I. The first is based upon the undoubted fact that the tides tend to r.e.t.a.r.d the rate of the earth"s rotation upon its axis. That this must be so is obvious, if one considers, roughly, that the tides result from the pull which the sun and the moon exert upon the sea, causing it to act as a sort of break upon the rotating solid earth.
Kant, who was by no means a mere "abstract philosopher," but a good mathematician and well versed in the physical science of his time, not only proved this in an essay of exquisite clearness and intelligibility, now more than a century old,[54] but deduced from it some of its more important consequences, such as the constant turning of one face of the moon towards the earth.
But there is a long step from the demonstration of a tendency to the estimation of the practical value of that tendency, which is all with which we are at present concerned. The facts bearing on this point appear to stand as follow:--
It is a matter of observation that the moon"s mean motion is (and has for the last 3,000 years been) undergoing an acceleration, relatively to the rotation of the earth. Of course this may result from one of two causes: the moon may really have been moving more swiftly in its...o...b..t; or the earth may have been rotating more slowly on its axis.
Laplace believed he had accounted for this phaenomenon by the fact that the eccentricity of the earth"s...o...b..t has been diminishing throughout these 3,000 years. This would produce a diminution of the mean attraction of the sun on the moon; or, in other words, an increase in the attraction of the earth on the moon: and, consequently, an increase in the rapidity of the orbital motion of the latter body. Laplace, therefore, laid the responsibility of the acceleration upon the moon; and if his views were correct, the tidal r.e.t.a.r.dation must either be insignificant in amount, or be counteracted by some other agency.
Our great astronomer, Adams, however, appears to have found a flaw in Laplace"s calculation, and to have shown that only half the observed r.e.t.a.r.dation could be accounted for in the way he had suggested. There remains, therefore, the other half to be accounted for; and here, in the absence of all positive knowledge, three sets of hypotheses have been suggested.
(a) M. Delaunay suggests that the earth is at fault, in consequence of the tidal r.e.t.a.r.dation. Messrs. Adams, Thomson, and Tait work out this suggestion, and, "on a certain a.s.sumption as to the proportion of r.e.t.a.r.dations due to the sun and the moon," find the earth may lose twenty-two seconds of time in a century from this cause.[55]
(b) But M. Dufour suggests that the r.e.t.a.r.dation of the earth (which is hypothetically a.s.sumed to exist) may be due in part, or wholly, to the increase of the moment of inertia of the earth by meteors falling upon its surface. This suggestion also meets with the entire approval of Sir W. Thomson, who shows that meteor-dust, acc.u.mulating at the rate of one foot in 4,000 years, would account for the remainder of r.e.t.a.r.dation.[56]
(c) Thirdly, Sir W. Thomson brings forward an hypothesis of his own with respect to the cause of the hypothetical r.e.t.a.r.dation of the earth"s rotation:--
"Let us suppose ice to melt from the polar regions (20 round each pole, we may say) to the extent of something more than a foot thick, enough to give 1.1 foot of water over those areas, or 0.006 of a foot of water if spread over the whole globe, which would, in reality, raise the sea-level by only some such undiscoverable difference as three-fourths of an inch or an inch. This, or the reverse, which we believe might happen any year, and could certainly not be detected without far more accurate observations and calculations for the mean sea-level than any hitherto made, would slacken or quicken the earth"s rate as a timekeeper by one-tenth of a second per year."[57]
I do not presume to throw the slightest doubt upon the accuracy of any of the calculations made by such distinguished mathematicians as those who have made the suggestions I have cited. On the contrary, it is necessary to my argument to a.s.sume that they are all correct. But I desire to point out that this seems to be one of the many cases in which the admitted accuracy of mathematical processes is allowed to throw a wholly inadmissible appearance of authority over the results obtained by them. Mathematics may be compared to a mill of exquisite workmanship, which grinds you stuff of any degree of fineness; but, nevertheless, what you get out depends on what you put in; and as the grandest mill in the world will not extract wheat-flour from peascods, so pages of formulae will not get a definite result out of loose data.
In the present instance it appears to be admitted:--
1. That it is not absolutely certain, after all, whether the moon"s mean motion is undergoing acceleration, or the earth"s rotation r.e.t.a.r.dation.[58] And yet this is the key of the whole position.
2. If the rapidity of the earth"s rotation is diminishing, it is not certain how much of that r.e.t.a.r.dation is due to tidal friction,--how much to meteors,--how much to possible excess of melting over acc.u.mulation of polar ice, during the period covered by observation, which amounts, at the outside, to not more than 2,600 years.
