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PART II. ON THE ORIGIN AND TOTAL AMOUNT OF PLUTONIC ENERGY.

30. By Plutonic action, I mean any disturbance of underground equilibrium. Volcanoes, earthquakes, and subsidences are the phenomena most commonly understood when plutonic activity is spoken of. The store of energy to which these phenomena are due is properly called plutonic energy, and according to the clear and simple, but thoroughly rigorous, language of modern dynamics, plutonic energy is to be distinguished from plutonic activity.

31. The action of a dynamical agent was defined by Newton, as something to be measured numerically, by the number measuring simple force or pressure, multiplied into the number measuring the velocity with which the matter experiencing it yields in the direction of the force. In the nineteenth century dynamical vocabulary, Newton's "action of an agent" is simply a performing of

work, and we distinguish between action, or rate of action, as defined by Newton, and the integral amount of action or integral amount of work done after any operation of force is completed. Again, in modern physical dynamics we have learned that every performance of work consists in merely a transformation or intertransposition of materials, or a stopping of some motion and generating of other instead, and that when work is performed in one locality, another locality must on that account be left with so much less of the wherewithal for the farther performance of work. This "wherewithal' is called energy; and thus the performance of work is simply the drawing of energy from one store and laying it out elsewhere. Any irreversible transformation of energy is called a dissipation of energy; of which the most prominent examples are the conduction of heat from warmer to colder parts of a body, or of the matter occupying any portion of space, and the generation of heat by friction or collision.

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32. Plutonic action is, therefore, to be defined as any transformation of energy going on within the

earth. No natural operation is thoroughly reversible, and therefore, every plutonic action involves something in the way of dissipation of energy. But the grand and awful phenomena of volcanoes and earthquakes, results of abnormal plutonic activity, give rise probably to much less dissipation of energy, summed for all parts of the earth from age to age, than the continual silent action of the conduction of heat outwards, the amount of which we are able to estimate in a thoroughly definite manner. Thus we find that from year to year the earth, at the present time, is parting with heat at the rate of 92 horse-power 1 per square kilometre.2 That is to say, from a square metre of surface the loss of energy is at an average rate of seven metre

1 "One horse-power" is a rate of performing work equal to (33,000 foot pounds, or) 4'563 metre-tons per minute; the French ton of 1000 kilogrammes understood, being 9842 of the British

ton.

2 The kilometre is 62138 of that very inconvenient measure, the British statute mile. The square kilometre is 247°11 of that, if possible worse measure, the acre. Experts can tell how many square yards are in an acre: but of all the men in England accustomed to reckon their land in acres, and to state, or read, or hear reckonings of political statistics in square miles, very few could readily answer the question, How many acres are there in a square mile?

tons per million seconds, or 220 metre-tons per annum. The whole area of the earth is 510,000,000 square kilometres; and therefore the loss from the whole earth is 3600 millions of metre-tons per second, or 112 x 1015 metre-tons per annum. This statement is not hypothetical in any respect. But the numerical data assumed in it, being 005 gramme-water-units per centimetre per second for conductivity, and 1° cent. per 30 metres for the rate of increase of underground temperature downwards, are what Professor Huxley would justly call loose, because we do not know the true average conductivity of the upper strata for the whole earth, nor the true average value of the rate of augmentation of temperature per metre downwards; and a considerable margin of probable error must be allowed for any estimate that can yet be made of the true rate at which energy is being lost from the earth. This, however, does not at all affect the principles in illustration of which I adduce the numbers, or the importance of these principles for the success of geology as a science.

33. The store of energy, transformations of

which constitute plutonic action, consists certainly at the present time in a great measure, if not altogether, of terrestrial heat. This indeed is the only description of energy proved to exist in any considerable quantity within the earth; but it is possible that there may be great masses of uncombined chemical elements, and that the potential energy of their mutual affinities may constitute a considerable portion of the plutonic energy in store, whether for the generation of future underground heat, or for immediate application to some of the more violent manifestations of plutonic activity. Now, there are two ways of estimating the possible total amount of plutonic energy; one by taking the earth as it is, and not reasoning from antecedent conditions, but simply estimating from known properties of matter; how much heat it is conceivable may exist in it in its present condition; the other by tracing the history of the earth backwards.

34. From experiments such as have not yet been made, but could be made with very great ease, on the total heats of fusion of ordinary rocks and

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