The same remarks apply to the gravitation theory of the cause of currents. Viewed simply as a theory it looks very reasonable. There is no one acquainted with physics but will admit that the tendency of the difference of temperature between the equator and the poles is to cause a surface current from the equator towards the poles, and an under current from the poles to the equator. But before we can prove that this tendency does actually produce such currents, another question must be settled, viz. is this force sufficiently great to produce the required motion? Now when we apply the method to which I refer, and determine the absolute amount of the force resulting from the difference of specific gravity, we discover that not to be the powerful agent which the advocates of the gravitation theory suppose, but a force so infinitesimal as not to be worthy of being taken into account when considering the causes by which currents are produced. CHAPTER XIII. THE WIND THEORY OF OCEANIC CIRCULATION. Ocean-currents not due alone to the Trade-winds.—An Objection by Maury.—Trade-winds do not explain the Great Antarctic Current.—Ocean-currents due to the System of Winds.—The System of Currents agrees with the System of the Winds.—Chart showing the Agreement between the System of Currents and System of Winds.—Cause of the Gibraltar Current.—North Atlantic an immense Whirlpool.—Theory of Under Currents.—Difficulty regarding Under Currents obviated.—Work performed by the Wind in impelling the Water forward.—The _Challenger’s_ crucial Test of the Wind and Gravitation Theories.—North Atlantic above the Level of Equator.—Thermal Condition of the Southern Ocean irreconcilable with the Gravitation Theory.