It follows from this, then, that as the winds from both sides approach the equator, their easterly tendency must diminish ; a fact which though inevitably resulting from the principle maintained by Halley, was reserved for the late Captain Basil Hall to reason out, and, though thus for the first time, in a very distinct manner, in his Fragments of Voyages and Travels.' The lengths of the diurnal circles increase very slowly in the immediate vicinity of the equator, and for several degrees on either side of it hardly change at all. Thus the friction of the surface has more time to act in accelerating the velocity of the air, bringing it towards a state of relative rest, and diminishing thereby the relati,ve set of the currents from east to west, which, on the other hand, is feebly, and, at length, not at all, rein forced by the cause which originally produced it. Arrived, then, at the equator, the trade winds must be expected to lose their easterly character altogether. And not only this, but the northern and southern currents here meeting and opposing, will mutually destroy each other, leaving only such preponderancy as may be due to a difference of local causes acting in the two hemispheres—which in some regions around the equator may lie one way, in some another— as will presently be seen. " The result, then," says Sir John Herschel, "must be the production of two great tropical belts, in the northern of which a constant north-easterly, and in the southern a south easterly, wind must prevail, while the winds in the equatorial belt, which separates the two former, should be comparatively calm, and free from any steady prevalence of easterly character. All these con sequences are agreeable to observed fact, and the system of currents above described constitutes in reality what is understood by the regular trade winds." On the subject of the region of calms, intervening between the trade.
- wincia, Mita:, in his Manual of Meteorology; remarks, " In the space betneen the two trade-winds the air is heated to the highest degree. There the ascending currents of air are the moat rapid ; and by this quick ascent the velocity of the wind blowing along the surface of the eta is greatly diminished. Besides it would seem that the upper and lower curreot of the air conic into contact with one another at a com paratively small elevation above the surface of the globe. These two circumstances appear to be the reason why no regular winds are met with within the region of the calms." We now proceed to review the actual and local phenomena of these winds. Two currents of air are found within their limits, of which the lower runs to the south-west, north of the equator, and to the north-west, south of it, and the upper in the opposite direction. In this manner a kind of atmospherical circulation is formed, which is admirably adapted for the preservation of animal life. The existence of this countercurrent of air in the upper regions had only been inferred front the theory, and nothing could be addncod to prove it, except that within the trade-winds the clouds, which rarely make their appearance io this region, generally take a direction which corresponds to that of the supposed current of air. But in 1812 an event took place which was rather more decisive in favour of the theory. It, the
eruption of the volcano of St. Vincent, considerable quantities of ashes and other volcanic matter descended on and spread over the island of Bari adoes. This event certainly excited a high degree of surprise, as in this part of the Caribbean Sea the trade-wind always blows with a cousiderable force, so that vessels sailing from St. Vincent to Barbadocs are obliged to make a circuitous course of sonic hundred miles to reach the place of their destination. It can hardly be questioned that the volcanic matter was raised, by the eruption cf the volcano, to such an elevation that it reached the counter-current, which, blowing from the west, carried it to Barbadom. Humboldt adduces also, in support of the theory, the strong south-western wind which he experienced at the top of the Peak of Teneriffe, whilst all the other parts of the irdatsis were under the sway of the trade•wind ; and this observation is also confirmed by Glass, who, in his ' History of the Canary Islands,' states that during the trade-winds the most elevated parts of those islands experience a continual westerly wind, which blows with considerable force. Confirmatory facts on this point, as observed by Professor C. Plazzi Smyth, will be found at the end of the article Omuta Lastly, we may adduce, in corroboration of the theory, the instantaneous change of wind which is frequently experienced when the limits of the trade-winds are passed.
It has been already observed that the boundary of the trade-winds towards the nearest polo does not always occur in the same part of the ocean, but changes with the seasons. The difference is considerable. At the greatest southern declination of the sun, in December and Janu ary, the northern boundary of the north-east trade-wind of the Atlantic occurs to the south of 25° N. lat., whilst in the opposite season, from June to September, it occurs about 32° N. lat.. Thus we find a tract of sea, seven degrees of latitude in width, which is alternately exposed to the sway of the trade-winds and of variable winds, and nearly in the middle of this tract the Canaries are situated. These islands, there fore. are within the trade-winds for six months, and for the remainder of the year without them. Von Buch, in his description of these islands, has given an account of the regular manner in which the trade-wind advances towards the north, with the progress of the sun in the northern hemisphere, and in which it recedes when the sun purses the equator on his return to the southern hemisphere, observing that the south-western wind, which is always found in the upper regions of the atmosphere above the trade-winda, does not make its appearance on the south, as may be inferred from the direction in which it blows, but is first experienced at Madeira, whence it gradually advances to Teneriffe and the other Canaries. Whilst this south western wind advances from north to south, it also descends by degrees from the upper to time lower regions, and to the surface of the globe. On Teneriffe this takes place in October, when the south-west wind is experienced on all mountains 6000 feet high, but generally one week passes, and sometimes several weeks, before the south-western wind sinks to the level of the sea.