In the books reckoning as being entirely the effect of currents." than that of currents. which treat of that most important subject, we meet with irreconcilable and conflicting theories. In the infancy of navigation, when nautical instruments, books, charts, &c. were in a rude and imperfect state, the errors committed by navigators not only excited sympathy and compassion, but also unluckily obtained the sanction and belief of wellinformed men. While the art remained in this state, we cannot be surprised to find, in the writings of many eminent authors, theories of currents which had no other existence than in the minds of ignorant and superstitious seamen, or in the reveries of the imagination of the learned. As the art of navigation improved, and nautical instruments, &c. became more accurate, it might have been imagined, that the knowledge of the phenomena of currents would have kept pace with the progress of science. This, however, is not the case ;—and however extraordinary it may appear, it is undoubtedly true, that our present knowledge of currents in the Atlantic Ocean, is not much better than it was in the 17th century. In confirmation of this, we shall quote a few passages from the writings of the celebrated Humboldt, and contrast them with those of an experienced, and I may add, scientific navigator. "In latitude," says Humboldt, in his Personal Narrative, p. 44, " 39° 50′ north, longitude 16° 10' west from Paris, we began to feel the effects of the great current, which, from the Azores, directs itself to wards the straits of Gibraltar and Canary Islands. Comparing the place deduced by the time-keeper with the pilot's reckoning, I was able to discover the smallest variation in the direction and velocity of the currents; and from 37° to 30° of latitude I found a current from 18 to 26 miles to the east." Such evidence, particularly from so able an observer as Humboldt, would, it might be imagined, be conclusive. But it is not so; for up starts Mr Bain, the navigator above alluded to, who says, in his ingenious and original essay on the variation of the compass, in answer to the above passage, "Now, without attempting to derogate from the universally recognised abilities of this celebrated philosopher, I am inclined to suspect, that had he been aware of the local attraction of his ship acting on the magnetic needle directing his course (which must have been west), he would not have considered this 18 or 26 miles error in the After this, what are we to think? Humboldt stands unrivalled in this, or any other age, for extent of knowledge, variety of learning, indefatigable activity, zeal, and enterprize; yet, with all his acknowledged acquirements and talent, he seems to have been ignorant of the phenomenon in the magnetic system, originally developed by the genius of Flinders, and since more fully illustrated by Bain, which, on ship-board, exerts a local influence over the magnetic needle, according as the ship's head is more or less deflected from the magnetie meridian, thereby producing an error in the reckoning to the east, in the northern hemisphere, and to the west, in the southern hemisphere. If Humboldt was ignorant of this law, and if the compasses on board his ship were so influenced at the time he made these remarks, (and we have little doubt but they were,) the 18 or 26 miles of current alluded to, must be reduced to 6 or 8, according to the necessary allowance pointed out in Bain's Essay for correcting this error. If Flinders' system be true, Humboldt has ascribed to currents that which is attributable only to a phenomenon in the magnetic stystem on ship-board. If, on the other hand, Humboldt is correct, and Flinders' and Bain's system a mere chimera, his beautifully illustrated theory of currents in the Atlantic Ocean, is to the philosopher, as well as to the pracThe system of Flinders and Bain may tical navigator, a valuable treasure. yet require to be authenticated by a further series of observations; and we are in confident hopes, that the period is not far distant when this subject will receive, from the illustrious Humboldt, the attention due to its great importance. As, however, the generality of our readers may not have had an opportunity of acquainting themselves with the different theories here alluded to, we shall first state the substance of Humboldt's theory, and make some observations on it, and then give that of Bain's, that the reader may draw his own conclusions. "My chief view," says Humboldt, in his Personal Narrative," in tracing a sketch of the currents of the Atlantic, is to prove, that the motion of the waters towards the south ! east, from Cape St Vincent to the Canary Islands," (from Canary Islands towards Cape St Vincent,) is the effect of the general motion which the surface of the ocean feels at its western extremity. We have just seen, that between the parallels of 11 and 43 degrees, the waters of the Atlantic are drawn on by the currents in a continual whirlpool. Supposing that a molecule of water returns to the same place from which it departed, we can estimate, from our present knowledge of the swiftness of currents, that the circuit of 3800 leagues is not terminated in less than two years and ten months. A boat, which may be supposed to receive no impulsion from the winds, would require thirteen months from the Canary Islands to reach the coast of Carracas, ten months to make the tour of the -Gulf of Mexico and reach Tertoise shoals opposite the port of Havannah,-while forty or fifty days might be sufficient to carry it from the Straits of Florida to the bank of Newfoundland. It would be difficult to fix the rapidity of the retrograde current from this bank to the coast of Africa; estimating