Owing to the fact that the air rises in the heated zone there is always a region of calms at this point where there is no wind and no rain. There are two other regions of calms in the ocean, one at the north at the tropic of Cancer and another at the south near the tropic of Capricorn. As has been stated, there are currents flowing back in the upper regions at the equator north and south, and these are called the upper trades--the lower currents being called the lower trades. These upper trades gradually fall till they reach the tropic of Cancer on the north, where the lower part of the current stops and bends back toward the equator, now becoming a part of the lower trade wind. This causes a calm at that point where it turns. The upper parts of this current continue on, in a northerly and southerly direction, on the surface until they meet with the cold air of the north and south polar regions, where there is a conflict of the elements--as there always is when cold and warm currents meet. The only point where the trade wind has free play is in the South Indian Ocean, and this is called the "heart of the trades." If the whole globe were covered with water there would be a more constant condition of temperature; but owing to the great difference between the land and water, both as to altitude and the ability to absorb and radiate heat, we have all of these varied and complicated conditions of wind and weather. The trade winds shift from north to south and vice versa with the advancing and receding seasons, due to the fact that the earth has a compound motion. It not only revolves on its axis once in twenty-four hours, but it rocks back and forth once a year, which is gradually changing the direction of its axis; and in addition to these motions it is traveling around the sun as well. CHAPTER XI. WIND--CONTINUED. In our last chapter we discussed the winds that prevail in the regions of the tropics called trade winds, because they follow a direct course through the year, with the exceptions noted in regard to their shifting to the north or south with the changing seasons; we also described the phenomena of land and sea breezes, which during certain seasons of the year reverse their direction twice daily. We will now describe another kind of wind, called monsoons, that prevail in India.