"That principle explains the transfer of heat by winds. If the heated air rose to the upper regions, and there radiated its heat, nothing would be gained; the heat would be simply radiated into space. But as the warm air rises pressure is more and more removed from it; it expands; its sensible heat becomes latent and is thus kept from radiation; its temperature falls, but not from loss of heat. This rarefied air forms the upper current flowing away from the heated centre. In due time this air must come to the surface of the earth again. Whenever this takes place the air is brought again under pressure; it is compressed, and its latent heat becomes again sensible. Heat is thus transferred from the warmer region to the colder in a latent condition, so that it cannot be lost. We must now apply this to the trade-winds. What are the trade-winds, Mr. Hume?" "They are regular winds blowing from a little north and south of the tropics of Cancer and Capricorn south-west and north-west toward the equator." "These winds are called _trade-winds_," continued Mr. Wilton, "on account of their great advantage to trade or commerce. The regular and steady sweep of these winds bears the merchantmen rapidly and safely on their way. The formation of 'the trades' is easily explained. By the intense heat of the sun under the equator the air is greatly expanded and rarefied; the heated air rises along the whole line of the equator; from both sides the cooler air presses in, is heated, and rises; thus steady winds are formed from the tropics, or a little beyond the tropics, toward the equator. If the earth had no rotation upon its axis, these winds would blow directly toward the equator, exactly south and north. The rotation of the earth gives the trade-winds their oblique, south-west and north-west direction. Suppose that a single particle of air at the tropic of Cancer starts upon its journey toward the equator. At its starting it has the same motion eastward as the surface of the earth at that place, that is, about nine hundred and fifty miles per hour. But as it moves on southward the degrees of longitude become longer and the motion of the earth's surface becomes more rapid, till at the equator its motion is one thousand and forty miles per hour. But the particle of air we are watching is not fastened to the earth's surface, and as the earth moves more rapidly the nearer we come to the equator, the particle of air falls behind, that is, the air moves southward and eastward, but the earth moves eastward more rapidly than the air, so that the air falls behind and seems to be moving westward. The result is that the air upon the earth's surface moves south-west. That which takes place with a single particle takes place with the whole body of the air, and that which takes place north of the equator takes place south of it also, producing north-west winds. On reaching the equator the winds from the north and the south meet and stop, forming the