Page:The Atlantic Monthly Volume 1.djvu/283

1858.] air tends to move,) into the small circle of the latitude, in which the air actually does move. The force of this deflection, measured by the centrifugal force of the air as it circulates around the pole, retards the movement from the equator, and finally wholly suspends it; so that the upper air circulates around in the higher latitudes as water may be made to circulate in a pail; and the air is drawn away from the polar regions as this circulatory motion is communicated to it, and tends to accumulate in the middle latitudes, as the circulating water is heaped up around the sides of the pail. Hence, in the middle latitudes there is a greater weight of air than at the poles, and this tends to press the lower air to higher latitudes. Centrifugal force, however, balances this pressure, so long as the lower air moves with the velocity of the upper strata; but as the friction of the earth retards its motion and diminishes its centrifugal force, it gradually yields to the pressure of the air above it, and moves toward the poles. Near the polar circles it is again retarded by its increasing centrifugal force, and it returns through the middle regions of the atmosphere.

Thus there are two systems of atmospheric circulation in each hemisphere. The principal one extends from the equator to high middle latitudes and partly overlies the other, which extends from the tropical calms to the polar circles. These two circulations move in opposite directions; like two wheels, when one communicates its motion to the other by the contact of their circumferences.

In the middle latitudes the lower current of the principal circulation lies upon the upper current of the secondary circulation, and both move together toward the equator. This principal lower current first touches the earth's surface beyond the tropical calms, and having lost its relative eastward motion and now tending westward, it appears as the trade-wind, very regular and constant; while the upper secondary current returns, without reaching the tropics, as an undercurrent, and in our latitude appears as the prevailing northeastward wind,—a very feeble motion, usually lost in the weather winds and other disturbances, and only appearing distinctly in the general average.

Mr. Thompson illustrates the effect of the friction of the earth's surface on the eastward circulation of the air by a very simple experiment with a pail of water. If we put into the pail grains of any material a little heavier than water, and then give the water a rotatory motion by stirring it, the grains ought, by the centrifugal force imparted to them, to collect around the sides of the pail; but, sinking to the bottom, they do in fact tend to collect at the centre, carried inward by those currents which the friction of the sides and bottom indirectly produces.

Thus Mr. Thompson's beautiful and philosophical theory completes that of Halley, and explains all those apparent anomalies which have hitherto seemed irreconcilable with the only rational account of the trade-winds. The rainless calms of the tropics are explained by this theory without that crossing and interference of winds which Lieut. Maury supposes; for the secondary circulation returns as an under-current toward the poles without reaching the tropics, and the dry lower current of the principal circulation passes over the tropical latitudes, in its gradual descent, before it reaches the earth as the trade-winds.

These trade-winds, absorbing moisture from the sea, precipitate it as they rise again, and produce the constant equatorial rains; and these rains, doubtless, tend much more powerfully than the mere unequal distribution of heat to direct the wind toward the equator; for the fall of rain rapidly diminishes the pressure of the air and disturbs its equilibrium, so that violent winds are frequently observed to blow toward rainy districts. Thus, primarily, the unequal distribution of heat, and, more immediately, the equatorial rains cause the principal circulation of our atmosphere; and this indirectly produces the secondary