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Mississippi. Sir C. Lyell, from the present amount of solid matter conveyed by it, and the area and depth of the accumulation near its mouth, inferred that 67,000 years would be needed for the Delta proper, and 33,000 more for the plain above to be transported to its present site. Hence he speaks of the whole period as “perhaps far exceeding 100,000 years." But in 1869 he says that “the data had considerably altered since first he wrote. Recent calculations had doubled the volume of water flowing into the sea, and thus the same effect might be produced in half the time previously calculated.” Thus 50,000 years were struck off by the first correction.

But now let us assume, instead of a fixed annual amount of detritus, that there has been a steady decrease of only one four hundredth part of the present quantity. The 50,000 years would then reduce themselves to 5,937, which would bring the commencement of the process within the limits of the known or biblical age of mankind.

8. Again, Mr. Croll makes a calculation, that the same river at its present rate would carry down the whole area drained by it to the sea-level in 41 million years. But, adopting a similar law, or supposing the decrease each year to be only one part in a thousand of the present amount, how long would have been needed to waste away a double quantity of land or rock to its present amount ? Rather less than 94,000 years.

The same principle applies to the mud of the Nile, and a vast number of cases of a similar kind. The doctrine of averages, when so applied, rests on a mere assumption, not only unproved, but highly improbable, and almost certainly untrue. In a single year of high flood a river may transport an amount and kind of material, which could not have been removed by a hundred years in which no flooding has occurred.

9. The case is the same as to upheaval and volcanic eruptions. It is plain that whenever the crust is broken through, and a stream of lava, before pent in, comes from below, the motive force must tend to exhaust itself by the effort. The heat, generated by internal pressure, will partly escape through the opening, while the pressure also is lessened by the rupture of the crust. The approach must be constantly towards a limit, when the upward and expansive force has spent itself, and though the renewal may have gone on through long ages, the first intensity or amount of action can never return. The process of condensation, with the generation of internal heat, and its conflict with the cooling ocean at the surface, or the intense cold of the interstellar spaces, has a natural limit, beyond which it cannot go, and to which it must approach more and more slowly as the change proceeds.

III.—THE THERMO-DYNAMIC THEORY.

initial energy

10. The doctrine of uniformity, in its extreme form, as held by Sir C. Lyell and many others, has found of late some strong opponents among our foremost analysts. Sir W. Thomson and Professor Tait would replace it by what may be called a Thermo-dynamic theory. They maintain that the solar energy is in process of constant dissipation, and that hypotheses assuming an average constancy of sun and storms for a million years

“cannot possibly be true.” It is quite certain, Sir William thinks, that the solar system cannot have gone on as

present for a few million years, without the irrevocable loss, by dissipation, of a very considerable portion of the entire

. He calculates, from Fourier's theory of the rate of conduction, and the specific heat of rocks at Edinburgh and Greenock, that the consolidation of the earth's crust cannot have taken place less than 20 nor more than 400 millions of years ago; also that the general climate cannot have been sensibly affected by conducted heat from the centre, except within the first 10,000 years after the solidification, and that in 96 millions of years the thickness of the crust, through which a given amount of cooling would be experienced, would have increased fivefold. He admits that a wholly different view is maintainable, that internal heat is due to chemical combination, going on slowly everywhere at great unknown depths, and creeping onward gradually as the chemical affinities of each layer are saturated. But he thinks also that " the less hypothetical view, that the earth is merely a warm, chemically inert body, cooling, is clearly to be preferred in the present state of science."

11. The objection may be urged, that the earth cannot well be supposed ever to have been a solid, uniformly heated, and 7,0000 warmer than the present heat of the surface, which is the hypothesis assumed. But Sir William replies that the solution may be easily modified, to meet the case of a liquid gradually becoming solid, at least when three fresh data hare been supplied. And he argues further that the earth, “although once all melted, did in all probability become a solid at its melting temperature all through, or all through the outer layer which had been melted; and that not until it was thus completely solidified, or nearly so, did the crust begin to cool."

12. It is clear, from this very statement, how much remains merely hypothetical in this solution, on which the calculation of the age of the earth's crust is to depend. Professor Tait has since replaced the estimate of the limits of 20 and 400 millions of years by a suggested period of 10 millions only. In the statement quoted it is owned that three further data must be supplied, before the solution can be altered so as to suit the real conditions. The view, which Sir William rejects as more hypothetical, that the heat is generated by chemical change, seems to me less hypothetical and more natural than his own; and needs only to be carried a step further and applied to the formation of the chemical elements themselves, by pressure, to supply a far more complete solution of the great problem.

The rejection of uniformity of action through many millions of years is justified, I conceive, on many grounds. But instead of grounding it on the certain steady decrease of solar heat by exhaustion and dissipation, I think it may be based more reasonably on the opposite ground of its increase. For if the present amount has ensued after solar condensation, and the sun was once a diffused mass of low temperature, variation by increase for long ages must be one constituent element of the theory; but a reversal of the process, and a greater loss than gain of heat for many millions of years must be wholly improbable in the absence of any direct experimental evidence.

