into their furnaces (stomachs), is however applied to two purposes. Part is expended in warming the machine and part is stored up in the various tissues of the body, to be consumed either for warmth I or motion, as occasion may require. But because it is thus I stored up, it must not be inferred that motion does not exist in it. It may or may not possess mass motion, according to the state of action or repose in which the animal chances to be; but in all cases where mass motion of a living body exists, as an act of the will, consumption of tissue also takes place, that is, a change of molecular motion into mass motion. After the crowd have rushed to the fire and rushed back again, their aggregate weight will have been considerably reduced, and they will find it necessary to " coal up" next morning at breakfast to make up for the loss. Thus we see that in the case cited there is no difficulty in referring the mass motion, suddenly resulting from the interposition of will, to previously existing molecular motion. In all other cases, although in some the connection between a mass motion and pre-existent molecular motions may be difficult to trace, there can be no reasonable doubt of its existence; and in the light of modern science it is certain that the sum of all the motions in the universe is a constant quantity.