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Figure the 1st represents the length of a 74-gun ship, divided into five sections, and the excess of weight or pressure in each.
Figure the 2nd also represents the length, divided into sections, and the positions of the masts, by which it will be seen that they are situated in those parts where the weights preponderate; and that they greatly contribute to it is a very natural conclusion.
In the foremost section there is, unavoidably, not only the weight of the foremast and its rigging, but the bowsprit and anchors also; the latter stowed at the extremity of the section, and the bowsprit projecting considerably beyond it; which, on the lever principle, assisted by the action of the ship and the excess of weight, and being situated in a part which from its formation affords but little resistance, and which is frequently left unsupported, must considerably augment the pitching motion, and contribute to the tendency to arch or hog.
In the aftermost section, the mizen-mast may be considered as contributing to the excess of weight; but it is not of such magnitude as the fore or mainmast; and its position is much more favourable for the ship than the foremast. The strain also on the after part is not so great as forward, as it is seldom
left so entirely unsupported by the passing of a sea, and the sending motion is not so sudden or violent as the pitching.
In the extreme sections, and from the formation of those parts below the surface of the water, the upper works may also be considered as contributing to the excess of weight.
In the middle section, the mainmast, from its magnitude, may be considered as materially contributing to the excess of weight; and that, according to the ordinary distribution of the weights in the main hold, a large proportion of it may be supposed to fall in the vicinity of the mast. If this conclusion be admitted, it must appear prejudicial, and the more so, in consequence of the pressure preponderating so much at both extremities of the section. Among other weights concentrated about the mast is the shot locker, and which must considerably add to the unavoidable and too great weight of the mast alone acting on so small a space.
I shall now endeavour to explain, how far the alterations proposed in my former remarks would be likely to remedy the inequality of the distribution of the weight and pressure specified in the calculation.
In the foremost section No. 5, where there is at present an excess of weight, are placed the bower anchors, and the gunners', boatswains', and carpenters' sea-stores, which I proposed transferring; the anchors further aft, by removing the catheads to the fore-parts of the channels, which, although by the removal still situated in the same section, would considerably lessen the power of their weight on the ship in a sea; and the sea-stores to the after cock-pit, which will be found situated in section No. 2, where the pressure now preponderates.
In the after section No. 1, where there is also an excess of weight, the bread is stowed at the extremity; a part of which, together with beds, slops, marine clothing, &c., (at present stowed in the after cock-pit) I proposed should occupy the present place of the sea-stores in section No. 5; which would reduce the excess of weight at present in the after extremity, and place less weight, more lively, and more speedily consumed than the sea-stores, in the place now occupied by them.
Such arrangements would also be likely to contribute to a
more equal distribution of the weights and pressure in sections 3 and 4; as it would most probably admit of weight being removed from the one where there is at present an excess, to that where pressure preponderates,—and, in fact, to a much more equal distribution throughout the whole body.
The following alterations would also contribute to a better distribution than the present.
Let the bower and sheet cables be stowed further aft; the stream cable and hawsers abaft them, in the space now occupied by cabins, slop-room, &c.; and the stores, at present stowed in the foremost section No. 5, removed aft to the fore part of the cable tiers; a part of the bread, slops, beds, marine clothing, &c., as in the former proposal, to occupy the present place of the sea-stores in section No. 5. By such arrangements the weight of the stream cable and hawsers, and part of the bower cables, would be removed to section No. 2; the sea-stores to No. 4; and the weights reduced at both extremities.
The excess of weight would not only be considerably reduced in the fore extremity, by the proposed alterations; but at sea, if desirable to lighten this part as speedily as possible, a regular decrease, in a 74-gun ship, of two tons per week, may be calculated on, till the expiration of half the cruise, when the fore bread-room would be cleared.
By a minute inspection and measurement of the different parts, and a correct calculation of the different weights and their bulk, the distribution may be made with great exactness.
Removing the anchors where they will stow perfectly clear of the ports, and similar to the sheet and spare anchors, will also leave the round of the bow perfectly clear, and remove every impediment to its being better fortified.
Plymouth, Feb. 14th, 1829.
Geological Notices of St. Helena suggested during a Visit to that Island in 1828, in a Voyage from India to England. BY CHARLES H. WESTON, Esq.
THE island of Saint Helena is interesting to the visitor on every account. Whether he may view it as the little spot which has formed the last and circumscribed retreat of one who in the days of his glory and ambition thought the wide world scarce sufficiently extensive; or whether he consider its peculiar scenery abounding in contrasts-of luxuriance and sterility, between whose rugged and naked hill-tops and grass-green vallies there is no such gradation as the English eye is accustomed to dwell upon; or whether he look upon its volcanic characters, and reflect upon the period when nature here exhibited such awful proofs of her irresistible power,-it will be found equally capable of exciting his interest. But it is in its last character that we are interested, and about which I would offer some remarks. These remarks, however, must unavoidably be few in number, and somewhat superficial in their nature, from the limited residence which the shipment of necessary supplies usually affords to passengers in their voyage from India to England.
But while stating the characters of the few geological specimens I collected and procured, I shall not hesitate to make use of the information of others, whose opportunities have afforded them better local knowledge; and if any hint should lead other and more able travellers to direct their attention. to the geology of this singular island, so as eventually to throw any light upon the interesting problem,-" Whether St. Helena is truly a volcanic product, or the remnant of a much larger portion of land now submerged," *-an object of no small importance will have been attained.
In approaching St. Helena from the southward, we sail
* Vide Beatson's "Tracts relative to St. Helena. Daubeny on Volcanoes. Philosophical Magazine, No. 342.
round the island from the south-east to the north-west, so that a great line of coast is presented to our view, and as the shores are so precipitous as to allow a ship to sail within almost two cables' length of the land, the whole coast is distinctly seen and may be easily examined. But I did not in any part recognise rocks belonging to the primitive formation; neither did I meet with any during my rides in the interior. From General Walker's geological notices also, it is evident that he had never met with any of this class; and as Governor Beatson is equally silent on this subject, we may conclude that primitive rocks do not exist in the island.
The perpendicular coast seems to give strong evidence of the convulsions which St. Helena must have experienced at some remote period; for not only is it intersected with deep ravines, but at the northern part there appeared to be nearly a complete separation of the cliff from the top to the bottom. The whole stratification also is much disturbed, and dips in every direction; at one spot I recollect seeing the strata of a hill to dip on either side to the contiguous vallies.
There are many parts which are beautifully intersected by red veins, composed of alumina, highly impregnated with the red oxide of iron.
In speaking of the general characters of St. Helena, I would only remark, that the volcanic matter chiefly consists of a rock of a basaltic naturet; and therefore, when we read, that "obsidian or pumice has never, I believe, been found here," we see a reason why the latter substance, at least, should not have been found, as it is the product of trachyte or feldspathic lava, and not of augitic or basaltic lava §.
But before describing the interior of the island, I will here subjoin a list of some geological specimens, with their localities:
No. 1. Basaltic lava, compact, 2. Ferruginous schistos,
* Vide Philosophical Magazine, No. 342.
+ See also Daubeny.
See Philosophical Magazine, No. 342. See Daubeny, p. 24, et passim. Analysis of different substances in Ure's Chemical Dictionary; and article" Pumice," Rees's Cyclopædia.