Page:Popular Science Monthly Volume 16.djvu/848

 the ocean to maintain its level, and naturally select the path of least resistance. Hence they are usually undercurrents.

The general agreement between the systems of winds and of oceanic circulation is shown on a chart. It may be mentioned that the direction of the winds, as laid down on this chart, does not precisely correspond with the direction determined by the late Professor Coffin, and indicated on the charts in his cyclopean work, "The Winds of the Globe," recently published by the Smithsonian Institution; and it is quite probable that more exhaustive observations will show that there are inaccuracies of as great magnitude in the representation of oceanic currents. It should be borne in mind, however, that any minor discrepancies between the two systems do not militate against the theory, unless it can be shown that they are not such as would be produced by the conformation of the coasts; for the wind system, as a cause of oceanic circulation, is modified by this important and frequently antagonistic factor.

In seeking for demonstrative evidence of the correctness of the wind theory. Dr. Croll seems to fall into an error which he has repeatedly had occasion to point out in others. Thus, in discussing one of Dr. Carpenter's sections, he shows, from Professor Muncke's coefficients of the density of sea-water, at various temperatures, that the surface of the Atlantic is, at north latitude 23° 10′, two feet six inches, and at north latitude 37° 54′, fully three feet six inches higher than at the equator, on account of the greater thickness at these points of the upper layer of warm water. It is urged that "gravitation never could have caused the ocean to assume this form," and hence that "gravitation can no more cause the surface-water of the Atlantic to flow toward the Arctic regions than it can compel the waters of the Gulf of Mexico up the Mississippi into the Missouri." Now, by Dr. Croll's own showing, it is gravity alone that causes the surface of the Atlantic to assume that form—that is, if that form is actually assumed; a fact not determined by measurements. It is merely calculated from the data given to what height above the level at the equator the waters of the North Atlantic would have to be raised in order to maintain a condition of static equilibrium; it is not shown that the waters are so elevated, but only that they would have to be in order that the influence of gravitation, in producing oceanic currents, should be eliminated. How the warm water, on which this supposed configuration depends, was conveyed thither, is a dynamical and not a statical problem.

Moreover, two important elements in the problem of oceanic circulation are disregarded by Dr. Croll: 1. The water borne from equatorial to polar regions by aerial currents, in the form of aqueous vapor, must exercise a powerful influence not only on the earth's temperature but also upon marine currents; for, as shown by Lieutenant Maury, it is sufficient to permanently render the Arctic seas much less saline than those of tropical regions. Sir Wyville Thomson strongly insisted upon