The southern depression is represented in somewhat higher latitudes, for the isobar of 29.4 ins. lies nearly in latitude 60° off Cape Horn, the position taken by that of 29.2 ins. in January.

Let us now look at the winds. The temperature in the northern hemisphere being higher than in January, the Trades are almost at their highest northern extension, and the South-east Trade crosses the equator both in the Atlantic and Pacific, causing a South-west wind, or Monsoon, over a small area of the sea near the west coasts of Africa and America, while in the Bay of Bengal and Arabian Sea we find, as already stated, the South-west Monsoon up as far as 20° N.

As the areas of high and low pressure over Asia and the North Atlantic respectively have disappeared, the strongly-marked circulation of the air induced by them is no longer observable. On the other hand, in the southern hemisphere, July being almost midwinter, the North-west winds are much more developed in force and extension than is the case in January.

This will suffice to give some idea of the broad features of the relations of wind to pressure over the globe. In the next two chapters I shall endeavour to deal with winds more minutely, and to show how they depend on temperature, and in their turn affect ocean currents and sea-surface temperature.

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CHAPTER XIV.

THE PREVAILING WINDS.

In the last chapter a general explanation has been given of the causes of atmospherical circulation, as exemplified by the phenomena of the Trades and Anti-trades. It will now be of interest to enter more into particulars as to the prevailing winds experienced in various parts of the earth, and as to their relations to the principal ocean currents and to sea-surface temperature. These latter relations will form the subject of the next chapter.

Following Dove, I shall divide these winds into three classes-Permanent Winds, Periodical Winds, and the Variable Winds of high latitudes.

The permanent winds are the Trades and Antitrades, which prevail all the year round over a great portion of their respective domains. As we have already seen, the limits of the Trade winds change their position with the season, and, accordingly, at both sides of each Trade-wind belt there are regions over which, for part of the year, the Trade extends, while for the rest of the year they lie outside its area; but over immense tracts in both the N. and S. Atlantic, the N. and S. Pacific, as well as in the South Indian Oceans, in what is called the Heart of the Trade, the wind blows more or less steadily as a North-east or

South-east wind respectively, according to the hemisphere, day after day, throughout the year. On approaching land, wherever that land may be, local influences come into play, affecting the force and direction of the wind, and at times even reversing the latter altogether.

It would be beyond the limits of a work like this to give the monthly changes in the geographical position of the Trade winds; still, it may be well to remark that, during the first nine months of the year, the equatorial limit of the North-east Trade in the Atlantic lies in a higher latitude near the west coast of Africa than it does farther to the westward, until the fortieth meridian is passed, when the limit again recedes from the equator. In the months from October to December, however, the Trade wind is in the lowest latitude on the African coast. The South-east Trade extends farther into north latitude in our summer (July to September), when from 25° to 45° W., its northern limit is about on the parallel of 5° N., and the intervening district of calms is at its narrowest for the year.

As to direction, too, that is modified with the longitude. Along the north-west coast of Africa the Trade is distinctly North-north-east inclining to North, while on the opposite side, along the West India Islands, the direction is Easterly, and even something South of East. These directions exhibit a distinct tendency in the air to circulate round the area of high barometrical pressure already noticed near the tropic of Cancer.

In the South Atlantic the South-east Trade blows with a greater steadiness, and over an area more extensive, than the corresponding wind in northern latitudes. It is decidedly more Southerly on the

African side than farther out at sea, and, even as far south as the parallel of 20°, the Admiralty Wind and Current Charts show that, along the coast, right up into the Gulf of Guinea, the influence of the land diverts the true motion of the air and produces a local Monsoon.

On the American side, however, south of Cape St. Roque, we do not find northing in the wind, which is represented as blowing true East; on to the coast.

In the North Pacific the North-east Trade is felt over an enormous extent of longitude, but its force is not equal to that experienced in the Atlantic Ocean. Its change of direction is strongly marked, being Northerly on the American shore, due East in midocean, while it exhibits a distinct South-easterly direction at certain seasons near the Philippine Islands, where it comes within the influence of the summer Monsoon system of China.

For the South Pacific the Admiralty Charts, already referred to, embody nearly all that is certainly known, and are the main source of information. They indicate during all months, except from July to September inclusive, an interruption in the Trade wind, which stretches in a north-west and south-east direction, and has a breadth of about twelve degrees in the first half of the year, the centre of the interruption lying near the Society Isles, in 20° S. and 150° W. In the three winter months of that hemisphere, July to September, the break disappears, and the Trade wind stretches across the ocean; and in the three spring months, October to December, the interruption between the two parts of the South-east Trade is not nearly so marked as it is from January to June, and it lies more in an east and west direction.

In the South Indian Ocean the South-east Trade prevails throughout the year. On the western coast of Australia it is distinctly more Southerly, and on the African coast more Easterly, than it is midway between these continents. On both sides, however, during the summer months of the southern hemisphere, the direction of the Trade near the coast is changed, under the influence of the Monsoon action.

In all oceans the Anti-trades or Westerly winds, with Northing or Southing in them according to the hemisphere, prevail on the polar sides of their Trade winds, though not with the same permanency as these latter, and extend far up towards the poles. They form a predominant feature of the meteorology of the North Atlantic, but in the Southern Ocean they attain a force and prevalence unknown at this side of the equator. These are the winds which blow in the Roaring Forties' of the South Indian Ocean, and whose domain extends rully up to 50° S.

We now come to the periodical winds, and of these by far the most important are the great Monsoons of the Indian and China Seas, known popularly as the North-east and South-west Monsoons, but more correctly termed by Blanford in his Vade Mecum,' the Winter and Summer Monsoons, for reasons which will at once appear.

Throughout the whole of this region, as far as 30° N. in India, and 20° S. on the coast of Australia,. the wind is reversed every six months. During our winter, from October to April, the North-east Trade blows down to the equator, with clear weather. It then crosses the Line, and blows as the North-west