PLATE III.—FROM RIO DI JANEIRO TO THE CAPE OF GOOD HOPE.
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At nine a. m., we cast off from the tug, not far from the little island of Razza, with its lighthouse, and spread our sails to the breeze, which gradually freshened, but blew from the N.E., which was foul for our course. However, we could still derive some advantage from even this as it was our intention to steer southerly from Rio, so as to be able to make almost exclusively a great circle course to the Cape of Good Hope, after we should have got further south than the Antarctic limit of the S.E. trades.

The near termination of the winter quarter in this southern hemisphere, the approximation of the sun towards the south pole, and the consequent tendency of the zones of wind and currents of air to pursue the same direction, gave us reason to hope, that when approaching the limits of the trades, we should find a change of wind, which should shorten the voyage, or at all events keep us clear of storms.

In the open ocean, where there are no hills or extraordinary conformations of land to break the uniformity of the earth's surface, and where the expanse of water is unbroken by any extensive group of islands, the disturbances in the atmospheric belt must necessarily be much less strongly marked than where continents are interposed, or in the narrow seas. The winds themselves, under such circumstances, display even in their shifts a certain amount of regularity, which is usually dependent upon the universal laws of nature.

Once any one is so fortunate as to comprehend the latter in all their extent, so as to be cognisant of their results, it becomes a mere question of the study of local conditions in order to be able to declare how these universal laws operate, and to elucidate by the most simple explanations many of the phenomena of nature that have till now baffled science. Thus, when a wind hitherto steady shifts its direction, there must necessarily be, certain active causes for its doing so; if these causes perpetually recur in well-marked periodical intervals, the change of the wind must follow a definite law. Under certain circumstances the direction of the wind is well-defined; as, for instance, at certain seasons in the open ocean it remains always the same, or changes with a certain regularity, whence it becomes apparent that the causes must remain unchanging, and the recurrence of the phenomenon must accordingly admit of explanation.

We know, for example, that in the case of hurricanes—those most terrific exemplifications of the tendency of the atmosphere to move in circles—the wind does not blow in straight lines, but rather in curves described round a central point, which again is not immovable, but has a regular progression along a definite curve. In that curved plane, however, which has been termed a cyclone, the wind always blows in one and the same direction, and in the Northern Hemisphere runs counter to the motion of a watch-hand, while in the Southern Hemisphere it, on the contrary, follows that motion.

These facts once granted as accounting for such phenomena, it follows as a natural consequence of the general principles laid down, that they hold good in minor cases, and must remain of the same efficacy, whether it be a hurricane or a dust-whirl which may be under consideration.

So, too, in conformity with those laws, light winds may be found subject to a variation in direction of a similar nature, such as may not perhaps be fully exemplified in every case, but simply serve to indicate the tendency of the wind to follow the same general direction as the hurricanes themselves.

The importance of ascertaining such curvilinearity in the direction of the winds will be especially manifest at the limits within which the regular winds prevail, and when they must necessarily become intermingled with other regular currents of the atmosphere.

Accordingly, as we neared the limit of the S.E. Trades, which always extend somewhat further south, as the sun's southern declination increases, we had to traverse regions where necessarily we encountered variable winds, owing to the increased area of the Trades. There are also found occasional spots at which a more rarefied atmosphere seems to fill the surrounding space, when there is seen a similar process to that in the case of hurricanes, first visible perhaps in the higher strata, but afterwards extending to those which are lower.