On taking a comprehensive view of their origin, we find that, like the oceanic currents, they are chiefly caused by the unequal influence of solar warmth upon the atmosphere under the line and at the poles. In the torrid zone, the air, rarefied by intense heat, ascends in perpendicular columns high above the surface of the earth, and there flows off towards the poles, in the same manner as in a vase filled with cold water and placed over the flame of a lamp, the warmed liquid rises from the bottom and spreads over the surface. But cold air-currents must naturally come flowing in an opposite direction from the poles to the equator to fill up the void, as in the example I have cited, colder and consequently heavier water comes streaming down the sides of the vase to replace the liquid which is rising in the centre under the influence of heat. Thus the unequal distribution of solar warmth over the surface of the earth evidently generates a constant circulation of air from the equator to the poles, and from the icy regions to the tropics, and by this means the purity of the atmosphere is chiefly maintained. The sun is not only the great fountain of warmth, he is also the universal ventilator; he not only calls forth animal life, but at the same time, by a simple and admirable mechanism, provides for its health by constantly renewing the air, which is essential to its existence. If caloric were the sole agent which influences the direction of the winds, or if the earth were one uniform plain, the opposite air-currents I have mentioned would naturally flow straight to the north and south; but their course is modified or diverted in the same manner as that of the ocean-currents by the rotation of the globe. Thus, the cold air-current (polar-stream) which comes rushing upon us from the Arctic regions, is felt in our latitude as the biting east or north-east wind, so trying to our nerves and organs of respiration, while we enjoy the warm air-current from the tropics as the mild western or south-western breeze. But besides the rotation of the earth, there are many other local influences by which the winds are deflected from their course, or by whose agency partial air-currents are called forth. Among these we particularly notice high chains of mountains, the unequal capacity of sea and land in absorbing and retaining heat, which gives rise to sea and land breezes; the increasing or diminishing power of the sun in different seasons by which the equilibrium of the air is modified in many countries, the difference of radiation from a sandy desert or a forest, electrical discharges from clouds, &c. &c. Although subject to many of these local disturbances, the winds generally blow with an astonishing regularity in the tropical zone; while in our variable climate the polar and equatorial stream are engaged in a perpetual strife, now bringing us warmth and moisture from the south and west, now cold and dryness from the north and east.