3. The effect of a different distribution of land and water in modifying the r.e.t.a.r.dation caused by tidal friction, and of reducing it, under some circ.u.mstances, to a minimum, does not appear to be taken into account.
4. During the Miocene epoch the polar ice was certainly many feet thinner than it has been during, or since, the Glacial epoch. Sir W.
Thomson tells us that the acc.u.mulation of something more than a foot of ice around the poles (which implies the withdrawal of, say, an inch of water from the general surface of the sea) will cause the earth to rotate quicker by one-tenth of a second per annum. It would appear, therefore, that the earth may have been rotating, throughout the whole period which has elapsed from the commencement of the Glacial epoch down to the present time, one, or more, seconds per annum quicker than it rotated during the Miocene epoch.
But, according to Sir W. Thomson"s calculation, tidal r.e.t.a.r.dation will only account for a r.e.t.a.r.dation of 22" in a century, or 22/100 (say 1/5) of a second per annum.
Thus, a.s.suming that the acc.u.mulation of polar ice since the Miocene epoch has only been sufficient to produce ten times the effect of a coat of ice one foot thick, we shall have an accelerating cause which covers all the loss from tidal action, and leaves a balance of 4/5 a second per annum in the way of acceleration.
If tidal r.e.t.a.r.dation can be thus checked and overthrown by other temporary conditions, what becomes of the confident a.s.sertion, based upon the a.s.sumed uniformity of tidal r.e.t.a.r.dation, that ten thousand million years ago the earth must have been rotating more than twice as fast as at present, and, therefore, that we geologists are "in direct opposition to the principles of Natural Philosophy" if we spread geological history over that time?
II. The second argument is thus stated by Sir W. Thomson:--"An article, by myself, published in "Macmillan"s Magazine" for March 1862, on the age of the sun"s heat, explains results of investigation into various questions as to possibilities regarding the amount of heat that the sun could have, dealing with it as you would with a stone, or a piece of matter, only taking into account the sun"s dimensions, which showed it to be possible that the sun may have already illuminated the earth for as many as one hundred million years, but at the same time rendered it almost certain that he had not illuminated the earth for five hundred millions of years. The estimates here are necessarily very vague; but yet, vague as they are, I do not know that it is possible, upon any reasonable estimate founded on known properties of matter, to say that we can believe the sun has really illuminated the earth for five hundred million years."[59]
I do not wish to "Hansardize" Sir William Thomson by laying much stress on the fact that, only fifteen years ago, he entertained a totally different view of the origin of the sun"s heat, and believed that the energy radiated from year to year was supplied from year to year--a doctrine which would have suited Hutton perfectly. But the fact that so eminent a physical philosopher has, thus recently, held views opposite to those which he now entertains, and that he confesses his own estimates to be "very vague," justly ent.i.tles us to disregard those estimates, if any distinct facts on our side go against them. However, I am not aware that such facts exist. As I have already said, for anything I know, one, two, or three hundred millions of years may serve the needs of geologists perfectly well.
III. The third line of argument is based upon the temperature of the interior of the earth. Sir W. Thomson refers to certain investigations which prove that the present thermal condition of the interior of the earth implies either a heating of the earth within the last 20,000 years of as much as 100 F., or a greater heating all over the surface at some time further back than 20,000 years, and then proceeds thus:--
"Now, are geologists prepared to admit that, at some time within the last 20,000 years, there has been all over the earth so high a temperature as that? I presume not; no geologist--no _modern_ geologist--would for a moment admit the hypothesis that the present state of underground heat is due to a heating of the surface at so late a period as 20,000 years ago. If that is not admitted, we are driven to a greater heat at some time more than 20,000 years ago. A greater heating all over the surface than 100 Fahrenheit would kill nearly all existing plants and animals, I may safely say. Are modern geologists prepared to say that all life was killed off the earth 50,000, 100,000, or 200,000 years ago? For the uniformity theory, the further back the time of high surface-temperature is put the better; but the further back the time of heating, the hotter it must have been. The best for those who draw most largely on time is that which puts it furthest back; and that is the theory that the heating was enough to melt the whole. But even if it was enough to melt the whole, we must still admit some limit, such as fifty million years, one hundred million years, or two or three hundred million years ago. Beyond that we cannot go."[60]
It will be observed that the "limit" is once again of the vaguest, ranging from 50,000,000 years to 300,000,000. And the reply is, once more, that, for anything that can be proved to the contrary, one or two hundred million years might serve the purpose, even of a thorough-going Huttonian uniformitarian, very well.
