The Catholic World, Vol. 01, April to September, 1865 / A Monthly Eclectic Magazine

by Various

Captain Maury supposes that when the air rises, at either side of the equator, it crosses over into the opposite hemisphere, so that there is a constant interchange of air going on between the northern and southern hemispheres. This he has hardly sufficiently proved, and his views are not generally accepted. One of the arguments on which he lays great stress in support of his theory is that on certain occasions dust has fallen in [{212}] various parts of western Europe, and that in it there have been discovered microscopical animals similar to those which are found in South America. This appears to be scarcely an incontrovertible proof; as Admiral Fitzroy observes: "Certainly, such insects may be found in Brazil; but does it follow that they are not also in Africa, under nearly the same parallel?"

This counter-current, or "anti-trade," as Sir J. Herschel has called it, is at a high level in the atmosphere between the tropics, far above the top of the highest mountains; but at the exterior edge of the trade-wind zone, it descends to the surface of the ground. The Canary islands are situated close to this edge, and accordingly we find that there is always a westerly wind at the summit of the Peak of Teneriffe, while the wind at the sea-level, in the same island, is easterly throughout the summer months. Professor Piazzi Smyth, who lived for some time on the top of that mountain, making astronomical observations, has recorded some very interesting details of the conflicts between the two currents, which he was able to observe accurately from his elevated position. In winter the trade-wind zone is situated to the south of its summer position in latitude, and at this season the southwest wind is felt at the sea-level in the Canary islands. Similar facts to these have been observed in other localities where there are high mountains situated on the edge of the trade-wind zone, as, for instance, Mouna Loa, in the Sandwich islands. There can, therefore, be no doubt that the warm, moist west wind, which is felt so generally in the temperate zones, is really the air returning to the poles from the equator, which has now assumed a south-west direction on its return journey, owing to conditions the reverse of those which imparted to it a north-east motion on its way toward the equator. This, then, is our south-west wind, which is so prevalent in the North Atlantic ocean that the voyage from Europe to America is not unfrequently called the up-hill trip, in contradistinction to the down-hill passage home. These are the "brave west winds" of Maury, whose refreshing action on the soil he never tires of recapitulating.

The south-west monsoons of Hindostan, which blow from May to October, and the north-west monsoons of the Java seas, which are felt between November and April, owe their westerly motion to a cause similar to that of the anti-trades which we have just described. To take the case of the monsoons of Hindostan: we have seen above how the rarefaction of the air in Central Asia attracts the southeast trade-wind of the southern hemisphere across the equator. This air, when it moves from the equator into higher latitudes, brings with it the rate of motion, to the eastward, of the equatorial regions which it has lately left, and is felt as a south-west wind. Accordingly, the directions of the monsoons are thus accounted for. In the winter months the true north-east trade-wind is felt in Hindostan; while in the summer months its place is taken by the south-east trade of the southern hemisphere, making its appearance as the south-west monsoon. In Java, conditions exactly converse to these are in operation, and the winds are south-east from April to November, and north-west during the rest of the year.

The change of one monsoon to the other is always accompanied by rough weather, called in some places the "breaking out" of the monsoon; just as with us the equinox, or change of the season from summer to winter, and vice versa, is marked by "windy weather," or "equinoctial gales."

The question may, however, well be asked, why there are no monsoons in the Atlantic Ocean?

In the first place, the amount of rarefaction which the air in Africa and in Brazil undergoes, in the respective hot seasons of those regions, is far less considerable than that which is [{213}] observed in Asia and Australia at the corresponding seasons.

Secondly, in the case of the Atlantic ocean, the two districts toward which the air is attracted are situated within the torrid zone, while in the Indian ocean they are quite outside the tropics, and in the temperate zones. Accordingly, even if the suction of the air across the equator did take place to the same extent in the former case as in the latter, the extreme contrast in direction between the two monsoons would not be perceptible to the same extent, owing to the fact that the same amount of westing could not be imparted to the wind, because it had not to travel into such high latitudes on either side of the equator. A tendency to the production of the phenomena of the monsoons is observable along the coast of Guinea, where winds from the south and south-west are very generally felt. These winds are not really the south-east trade-wind, which has been attracted across the line to the northern hemisphere, They ought rather to be considered as of the same nature as the land and sea breezes before referred to, since we find it to be very generally the case, that in warm climates the ordinary wind-currents undergo a deflection to a greater or less extent along a coast-line such as that of Guinea, Brazil, or north of Australia.

Our readers may perhaps ask why it is, that when we allege that the whole of the winds of the globe owe their origin to a regular circulation of the air from the Polar regions to the equator, and back again, we do not find more definite traces of such a circulation in the winds of our own latitudes? The answer to this is, that the traces of this circulation are easily discoverable if we only know how to look for them, In the Mediterranean sea, situated near the northern edge of the trade-wind zone, the contrast between the equatorial and polar currents of air is very decidedly marked. The two conflicting winds are known under various names in different parts of the district. The polar current, on its way to join the trade-wind, is termed the "tramontane," in other parts the "bora," the "maestral," etc.; while the return trade-wind, bringing rain, is well known under the name of the "sirocco." In Switzerland the same wind is called the "Fohn," and is a warm wind, which causes the ice and snow to melt rapidly, and constantly brings with it heavy rain.

In these latitudes the contrast is not so very striking, but even here every one knows that the only winds which last for more than a day or two at a time are the north-east and the south-west winds, the former of which is dry and cold, the latter moist and warm. The difference between these winds is much more noticeable in winter than in summer, inasmuch as in the latter season Russia and the northern part of Asia enjoy, relatively to the British Islands, a much higher temperature than is the case in winter; so that the air which moves from those regions during the summer months does not come to us from a climate which is colder than our own, but from one which is warmer.

So far, then, we have attempted to trace the ordinary wind-currents, but as yet there are very many questions connected therewith which are not quite sufficiently explained. To mention one of these, we hear from many observers on the late Arctic expeditions, that the most marked characteristic of the winds in the neighborhood of Baffin's Bay, is the great predominance of north-westerly winds. It is not as yet, nor can it ever be satisfactorily, decided how far to the northward and westward this phenomenon is noticeable. The question then is, Whence does this north-west wind come?