the mean velocity of the waters at seven or eight miles in the 24 hours, we find ten or eleven months for this last distance." This view of the continued rotation of the currents of the Atlantic Ocean is at first sight plausible; yet, when we contemplate the immensity of water which, even at the moderate calculation of seven or eight miles in the twenty-four hours, must arrive from the east coast of America upon that of the west of Europe and Africa, we find some difficulty in adopting it. It seems natural enough, that the body of water which the opening of the Straits of Gibraltar refuses, may find an easy egress, by bending its direction along the coast of Africa, and may thence again return to the shores of America. Such, indeed, may be the true theory on which the currents in the Atlantic are constituted; but we apprehend, if the whole of this immense body of water was prevented from entering the Straits of Gibraltar, and the residue obliged to take the course assigned it, the torrent continually rushing through the Pillars of Hercules would as far exceed the torrent issuing out of the Gulf of Florida, as the width of the latter exceeds that of the former. This, however, is by no means the case; for, on the contrary, Humboldt himself assures us, (and we know that he is correct,) that the mean velocity of the currents, in This error must have originated either with the translator or the printer. the parallel of Cape Cannaveral, re sembles that of a torrent, and runs five miles an hour; and, in the 26th and 27th degrees of latitude, at the rate of five feet every second, or eighty miles in the twenty-four hours: whereas, we believe, the mean velocity of the currents running through the Straits of Gibraltar into the Mediterranean Sea, even in the narrowest place, seldom exceeds two or three miles an hour. Besides, from the bearing of the coasts between the British Channel and Cape St Vincent, may it not with great probability be assumed, that the northern part of the stream, which is separated in its course by Cape St Vincent on the north, Cape Cantin on the south, and the Straits of Gibraltar in the centre, would, like the southern branch, pass to the northward with the bending of the coasts of Portugal and Spain? Experience, however, proves this not to be the fact; and navigators, at least such as can be relied on, find the currents in that part of the Atlantic setting one way as often as they do the other. But this, like the other point in dispute, must also be left to time and further experience to determine. We may however remark, before quitting this part of the subject, that the most careless observer of the phenomena of currents must have often witnessed one or two tides, during a N.W. gale on the eastern coast of Scotland, and during a S.W. gale on the western coast of England, rise so unusually high as to be productive of much serious mischief. Both phenomena are entirely owing to the first impulse of the gale forcing the surface of the sea, until turned by the windings of the coast, towards that point of the horizon to which the wind is directed; but no sooner does the gale cease, than the tides assume their usual appearance,nay, often much sooner; for the gale may continue weeks, without producing more than one or two extremely high tides; which sufficiently shews, that the body of waters thus impelled out of its course by the beginning of the gale, returns almost instantaneously to fill the vacuity made by its own unnatural departure. A S.E. gale on the east coast of Scotland, and a N.E. gale on the west coast of England, produces a contrary effect at these places respectively; which is a still further corroboration of the causes being entirely local, and totally unconnected with any hypothesis founded or theory whatever. We still remember the dreadful effects produced on the weak and ignorant minds of seamen by "Rennell's current." This fable, which got an easy admission into nautical books, and, strange to tell, occupies fifteen pages of the seventh volume of "Campbell's Lives of the Admirals," just published, was calculated to do, and certainly did do, incalculable mischief. It is now happily exploded; and we have alluded to it now, merely to shew how very cautious navigators ought to be, before they adopt into their calculations systems and theories which are not founded in truth, and verified by experience. On these principles we have no hesitation in saying, that whatever may be the laws which give effect to currents without the tropics,-whether by external impulse, by a difference of heat or saltness, by the periodical melting of the polar ice, by the inequality of evaporation, or by the variable pressure of the atmosphere on the surface of the sea,-this we are pretty certain of, that the currents within the tropics undoubtedly receive the first impulse from, and are set in motion by, the trade and monsoon winds. This is a fact so universally acknowledged, since first stated by Dampier, as to require no illustration. "Local situations," says Mr Bain, "produce local currents; and there are few capes or promontories where currents are not more or less experienced. But if those capes or promontories are situated within the influence of trade or monsoon winds, the direction of the waters follow that of the winds. These currents are, however, very superficial; and we frequently observe two different currents in the same place, the upper part of the cable of a ship at anchor being sometimes carried one way, whilst the lower part is carried another. Even on the shores of the Florida channel, the most remarkable in the world for its currents, a ship may pass the opposite way, by the revulsion of the waters close