The conjecture that the sun is a variable star is also negatived by the consideration that in this case there ought to have been periodical variations in the earth's temperature, and hot and cold climates recurring at regular intervals throughout geological time, which has certainly not been the case.
Again, the passage of the solar system through cold regions of space has been suggested, but it is a mere conjecture, unsupported by a particle of evidence, and opposed to all we know of the laws of heat, and of the constitution of the universe. It is hard to conceive how hot regions can exist surrounded by cold ones, or vice versâ, without walls of a non-conducting medium to separate them, or that the faint heat from the fixed stars can ever have perceptibly affected the temperature of space. And such a theory, if it were possible, would fail to account for the frequent vicissitudes of hot and cold at short intervals within the glacial period, and for the great differences of temperature prevailing in the same latitudes.
An alteration in the position, of the poles has also been suggested, but this also is clearly inadmissible. There is no evidence that the present position has ever materially varied, and there is no known law that could cause such a variation. On the contrary, all the elaborate mathematical calculations by which the motions of the sun, moon, and planets are deduced from Newton's law of gravity, tend to negative such a supposition.
And what is perhaps even more convincing to a nonmathematical mind, the position of the poles implies the position of the equator, and cannot change without a corresponding change in the earth's shape. Now the earth is not a sphere, but an oblate spheroid, of almost the exact shape which a fluid mass would take revolving about the present axis. The centrifugal force arising from the greater velocity of rotation in going from the poles to the equator would pile up a protuberant belt where the velocity was greatest, and in point of fact the earth's equatorial diameter is longer than the polar diameter by about twenty-eight miles. Any displacement therefore of the poles, which carried them away from their present position, must displace the present equator to a corresponding extent. This mass of twenty-eight miles in thickness of earth and ocean must be thrown out of the old position, and driven to establish a new equilibrium in a position many degrees north or south of it in order to affect climates materially, submerging all existing lands, and leaving, until removed by denudation, miles upon miles of solid earth in unsymmetrical belts, like the moraines of retreating glaciers, as the equator shifted into new positions. And all this must have occurred, not once, but twice at least, and that with many minor vicissitudes, within the narrow limit of the quaternary period. It is unnecessary to say that nothing of the sort could by any possibility have occurred. Some evidence has recently been adduced that some very slight changes in latitude are going on at the observatories of Dorpat and Greenwich, but if confirmed these can only be of very minute amount, arising from slight changes in the position of the earth's centre of gravity owing to partial elevations and depressions, and could never have been sufficient to account for great variations of climate.[11]
Neither could the precession of the equinoxes have been of itself a principal cause, for here also the limit of time negatives the supposition. This precessional circle carries the perihelion and aphelion, and with it the seasons, completely round, and brings them back to the old position, in about 21,000 years, and therefore if glacial periods were occasioned by them, there ought to be alternations from maximum of cold to maximum of warmth in each hemisphere every 10,500 years. But this has certainly not been the case even in recent times, and still less if we go back to the quaternary, tertiary, and earlier geological periods.
In fact it is only when combined with periods of high eccentricity of the earth's orbit, according to Croll's theory, that precession can pretend to have any claim to be an important factor in the production of glacial periods. And even then the question is not of its being the sole or principal cause, but only whether it has had such a perceptible auxiliary effect on other more powerful causes, as may enable us to use it as a chronometer in assigning approximate dates for some of the more important phenomena of the long and varied period between the close of the Tertiary and the establishment of the Recent period.
As man certainly existed throughout the whole of this period, the possibility of finding such a chronometer becomes intensely interesting, and I proceed to discuss the latest state of scientific opinion respecting it. But as Croll's theory if a real is clearly only an auxiliary cause, I will, in the first instance, point out what are the certain and admitted causes which account for variations of temperature irrespective of latitude.
They may be summed up, in Lyell's words, as different combinations of sea and land, for on these depend the secondary conditions which affect temperature. Thus elevation of land is as certain a cause of cold as high latitude, and even Kilimanjaro, under the equator, retains patches of unmelted snow throughout the year. It is estimated that a rise of 1000 feet in height is about equivalent to a fall of 3° F. in mean annual temperature, and that the line of perpetual snow is, on the average, a little higher than the line where this mean annual temperature is at 32° F., or freezing-point. If there is any mass of land so high as to be below this temperature, snow accumulates and forms glaciers, which descend some 4000 feet below the snow-line before the excess of ice pushing down is melted off by the summer heat unless it has been previously floated off in icebergs at a higher level. Now the mean temperature of the north of Scotland at sea-level is about 46° F., so that an elevation of 8000 or 10,000 feet would bring a great part of it well above the snow-line, and vast glaciers would inevitably accumulate, which would push down through the principal valleys almost to the sea-level; a state of things which actually exists in New Zealand, where glaciers from the Southern Alps at about this elevation descend, in some instances to within 700 feet of the sea-level, in the latitude of Devonshire. But a still more important factor of temperature is found in aërial and oceanic currents, which again, to a great extent, are a product of the configuration of sea and land. The most familiar instance is that of the Gulf Stream, which raises the temperature of Western Europe some 10°, and in Norway as much as 15° F., above that due to latitude, and which prevails on the other side of the Atlantic. The northern extremity of the British Islands in Shetland is on the same parallel of latitude as the southern extremity of Greenland, Cape Farewell. One is buried under perpetual ice, in the other there is so little frost in winter that skating is an unknown art.
What is the reason of this? We must go to the tropics to find it. A vast mass of vapour is raised by the sun's heat from the oceans near the equator, which being lighter rises and overflows, the trade winds rushing in from the north to supply its place, and being deflected to the west by the earth's rotation. This prevalence of easterly surface winds sweeps the waters of the Atlantic to the west, where they are intercepted by South America, turned northwards into the Gulf of Mexico, where they circle round under a tropical sun and become greatly heated, and finally run out through the Straits of Florida with a rapid current, and spread a surface return current eastwards over the Northern Atlantic. The shores of North-west Europe are thus in the position of a house warmed by hot-water pipes, while their neighbours over the way in North-eastern America have no such apparatus.
This oceanic circuit of warm water has a counterpart in the aërial circuit of heated air. The vapour which rose in the tropics overflows, and as it cools and gets beyond the region of the trade winds, descends mainly over the Northern Atlantic, carrying with it its greater velocity of rotation, and so causing westerly winds, which reach our shores after blowing over a wide expanse of ocean heated by the Gulf Stream, thus bringing us warmth and wet, while the corresponding counter-currents which blow over continental Europe and Asia from the north-east bring cold and drought. The extreme effects of this may be seen by comparing the Black Sea at Odessa, where ice often stops navigation, with the North Sea at the Lafoden islands, where the cod-fishing is carried on in open boats in the middle of winter. We in England are in the happy position where on the whole the mild and genial west winds prevail, though not exclusively, so as to give us the drenching rains of Western Ireland and Scotland, or to prevent spells of a continental climate which give us bracing frosts in winter, and alternations of cold and heat in summer.