The indian ocean, the setting for much that is to follow, is a world by itself, with winds peculiar to it and but few ways by which it may be entered. In the years with which we are concerned these winds were of greater moment and the entrances fewer than they are now. From the westward the route round the Cape was the only gateway. Of the entrances from the Pacific only four or five at most were used. Between these narrow doors and northwards about the lands to which they lead, all maritime trade and war was dominated by the winds; peculiarly so, in that the winds tend to be both constant and predictable. Without a knowledge of these winds there can be no understanding of Eastern commerce or naval strategy in the days of sail.

In the Eastern Seas the year is divided into two seasons according to the direction of the prevailing wind. From April to October is the period of the South-West Monsoon. The effect of this wind may be said to extend between the Equator and the Tropic of Cancer, and from the east coast of Africa to the Sea of Japan; beyond the Philippines also, as far to the eastward as long. 145° E. The North-East Monsoon prevails throughout much the same area from October to May. These monsoons, however, are limited in two respects; in time, that is to say, and in space.