in shore. "The trade-winds which blow between the coast of Africa and Brazil, and the southerly winds which prevail on the former, (both of which phenomena are perhaps owing to the opposite rotation of the earth,) undoubtedly force along the surface of the ocean towards the coast of Brazil, which, stopping in its direct progress, it diverges towards the north and south. The greater part passes into the Gulf of Mexico, and hence, by the famous gulf stream, is again thrown into the Atlantic. A lesser portion passes west, through the Straits of Magellan, and round Cape Horn. On these different coasts of America, the currents are generally pretty strong; but twenty, thirty, and fifty leagues off the coast, they are scarcely discernible. At Barbadoes there is very little current. "In the East Indies, the N.E. monsoon, which blows from October to April, and the S.W. monsoon, which blows from April to the waters over which they respectively pass. October, have very considerable effect on From the east coast of China, during the N.E. monsoon, the waters are impelled with considerable velocity along the coasts of Tonquin, Cochin-China, across the Gulf of Siam, towards the Malacca peninsula ; and from thence they pass through the different straits west of Borneo, into the Java Sea, and are again carried by the N.W. winds (which prevail during the period the N.E. monsoon continues in the China Sea) eastward. Hence the impracticability of making a passage, during the N.E. monsoon, up the China Sea, or one down the Java Sea during the same period; although a passage is to be made either way, by going out of the one sea into the other. "The S.W. monsoon has the same effect, but in a contrary direction. But it is to be observed, that at some distance from the respective shores just mentioned, the currents are weak and feeble, when compared with their force in shore, and in the different straits through which they pass. "From the west side of the great Asian Archipelago, and eastern side of the Bay of Bengal, the waters, during the N.E. monsoon, are forced round the island of Ceylon, across the Arabian Sea, until turned to the S.W. by Cape Guardafici. From thence they follow the direction of the coast, and run with considerable rapidity towards the Cape of Good Hope, where they are again turned, by the westerly winds, eastward towards New Holland; but in this last track their force is extremely weak; for it has been repeatedly observed, that after passing to the southward or eastward of the great bank of Agulhas, the currents are hardly perceptible; whereas a ship on the bank will be sometimes set twenty, forty, and often sixty miles in the twenty-four hours to the S.W., against a heavy gale of wind from that quarter. On the coast of Chili and Peru, from 30° of south to 4° of north latitude, the constant southerly winds force a current along that coast; and off Cape St Francis, Cape Passao, Cape St Lawrence, and Cape Blanco, they are generally very strong; but forty or fifty leagues from the shore they are extremely weak. At the Gallapagos Islands they are hardly perceptible. On the coasts of Peru and Mexico the same thing is observed." DISCOVERY OF HAUYNE, IN THE ISLAND OF TIREE. HITHERTO in Scotland the attention of mineralogists has been principally directed to the investigation of the structure, relative position, and mode of formation of mountain rocks. This branch of mineralogy, it must be confessed, is more generally interesting than any of the others. The mind delights more in tracing out those grander features and relations in the mineral kingdom, exhibited in the structure and arrangement of mountainous and alpine country, than in decyphering the minute, although very interesting, connexions observable among simple minerals. We have accurate geognostical descriptions of many extensive tracts of country in Great Britain, but the history of the simple minerals contained in the rocks of these districts is but imperfectly known. It is therefore with pleasure that we communicate to our readers the following notice, given to us by the celebrated professor Pictet of Geneva. Description of a Mineral nearly resembling Hayne, found in primitive Limestone, in the Island of Tiree, one of the Hebrides. By Professor L. A. NECKER: Colour-Pure sky-blue, sometimes slightly greenish. Lustre Shining and vitreous. Chemical Characters. Before the blow-pipe it becomes white and opaque, but does not melt. It dissolves in acids, but we could not, from the smallness of the quantity, determine if it formed with them a jelly. Geognostic Situation. It occurs in very minute grains in the contemporaneous masses of felspar, mica, sahlite, and augite, which are imbedded in a primitive limestone contained in gneiss. Geographic Situation. It occurs in the limestone or marble rocks at the farm of Balephetrich in Tiree. Haüyne, it will be the first instance of its occurring in a primitive district, the varieties hitherto described having been met with in lavas, basaltes, and rocks thrown out by volcanoes*. A Word to the rival Huttonian and It may appear somewhat like presumption in a nameless observer to attempt to direct the attention of two sects so violent in controversy, and so impatient of contradiction, to any geological appearances that equally support the truth of both the theories of the earth contrived by their ingenuity. But a natural wish on my part to repress the over-weening confidence of both, and thus to lead more certainly to a true judgment in mineralogical subjects, induces me to request, Mr Editor, your countenance to the short statement I am about to give. If the phenomena I propose to mention were distant from this city, and concealed in a wild and