Page images
PDF
EPUB

Hourly observations, or, at all events, those provided for by the Board of Trade, should be made off the Western Coast of Africa, having especial reference to this depression, also to the elucidation of the origin of the great system of southwesterly atmospheric waves that traverse Europe. They should also furnish data for comparison with the amount of oscillation and other barometric phenomena in the Gulf of Mexico and the Caribbean Sea, a portion of the torrid zone essentially different in its configuration and in the relations of its area to land and water, as contradistinguished from the northern portion of the African continent. Such observations are the more desirable as the vessels may approach the land, and should be carefully made in both localities, viz. off the continent of Africa and amongst the West Indian islands.

Southern Atlantic.-There are two points in the southern hemisphere, between 80° west longitude and 30° east longitude, that claim particular attention in a barometric point of view-viz. Cape Horn and the Cape of Good Hope. Too much attention cannot be paid to the indications of the barometer, combined with the direction and force of the wind, as vessels are approaching or leaving the Cape of Good Hope, especially for contrasting the meteorology of the southern extremity of Africa with the northern part of the South Atlantic Ocean, which has been termed the true Pacific Ocean of the world. At St. Helena a gale is scarcely ever known; it is also said to be entirely free from actual storms (Col. Reid's Law of Storms,' 1st edition, p. 415). It may therefore be expected that the barometer will present in this locality but a small oscillation, and ships in sailing from the Cape to St. Helena will do well to ascertain the decrease of oscillation as they approach that island. The same thing will hold good with regard to Cape Horn: it appears, from previous observation, that a permanent barometric depression exists in this locality, most probably

[ocr errors]

hundredths of an inch, and this without incurring risk by moving the instrument, and without any trouble beyond making the usual observations."

in some way connected with the immense depression noticed by Captain Sir James Clark Ross, towards the Antarctic Circle. The general character of the atmosphere off Cape Horn is also extremely different from its character at St. Helena. It would, therefore, be well for vessels sailing into the Pacific by Cape Horn to adhere as closely as possible to the instructions issued by the Board of Trade.

Before quitting the Atlantic Ocean it may be well to notice the marine stations mentioned in my Third Report on Atmospheric Waves,* as being particularly suitable for testing the views advanced in that report, and for tracing a wave of the south-westerly system from the most western point of Africa to the extreme north of Europe. A series of hourly observations off the western coast of Africa has already been suggested. Vessels staying at Cape Verde Islands should not omit to make observations at two hours' interval during the whole of their stay, and, when circumstances will allow, to make them hourly. At the Canaries, Madeiras, and the Azores, similar observations should be made. Vessels touching at Cape Cantin, Tangier, Gibraltar, Cadiz, Lisbon, Oporto, Corunna, and Brest, should also make these observations while they are in the localities of these ports. At the Scilly Isles we have sixhourly observations, made under the superintendence of the Honourable the Corporation of the Trinity House. Ships, in nearing these islands, and making the observations already pointed out, will greatly assist in determining the increase of oscillation proceeding westward from the nodal point of the two great European systems. Vessels

* Reports of the British Association for the Advancement of Science, 1846, p. 139.

Nodal point. The two systems of north-east and north-west winds, with their compensatory south-west and south-east winds, as explained on pp. 171, 172, very materially influence the barometer, so that in the neighbourhood of Brussels, for instance, the variations of pressure as manifested by the instrument are very much less than in localities removed from that city towards the north-west. Those points on the terrestrial or oceanic surfaces where the oscillations of the mercury are smaller than in the localities around them are called nodal points.

navigating the North Sea, the Baltic, and the coast of Norway, as far as Hammerfest, may greatly contribute towards the same object; nor is the Mediterranean without considerable interest, both as regards these particular waves and the influence which its waters exert in modifying the two great systems of central Europe. We have already alluded to the influence of its waters in originating small atmospheric waves, and no opportunity should be lost for increasing our knowledge of its meteorology, especially in the Straits of Gibraltar, the neighbourhood of Sicily and Italy, and in the Grecian Archipelago.

