Sir John Herschel, in speaking of the solar spots, says: "We may also observe that the tranquillity of the sun's polar, as compared with his equatorial regions (if his spots be really atmospheric), cannot be accounted for by its rotation on its axis only, but must arise from some cause external to the sun, as we see the belts of Jupiter and Saturn and our trade winds arise from a cause external to these planets combining itself with their rotations, which _alone_ (and he lays an emphasis on the word) can produce no motions when once the form of equilibrium is attained."
With respect to the origin of the solar spots, we have no disposition to question the conclusion; but, as regards the _principle_ laid down, that rotation can produce no motions when once the form of equilibrium is attained, we must unequivocally dispute it. If our atmosphere were of uniform density, the rotation of the earth would cause no current such as we have described; with our atmosphere as it is, the result will be different. The momenta of two portions of matter are the products of their inertiæ by their motions, and, in the present case, we must take the inertiæ of equal spaces. A cubic inch of air at the surface, and at three miles above the surface, is as 2 to 1; but their centrifugal velocity varies only as the radii of the respective spheres, or as 1320 to 1321. In the polar regions, therefore, the momentum of the surface air preponderates, and, in this case, the _surface_ current is towards the equator, and the upper current towards the poles. When, however, the centrifugal velocity is considerably increased in a lower latitude, and the curvature of the surface becomes more and more inclined to the direction of that resolved part of the centrifugal force, which is always _from_ the axis, the surface layers will evince a tendency to leave the surface, and an intermingling will then take place in the space between latitude 70° and 50°, or in latitude 60°. As this layer is continually urged on in the same direction by the surface layer of latitudes above 60°, the upper layer now becomes a current setting _towards_ the equator, and, consequently, the back current occupies the surface. Now, considering that the rarefying action of the sun is elevating the air under the equator, there must necessarily be an upper current from the equator to the poles; so that if we conceive the two currents to meet about latitude 30°, there will be a second intermingling, and the current from the poles will again occupy the surface. Thus, we regard a part of the effect of the trades to the rotation of the earth, which is the chief impelling power at the poles, as the sun is at the equator; and the latitudes 60° and 30° will be marked by some especial phenomena of temperature, and other meteorological features which do actually obtain. These would be much more marked if the irregular configuration of land and sea, the existence of mountain chains, and the different heating power of different latitudes, owing to the unequal distribution of the land, did not interfere; and the currents of the air (disregarding the deflection east and west) might then be represented by a treble link or loop, whose nodes would vary but little from latitudes 30° and 60°. As it is, it has, no doubt, its influence, although unimportant, when compared with the disturbing action of the ethereal vortices.