inaccessible part of the country, I know well what would happen; both fire and water philosophers would join against me, and would say this anonymous scribbler knows nothing of the matter-he cannot distinguish one rock from another, and it is quite evident that he is ignorant of the most obvious distinctions known to every tyro in mineralogy. They would spurn from them such information as that we are about to communicate, because it would shew that both are in the right, and both in the wrong, and thus force them to abandon the fantastic field of geological romance. But the geological appearances are neither distant nor difficult of access; on the contrary, they are open to the hourly inspection of all the numerous geologists of this intellectual city. Note. Some mineralogists maintain taining Hayne are true lavas. This we that all the basaltes and ejected rocks condeny, because these basaltes and ejected rocks are different in their external and physical characters from lavas. It is said, for instance, that the Monte Somma at Vesuvius, which contains these ejected masses, is the old crater of that volcano, and in some aspect. We see no evidence for the voldrawings it has certainly a very crater-like canic origin of Somma, and, until such is produced, shall be contented to consider it as an unaltered portion of the general crust of the earth. Translator. 4 E They are to be seen in the grand section of the Calton Hill, at the site of the proposed Waterloo hotel. The whole mass of rock in that part of the hill is composed of a substance known under the popular name of whin-stone, or, to speak more scientifically, greenstone. Now this whin-stone, or greenstone, according to the Huttonians, is a lava which has been projected from the grand repository of that matter in the interior of the earth,-while the Wernerians maintain, that it is a rocky substance deposited from the original waters of the globe, when this submundane system was emerging from its chaotic state. This greenstone is traversed by three veins of green-stone, that exhibit many interesting phenomena, of which the most striking are those which are at present to claim our attention. For the previous information of your readers it may be remarked, that both theories are considered to depend very much on the appearances presented by veins. The Huttonians maintain, that veins were formerly open fissures or rents, widening towards the interior of the earth, but becoming gradually narrower, and terminating upwards; these fissures are supposed to have been filled with the matter they now contain by the injection of melted lava from below. Hence, according to the Huttonian view, veins always widen as they go downwards, but become narrower and narrower, and at length terminate above in some rock or other. The Wernerians agree with the Huttonians, in considering all veins as having been originally open fissures or rents, but maintain, that these rents or fissures are wide above, but become gradually narrower and narrower, at length terminate below. These rents are supposed to have been filled from above, from the waters of the ancient ocean, with the minerals they now contain. What then are the peculiarities of the veins of the Calton Hill? These may be stated in a few words. One of the veins is wide above, but as it descends, becomes narrower, and at length terminates in a narrow wedge. This is an example of a vein filled from above, or of a Wernerian vein. Another vein of green-stone, which is wide below, but narrow and terminated above, is a short distance from that just mentioned, and is an example of a Huttonian vein, or one filled from below. The third vein, in its present condition, cannot be considered either as Wernerian or Huttonian, for it has no visible beginning nor end. If we had an opportunity of seeing it in its whole length, it might either terminate above or below, and thus afford, according to circumstances, an illustration of either of the rival theories. The facts we have just stated are very fearful in their consequences to both theories, in short, involving nothing less, in my feeble comprehension, than their utter destruction. But we must have something in their place, we cannot exist, geologically, without a theory of veins. We have heard it whispered, that all veins of this description terminate, both above and below, in the rock in which they are contained, and that in their original state neither communicated with the infernal regions nor with the wild and tumultuous water of the superincumbent ocean. On the Use of Petrifactions as a Character in the Discrimination of Rock Formations. have indulged in this Number of your It was my intention, Mr Editor, to Magazine, in a series of observations on petrifactions, with the view of proving, that rock formations are in every case to be determined by their mineralogical characters alone,*_and_that therefore the attempts which have been lately made of separating and classifying them by the fossil organic remains they contain, must be abandoned as incorrect and unphilosophical, but want of leisure prevents me entering on the subject at present. vier, Blumenbach, Spix, and many I am aware of the opinions of Cuothers, and still I believe in the accuracy of the opinion just stated. readily acquiesce in a doctrine which These zoological gentlemen will not takes so much from the importance they have lately assumed in mineralogical matters; but I am inclined to think, that our friends the Huttonians and Wernerians, who are sensible and well disposed as long as they are kept out of reach of fire and water, will hear reason, and agree with us in en A position I have heard proposed and illustrated in Professor Jameson's lectures. |