13. Those theorems of Fourier, on which the reckoning is based, all rest on the hypothesis that the heat transferred from a hot to a cool body is strictly as the difference of their temperatures, and that the temperature is the quotient of the amount of heat in any body, divided by the mass. This implies the hypothesis that heat is a specific fluid. For it reasons as if the total heat of the system, between the parts of which conduction takes place, were a fixed quantity, not capable of increase or diminution, by forces generating motion, or motion being extinguished by expansion. But the opposite view, the doctrine of Bacon and Rumford, that heat is simply atomic motion, is now fully established, and Sir William is one of those who have had no mean share in its confirmation. Hence the conditions of the problem of conduction, for long periods, must be wholly altered. There is no longer a fixed amount of heat, of which a small part is transferred by a definite law from a hot to a cooler body. It may be generated in the one by condensation, and conversely by expansion be destroyed in the other to an unknown extent. Potential may be turned into kinetic energy on one side, and on the other kinetic into potential. There may thus be both an indefinite demand, and an equally unlimited supply. The real problem will depend mainly on these two elements, which are entirely absent in the solution Sir William has proposed. The calculation is really a partial survival from that fluid-caloric theory which is now universally abandoned.

14. The doctrine of uniformity, as held by Sir C. Lyell, rests on a confusion of two things wholly distinct,-the constancy of natural laws, such as gravitation and cohesive affinity, and the sameness of the conditions under which they operate at widely separated periods of time. But these conditions are changing hourly through the action of the laws themselves, and the difference in the course of ages becomes so great as wholly to falsify any conclusions which are based on the assumption of their near approach to identity. I fully agree, then, with Sir W. Thomson, in his protest against that theory; but I cannot accept, as reasonable or true, the special ground on which he bases his opposition. Mr. Croll sets the two doctrines in contrast in the following passage, which shows the immense scale of time adopted by uniformitarian theorists.

" It was the modern doctrine that the great changes undergone by the earth's crust were produced not by convulsions of nature, but by the slow and almost imperceptible action of sun, rivers, snow, frost, ice, which impressed so strongly on the minds of geologists the vast duration of geological periods. When it was considered that the rocky face of our globe had been carved into hill and dale, and worn down to the sea-level by these apparently trifling agents, not once or twice but many times, in past ages, it was not surprising that the views entertained by geologists on the immense antiquity of our globe should not have harmonized with the deductions of physical science. It had been shown by Sir W. Thomson and others, from physical considerations of the sun's heat and the secular cooling of our globe, that the history of the earth's crust must be limited to a period of something like a hundred millions

But these speculations had little weight when pitted against the stern and undeniable fact of subaërial denudation. How were the two to be reconciled ? Was it the physicist who had under-estimated geological time, or the gelogist who had over-estimated it? Few familiar with modern physics, who have given attention to the subject, would admit that the sun could have been dissipating his heat at the present enormous rate for a period much beyond a hundred millions of years.'

15. In this conflict of the two theories, I believe that there

of years.

mass,

is an almost equal error on each side. Each theory is based on data wholly insufficient to establish its truth. The doctrine of uniformity, I believe, is untrue for many reasons, but not for the reason which Mr. Croll, following Sir W. Thomson, has assigned. There is no proof that the sun was much hotter a hundred or fifty millions of years ago than at present. If there be a difference, which is probable, I think it much more likely that it would be of an opposite kind, and that its heat has increased by condensation, more than it has lost by dissipation. In the " Theory of Helmholtz,” which Sir William has latterly espoused, having abandoned Meyer's meteoric hypothesis, the heat of the sun is now thought to be supplied by condensation, which replaces the ceaseless waste from dissipation or radiation into space. Now if the sun has reached its present high state of heat and light from an earlier stage, when it was neither hot nor luminous, what proof can there be that the process has been reversed for the last million of years, and the waste exceeded the supply for so long? But this very idea, that all the heat radiated into space is dissipated and lost, is an assumption without solid reason. If it arose at first from a transformation of potential into kinetic energy, or attractive force into motion, by the condensation of the solar

it can only cease or be lost by a reconversion of this kinetic energy into potential energy of another kind; namely, the condensation of repulsive ether. Thus the energy which flows out from the sun as sensible heat and light, in the sector of space bordering on the sun's equator, will return to it in. visibly and insensibly, in the neighbourhood of the poles, and-the sun would thus be an immense magnet by virtue of its revolution.

16. The general climate of the earth, Sir W. Thomson further remarks, “cannot have been sensibly affected by conducted heat, at any time more than ten thousand years after the solidification of the surface.” This may be true, if we take the phrase "conducted heat” in a rigorous sense, and exclude all liquefaction, convection, regelation, or fresh generation of heat by condensation from pressure or chemical change. But these omitted or excluded elements are those of chief importance in the actual problem. A solution which omits them may be true as an abstract dynamical theorem, but can have little bearing on the actual course of geological change.

17. The first volume of Sir W. Thomson's and Professor Tait's comprehensive “Treatise on Natural Philosophy closes with these remarks on the once current hypothesis of the earth's fluidity below a thin superficial crust.

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