But if, on the other hand, the 100,000,000 or 200,000,000 years appear to be insufficient for geological purposes, we must closely criticise the method by which the limit is reached. The argument is simple enough.
_a.s.suming_ the earth to be nothing but a cooling ma.s.s, the quant.i.ty of heat lost per year, _supposing_ the rate of cooling to have been uniform, multiplied by any given number of years, will be given the minimum temperature that number of years ago.
But is the earth nothing but a cooling ma.s.s, "like a hot-water jar such as is used in carriages," or "a globe of sandstone?" and has its cooling been uniform? An affirmative answer to both these questions seems to be necessary to the validity of the calculations on which Sir W. Thomson lays so much stress.
Nevertheless it surely may be urged that such affirmative answers are purely hypothetical, and that other suppositions have an equal right to consideration.
For example, is it not possible that, at the prodigious temperature which would seem to exist at 100 miles below the surface, all the metallic bases may behave as mercury does at a red heat, when it refuses to combine with oxygen; while, nearer the surface, and therefore at a lower temperature, they may enter into combination (as mercury does with oxygen a few degrees below its boiling-point) and so give rise to a heat totally distinct from that which they possess as cooling bodies?
And has it not also been proved by recent researches that the quality of the atmosphere may immensely affect its permeability to heat; and, consequently, profoundly modify the rate of cooling the globe as a whole?
I do not think it can be denied that such conditions may exist, and may so greatly affect the supply, and the loss, of terrestrial heat as to destroy the value of any calculations which leave them out of sight.
My functions as your advocate are at an end. I speak with more than the sincerity of a mere advocate when I express the belief that the case against us has entirely broken down. The cry for reform which has been raised without, is superfluous, inasmuch as we have long been reforming from within, with all needful speed. And the critical examination of the grounds upon which the very grave charge of opposition to the principles of Natural Philosophy has been brought against us, rather shows that we have exercised a wise discrimination in declining, for the present, to meddle with our foundations.
FOOTNOTES:
[39] On Geological Time. By Sir W. Thomson, LL.D. Transactions of the Geological Society of Glasgow, vol. iii.
[40] The Theory of the Earth, vol. i. p. 173, note.
[41] Ibid. p. 281.
[42] Ibid. p. 371.
[43] The Theory of the Earth, vol. i. p. 200.
[44] The Theory of the Earth, vol. i. pp. 16, 17.
[45] The Theory of the Earth, vol. i. p. 223.
[46] Principles of Geology, vol. ii. p. 211.
[47] Principles of Geology, vol. ii. p. 613.
[48] "Man darf es sich also nicht befremden la.s.sen, wenn ich mich unterstehe zu sagen, da.s.s eher die Bildung aller Himmelskorper, die Ursache ihrer Bewegungen, kurz der Ursprung der ganzen gegenwartigen Verfa.s.sung des Weltbaues werden konnen eingesehen werden, ehe die Erzeugung eines einzigen Krautes oder einer Raupe aus mechanischen Grunden, deutlich und vollstandig kund werden wird."--KANT"S _Sammtliche Werke_, Bd. I. p. 220.
[49] Grant ("History of Physical Astronomy," p. 574) makes but the briefest reference to Kant.
[50] "Allgemeine Naturgeschichte und Theorie des Himmels; oder Versuch von der Verfa.s.sung und dem mechanischen Ursprunge des ganzen Weltgebaudes nach Newton"schen Grundsatzen abgehandelt."--KANT"S _Sammtliche Werke_, Bd. i. p. 207.
[51] Systeme du Monde, tome ii. chap. 6
[52] Kant"s "Sammtliche Werke," Bd. viii. p. 145.
[53] Sir William Thomson implies (loc. cit. p. 16), that the precise time is of no consequence: "the principle is the same;" but, as the principle is admitted, the whole discussion turns on its practical results.
[54] "Untersuchung der Frage ob die Erde in ihrer Umdrehung um die Achse, wodurch sie die Abwechselung des Tages und der Nacht hervorbringt, einige Veranderung seit den ersten Zeiten ihres Ursprunges erlitten habe, &c."--KANT"S _Sammtliche Werke_, Bd. i. p. 178.
[55] Sir W. Thomson, loc. cit., p. 14.