The Indian and Southern Oceans.-There is perhaps no portion of the aqueous surface of the globe with the meteorological character of which we are better acquainted than the Indian Ocean, traversed as it is by vessels trading to our Indian territories, the empire of China, and our flourishing colonies in Australia; its regions of trade-winds and monsoons, its calms and variables, its rains and storms, have been tolerably well marked out. Still much remains to be done, and the example so well set forth in the very bosom of the Indian Ocean itself (in that island gem which rises above its waters and stands as a friendly resting-place between the Cape, stormy as it is at times, and the Bay over which the revolving cyclone sweeps with devastating violence, or that sea in which the typhoon stalks abroad, spreading around and within its circumference destruction and death)-an example, likely to be attended with results of almost incalculable benefit to seamen-should be most assiduously followed, especially in that locality the winds and storms of which have contributed in no small degree to develope to a greater extent that peculiar power of the human mind which seeks to apprehend at a glance the varied and complicated dispositions of natural phenomena spread over an extensive surface, and to grasp that thread of relationship which binds. the whole together, and shows that each is but a part of one mighty and stupendous whole.

These reflections have been forced upon us by the consideration of Professor Meldrum's paper on the Meteorology

of the Indian Ocean. It forms one of the Meteorological papers issued by the Board of Trade, and should be in the hands of every commander who sails eastward of the Cape. The portion more particularly interesting to us in connexion with this article is that which treats of the N.E. wind eastward of the Cape, and the S.W. wind which blows between it and the Cape. If our readers will take the trouble to compare Professor Meldrum's arrows indicating these winds on his chart with our diagram of an atmospheric wave, they will at once see that the winds in question form a well-marked instance of a wave rolling from the eastern coasts of Africa to about 70° E. long., on which meridian it is generally broken up. The elements of the wave appear to be unmistakeable; but, as the phenomena of the cyclone are reversed in the opposite hemispheres, so are those of the wave. In the northern hemisphere, the barometer, during the passage of an atmospheric wave, rises with N.E. winds; in this instance, in the southern hemisphere, it falls; in the northern hemisphere, as the N.E. winds pass they decrease in force; in the southern hemisphere they appear to increase; and it is highly probable that, as over so large an area as 50° of E. long. and 20° of S. lat. these waves appear continually to roll, so the phenomena exhibited will be exactly the opposite of those characterising the atmospheric waves of western Europe. The area above marked out, viz. from 20° to 70° E. long., and from 20° to 40° S. lat., will consequently claim great attention while sailing through it.

A highly practical question, however, arises here. Professor Meldrum shows that at the Mauritius, lat. 20° 10′ S., long. 57° 29′ E., on the 6th of March, 1853, the barometer was low, 29-738, and falling, with wind at E.S.E. On the 7th it fell calm, while at the same time the 'Fanny Fisher,' in 25° 22′ S. lat., 80° 57' E. long., had fresh breezes from E.S.E.; her barometer, which had been falling, was quoted at 29-82, and she took in a single reef of her topsails. On the 8th, in 24° 10' S. lat., 77° 17′ E. long., she experienced a fresh gale between E. and E.S.E.; was reduced to close-reefed topsails; hove-to, on the port

tack, gale increasing and barometer falling (29-40). On the 9th, a heavy gale, with constant wind E.N.E., barometer rising rapidly. At noon her position was 24° 59′ S. lat., 76° 39' E. long.

From these quotations it would appear that a circular storm had passed to the northward of the ship. Professor Meldrum finds, however, that the bad weather is not to the northward, but to the westward, and he thus reasons that while a captain, implicitly relying on the instructions of cyclonologists, would endeavour to pass in front of the storm, and steer W. or W.N.W., by so doing he would get the worst of it, and steer into instead of away from the storm in fact, he would run into the very heart of the gale. If, then, the phenomena of the storms and waves are, on the one hand, identical, while, on the other, the fury of the cyclone is raging in one direction and that of the atmospheric wave in the opposite or nearly so, what criterion has the commander to guide him in the determination that his gallant vessel is ploughing through the sea raised by the winds of the atmospheric wave, and not battling with that in the dangerous quadrant of a cyclone?

This is an important question, and one that must be answered before a commander can find himself perfectly at ease in the Indian Ocean anywhere eastward of the Cape and southward of Madagascar and the Mauritius. Hereabout cyclones generally move towards the S.E. Professor Meldrum considers that they are generated between the N.E. and S.W. winds, i. e. in the trough between the two compensating winds. If so, there would be two classes of phenomena that would at once decide the question. With a N.E. wind, a cyclone approaching from north-west, and catching the ship on the southern quadrant, the wind would veer to E., increase in force, veer to S.E., and pass off at S.; but if the ship were caught on the eastern quadrant, the wind would veer to N., increase in force, then veer to N.W., and pass off at W. All this is very different from S.E., E., to N.E. veering. It would be indeed but rarely that between 20° and 30° S. lat. and 50° and 70° E. long. cyclones would bear down from N.E.; a few might

« PreviousContinue »