To sum up, we may say that the trade-winds, like the currents of the ocean, are due, _first_, to the sun,--that is to the different measure in which the solar heat is distributed on the globe’s surface; and, _secondly_, to the earth’s axial rotation, which affects the direction of currents in the aerial ocean in manner precisely analogous to that in which it affects the like currents in the aqueous ocean. In truth, the ocean of water, and the ocean of air--in contact with one another, and possessing many properties in common--act and react upon one another, mutually imparting their respective temperatures, movements, and other conditions. This is only one among the instances of mutual harmony--one of the many mute sympathies--which abound in the natural world. The monsoons are winds which blow over the Indian Ocean, and the countries adjacent to its waters. In general terms, it may be said that they prevail within the same latitudes as those over which the trade-winds of the Atlantic and Pacific blow. But the monsoons differ from the trade-winds of the two greater oceans in the fact that they are _periodical winds_, not perennial. The monsoon blows for half the year from one quarter of the heavens, and for the other half from an opposite quarter. [Illustration: Charts Showing the General Directions of Wind and Tide Currents] Over the northerly portion of the Indian Ocean--from the neighborhood of the equator to the shores of the Asiatic Continent, including the Malay Archipelago and the adjacent China Sea--a northeast monsoon blows during the winter months of the Northern Hemisphere; that is, from October to March, inclusive. During the summer months--April to September--and within the same limits, the southwest monsoon blows. Southward from the equator to the neighborhood of the tropic of Capricorn, the southeast monsoon blows during the winter of those latitudes (April to September): this is exchanged, during the other half of the year, for a northwest monsoon in the neighborhood of the Australian coasts, and for a northeast monsoon along the line of the African shores. The term _monsoon_--derived from a Malay word which signifies “season”--expresses the periodical nature of these winds, and indicates to how large an extent the climate of Indian seas and lands is dependent upon their periodical recurrence. The change from the one monsoon to that from an opposite quarter is not accomplished at once. The breaking-up of the monsoon, as it is termed, is attended by thunderstorms and other meteorological phenomena, which prevail during some weeks, until the setting-in of the coming monsoon is fairly accomplished. The nature of these changes, and the general characteristics of the monsoon itself, are admirably depicted in the following passage, by a master hand: