Climate and Time in Their Geological Relations / A Theory of Secular Changes of the Earth's Climate

by James Croll

A Confusion of Ideas in Reference to the supposed Agency of Polar Cold.—It seems to me that Dr. Carpenter has been somewhat misled by a slight confusion of ideas in reference to the supposed agency of polar cold. This is brought out forcibly in the following passage from his memoir in the Proceedings of the Royal Geographical Society, vol. xv.

“Mr. Croll, in arguing against the doctrine of a general oceanic circulation sustained by difference of temperature, and justly maintaining that such a circulation cannot be produced by the application of heat at the surface, has entirely ignored the agency of cold.”

It is here supposed that there are two agents at work in the production of the general oceanic circulation. The one agent is heat, acting at the equatorial regions; and the other agent is cold, acting at the polar regions. It is supposed that the agency of cold is far more powerful than that of heat. In fact so trifling is the agency of equatorial heat in comparison with that of polar cold that it may be “practically disregarded”—left out of account altogether,—polar cold being the primum mobile of the circulation. It is supposed also that I have considered the efficiency of one of the agents, viz., heat, and found it totally inadequate to produce the circulation in question; and it is admitted also that my conclusions are perfectly correct. But then I am supposed to have left out of account the other agent, viz., polar cold, the only agent possessing real potency. Had I taken into account polar cold, it is supposed that I should have found at once a cause perfectly adequate to produce the required effect.

This is a fair statement of Dr. Carpenter’s views on the subject; I am unable, at least, to attach any other meaning to his words. And I have no doubt they are also the views which have been adopted by those who have accepted his theory.

It must be sufficiently evident from what has already been stated, that the notion of there being two separate agents at work producing circulation, namely heat and cold, the one of which is assumed to have much more potency than the other, is not only opposed to the views entertained by physicists, but is also wholly irreconcilable with the ordinary principles of mechanics. But more than this, if we analyze the subject a little so as to remove some of the confusion of ideas which besets it, we shall find that these views are irreconcilable with even Dr. Carpenter’s own explanation of the cause of the general oceanic circulation.

Cold is not a something positive imparted to the polar waters giving them motion, and of which the tropical waters are deprived. If, dipping one hand into a basin filled with tropical water at 80° and the other into one filled with polar water at 32°, we refer to our sensations, we call the water in the one hot and that in the other cold; but so far as the water itself is concerned heat and cold simply mean difference in the amounts of heat possessed. Both the polar and the tropical water possess a certain amount of energy in the form of heat, only the polar water does not possess so much of it as the tropical.

How, then, according to Dr. Carpenter, does polar cold impart motion to the water? The warm water flowing in upon the polar column becomes chilled by cold, but it is not cooled below that of the water underneath; for, according to Dr. Carpenter, the ocean in polar regions is as cold and as dense underneath as at the surface. The cooled surface-water does not sink through the water underneath, like the surface-water of a pond chilled during a frosty night. “The descending motion in column C will not consist,” he says, “in a successional descent of surface-films from above downwards, but it will be a downward movement of the entire mass, as if water in a tall jar were being drawn off through an orifice at the bottom” (§ 29). There is a downward motion of the entire column, producing an outflow of water at the bottom towards the equatorial column W, which outflow is compensated by an inflow from the top of the equatorial column to the top of the polar column C. But what causes column C to descend? The cause of the descent is its excess of weight over that of column W. Column C descends and column W ascends, for the same reason that in a balance the heavy scale descends and the light scale rises. Column C descends not simply because it is cold, but because it is colder than column W. Column C descends not simply because in consequence of being cold it is dense and therefore heavy, but because in consequence of being cold it is denser and therefore heavier than column W. It might be as cold as frozen mercury and as heavy as lead; but it would not on that account descend unless it were heavier than column W. The descent of column C and ascent of column W, and consequently the general oceanic circulation, results, therefore, according to Dr. Carpenter’s explanation, from the difference in the weights of the two columns; and the difference in the weights of the two columns results from their difference of density; and the difference of density of the two columns in turn results from their difference of temperature. But it has already been proved that the difference of temperature between the polar and equatorial columns depends wholly on the difference in the amount of heat received by each from the sun. The equatorial column W possesses more heat than the polar column C, solely because it receives more heat from the sun than column C. Consequently Dr. Carpenter’s statement that the circulation is produced by polar cold rather than by equatorial heat, is just as much in contradiction to his own theory as it is to the principles of mechanics. Again, his admission that the general oceanic circulation “cannot be produced by the application of heat to the surface,” is virtually a giving up the whole point in debate; for according to his gravitation theory, and every form of that theory, the circulation results from difference of temperature between equatorial and polar seas; but this difference, as we have seen, is entirely owing to the difference in the amount of heat received from the sun at these two places. The heat received, however, is “surface-heat;” for it is at the surface that the ocean receives all its heat from the sun; and consequently if surface-heat cannot produce the effect required, nothing else can.

M. Dubuat’s Experiments.—Referring to the experiments of M. Dubuat adduced by me to show that water would not run down a slope of 1 in 1,820,000,[82] he says, “Now the experiments of M. Dubuat had reference, not to the slow restoration of level produced by the motion of water on itself, but to the sensible movement of water flowing over solid surfaces and retarded by its friction against them” (§ 22). Dr. Carpenter’s meaning, I presume, is that if the incline consist of any solid substance, water will not flow down it; but if it be made of water itself, water will flow down it. But in M. Dubuat’s experiments it was only the molecules in actual contact with the solid incline that could possibly be retarded by friction against it. The molecules not in contact with the solid incline evidently rested upon an incline of water, and were at perfect liberty to roll down that incline if they chose; but they did not do so; and consequently M. Dubuat’s experiment proved that water will not flow over itself on an incline of 1 in 1,000,000.

A Begging of the Question at Issue.—“It is to be remembered,” says Dr. Carpenter, “that, however small the original amount of movement may be, a momentum tending to its continuance must be generated from the instant of its commencement; so that if the initiating force be in constant action, there will be a progressive acceleration of its rate, until the increase of resistance equalises the tendency to further acceleration. Now, if it be admitted that the propagation of the disturbance of equilibrium from one column to another is simply retarded, not prevented, by the viscosity of the liquid, I cannot see how the conclusion can be resisted, that the constantly maintained difference of gravity between the polar and equatorial columns really acts as a vis viva in maintaining a circulation between them” (§ 35).

If it be true, as Dr. Carpenter asserts, that in the case of the general oceanic circulation advocated by him “viscosity” simply retards motion, but does not prevent it, I certainly agree with him “that the constantly maintained difference of gravity between the polar and equatorial columns really acts as a vis viva in maintaining a circulation between them.” But to assert that it merely retards, but does not prevent, motion, is simply begging the question at issue. It is an established principle that if the force resisting motion be greater than the force tending to produce it, then no motion can take place and no work can be performed. The experiments of M. Dubuat prove that the force of the molecular resistance of water to motion is greater than the force derived from a slope of 1 in 1,000,000; and therefore it is simply begging the question at issue to assert that it is less. The experiments of MM. Barlow, Rainey, and others, to which he alludes, are scarcely worthy of consideration in relation to the present question, because we know nothing whatever regarding the actual amount of force producing motion of the water in these experiments, further than that it must have been enormously greater than that derived from a slope of 1 in 1,000,000.