Page:Physical Geography of the Sea and its Meteorology.djvu/308

282 and carried up aloft still farther, to keep, at the proper distance from the earth, the lino of perpetual congelation. Were it possible to trace a thermal curve in the upper regions of the air to represent this line, we should no doubt find it mounting sometimes at the equator, sometimes on this side, and sometimes on that, but always so mounting as to overleap this cloud-ring. This thermal line would not ascend always over the same parallels: it would ascend over those between which this ring happens to be; and the distance of this ring from the equator, north or south, is regulated according to the seasons. If we imagine the atmospherical equator to be always where the calm belt is which separates the north-east from the south-east trade-winds, then the loop in the thermal curve, which should represent the line of perpetual congelation in the air, would be always found to stride this equator; and it may be supposed that a thermometer, kept sliding on the surface of the earth, so as always to be in the middle of this rain-belt, would show very nearly the same temperature all the year round; and so, too, would a barometer the same pressure, though the height of the atmosphere over this calm belt would, in consequence of so much heat and expansion, be very much greater than it is over the trade-winds or tropical calms.

521. Offices of the cloud-ring.—Returning and taking up the train of contemplation as to the office which this belt of clouds, as it encircles the earth, performs in the system of oceanic adaptations, we may see how the cloud-ring and calm zone which it overshadows perform the office both of ventricle and auricle in the immense atmospherical heart, where the heat and the forces which give vitality and power to the system are brought into play—where dynamical strength is gathered, and an impulse given to the air sufficient to send it thence through its long and tortuous channels of circulation.

522. It acts as a regulator.—Thus this ring, or band, or belt of clouds is stretched around our planet to regulate the quantity of precipitation in the rain-belt beneath it; to preserve the due quantum of heat on the face of the earth; to adjust the winds; and send out for distribution to the four corners vapours in proper quantities to make up to each river-basin, climate, and season its quota of sunshine, cloud, and moisture. Like the balance-wheel of an artificial machine, this cloud-ring affords the grand atmospherical machine the most exquisitely-arranged self-