So far as I am aware, the theory I am about to present in explanation of the phenomenon of a low antarctic barometer is original. It is sufficiently simple; -perhaps, if we remember how very seldom physical phenomena admit of a simple explanation, one may say that the theory labours under the disadvantage of simplicity.
It is obvious that the centre of gravity of the solid portion of the earth's globe lies somewhat to the south of the centre of figure. This arrangement has long been accepted as the explanation of two remarkable geographical features-the prevalence of water over the southern hemisphere, and the configuration of nearly all the peninsulas over the whole globe. Whether or not those parts within the antarctic regions which have not yet been explored, are occupied by land (chiefly) is a question which has little more bearing on our views respecting this point than has the counter question-whether the unexplored north-polar regions are or are not occupied by a north-polar ocean.1 Sup-
1 Captain Maury holds the affirmative on both points. I have already-had occasion to discuss in these pages his theory of a tidal north-polar ocean, and I think the theory cannot be maintained. But the theory of a polar ocean communicating with the Atlantic and Pacific is a sufficiently posing these arrangements to exist, it is evident that they form mere local peculiarities. The general tendency of water towards the southern hemisphere is very obvious, and, so far as I am aware, no other explanation of the peculiarity has ever been offered than that founded on a slight displacement southwards of the earth's centre of gravity. If, then, C is the centre of the black circle in Fig. 3, representing the solid part of the earth, the centre of gravity of this part lies (in the Fig.) slightly below C - between C and C, let us suppose.
Now we see that, owing to this slight displacement, the watery envelope of the earth tends southward. If the earth were a perfectly uniform spheroid, it is clear that there would be a tendency to some such arrangement as is represented (on a greatly exaggerated scale) in Fig. 3, in which the shaded part represents the sea - that is, a shell of water, thicker towards the south, probable one. The theory of an antarctic continent is hardly in the same position, since antarctic explorations have given us but faint indications, here and there, of the habitudes of the south-polar regions. But I may note, in passing, a very singular argument used by Captain Maury in favour of the existence of such a continent. He states it as a physical law that land is scarcely ever antipodal to land; ' therefore,' he says, ' since the north-polar regions are probably occupied by a vast ocean, the south-polar regions are probably occupied by a vast continent.' He seems to forget that it by no means follows that because land is seldom antipodal to land, water should seldom be antipodal to water. Since the extent of water is nearly three times that of land, it is absolutely necessary that nearly two-thirds of the water should be antipodal to water. The supposed peculiarity that nearly all the land is antipodal to water (one twenty-seventh only being antipodal to land), is in reality no peculiarity at all. It would have been far more singular if any large proportion of the land (which occupies little more than one-fourth of the globe) had been antipodal to land.
would surround the solid earth. For our present purposes it is sufficient to consider this supposed arrangement, as minor inequalities of the earth's surface-contour have clearly nothing whatever to do with the phenomenon* we are considering.
Let C be the centre of the spheroid which bounds the earth's fluid envelope. Then it is very clear that if this envelope were of the same specific gravity as the solid portion of the earth, the centre of gravity of the entire mass would lie very near C, but slightly south of that point, on account of the slight southerly disx placement of the centre of gravity of the solid portion. But when we consider that the specific gravity of the fluid envelope is less than one-fifth of that of the solid globe, it is perfectly clear that the centre of gravity of the entire mass will not be so far south as C. For, of the entire mass, the northern half is the heavier, and therefore the centre of gravity must lie north of the centre of the entire mass - that is, north of C. In fact, it must lie much nearer to C than to C'.
Thus, the centre of gravity of the solid globe, and that of the entire mass, solid and fluid, both lie between C and C. Now it is evident that the central point, about which the earth's atmospheric envelope tends to form itself as a spherical or spheroidal shell, is the centre of gravity of the entire solid and fluid terrestrial globe - that is, is a point north of C'. Therefore, precisely as the effect of the fluid envelope collecting itself centrally about a point south of C is to cause the mean depth of water to be greatest in the southern hemisphere, so the fact that the atmospheric envelope collects itself centrally about a point north of C' should result in giving a greater mean depth of air (referred to the sea-level) over the northern hemisphere. This arrangement is represented in Fig. 3, in which the unshaded part is supposed to represent the atmosphere.
I have endeavoured to make the above explanation of my theory in explanation of the low antarctic barometer as complete and exact as possible; but there is another way of presenting the theory, which, though less complete, may appear clearer to some minds: -
Variation of mean barometric pressure, as we proceed from one place to another, may be due either to a variation of circumstances of heat, moisture, and other like relations, or to difference of level. Maury's explanation assigns to the low antarctic barometer a cause or causes falling under the former category. My theory amounts to the supposition that the low barometer is due to an absolute difference of level. I say that the sea-level, to which we refer barometric pressure, is not a just level of reference when atmospheric pressure over the whole globe is the subject of inquiry, because the southern seas stand out to a greater distance than the northern seas from the true centre of gravity of the earth's solid and fluid mass.