Kitabı oku: «The Atlantic Monthly, Volume 12, No. 74, December, 1863», sayfa 11

We find great national significance in the history of bridges in different countries. Their costly and substantial grandeur in Britain accords with the solid qualities of the race, and their elegance on the Continent with the pervasive influence of Art in Europe. It is a curious illustration of the inferior economical and high intellectual development of Greece, that the "Athenians waded, when their temples were the most perfect models of architecture"; and equally an evidence of the practical energy of the old Romans, that their stone bridges often remain to this hour intact. Our own incomplete civilization is manifest in the marvellous number of bridges that annually break down, from negligent or unscientific construction; while the indomitable enterprise of the people is no less apparent in some of the longest, loftiest, most wonderfully constructed and sustained bridges in the world. We have only to cross the Suspension Bridge at Niagara, or gaze up to its aërial tracery from the river, or look forth upon wooded ravines and down precipitous and umbrageous glens from the Erie Railway, to feel that in this, as in all other branches of mechanical enterprise, our nation is as boldly dexterous as culpably reckless. As an instance of ingenuity in this sphere, the bridge which crosses the Potomac Creek, near Washington, deserves notice. The hollow iron arches which support this bridge also serve as conduits to the aqueduct which supplies the city with water.

Amid the mass of prosaic structures in London, what a grand exception to the architectural monotony are her bridges! how effectually they have promoted her suburban growth! Canova thought the Waterloo Bridge the finest in Europe, and, by a strangely tragic coincidence, this noble and costly structure is the favorite scene of suicidal despair, wherewith the catastrophes of modern novels and the most pathetic of city lyrics are indissolably associated. Westminster Bridge is as truly the Swiss Laboyle's monument of architectural genius, fortitude, and patience, as St. Paul's is that of Wren; and our own Remington's bridge-enthusiasm involves a pathetic story. At Cordova, the bridge over the Guadalquivir is a grand relic of Moorish supremacy. The oldest bridge in England is that of Croyland in Lincolnshire; the largest crosses the Trent in Staffordshire. Tom Paine designed a cast-iron bridge, but the speculation failed, and the materials were subsequently used in the beautiful bridge over the River Wear in Durham County. There is a segment of a circle six hundred feet in diameter in Palmer's bridge which spans our own Piscataqua. It is said that the first edifice of the kind which the Romans built of stone was the Ponte Rotto, begun by the Censor Fulvius and finished by Scipio Africanus and Lucius Mummius. Popes Julius III. and Gregory XIV. repaired it; so that the fragment now so valued as a picturesque ruin symbolizes both Imperial and Ecclesiastical rule. In striking contrast with the reminiscences of valor, hinted by ancient Roman bridges, are the ostentatious Papal inscriptions which everywhere in the States of the Church, in elaborate Latin, announce that this Pontiff built or that Pontiff repaired these structures.

The mediæval castle moat and drawbridge have, indeed, been transferred from the actual world to that of fiction, history, and art, except where preserved as memorials of antiquity; but the civil importance which from the dawn of civilization attached to the bridge is as patent to-day as when a Roman emperor, a feudal lord, or a monastic procession went forth to celebrate or consecrate its advent or completion; in evidence whereof, we have the appropriate function which made permanently memorable the late visit of Victoria's son to her American realms, in his inauguration of the magnificent bridge bearing her name, which is thrown across the St. Lawrence for a distance of only sixty yards less than two English miles,—the greatest tubular bridge in the world. When Prince Albert, amid the cheers of a multitude and the grand cadence of the national anthem, finished the Victoria Bridge by giving three blows with a mallet to the last rivet in the central tube, he celebrated one of the oldest, though vastly advanced, triumphs of the arts of peace, which ally the rights of the people and the good of human society to the representatives of law and polity.

One may recoil with a painful sense of material incongruity, as did Hawthorne, when contemplating the noisome suburban street where Burns lived; but all the humane and poetical associations connected with the long struggle sustained by him, of "the highest in man's soul against the lowest in man's destiny," recur in sight of the Bridge of Doon, and the two "briggs of Ayr," whose "imaginary conversations" he caught and recorded, or that other bridge which spans a glen on the Auchinleck estate, where the rustic bard first saw the Lass of Ballochmyle. The tender admiration which embalms the name of Keats is also blent with the idea of a bridge. The poem which commences his earliest published volume was suggested, according to Milnes, as he "loitered by the gate that leads from the battery on Hampstead Heath to the field by Camwood"; and the young poet told his friend Clarke that the sweet passage, "Awhile upon some bending planks," came to him as he hung "over the rail of a foot-bridge that spanned a little brook in the last field upon entering Edmonton." To the meditative pedestrian, indeed, such places lure to quietude; the genial Country Parson, whose "Recreations" we have recently shared, unconsciously illustrates this, when he speaks of the privilege men like him enjoy, when free "to saunter forth with a delightful sense of leisure, and know that nothing will go wrong, although he should sit down on the mossy parapet of the little one-arched bridge that spans the brawling mountain-stream." On that Indian-summer day when Irving was buried, no object of the familiar landscape, through which, without formality, and in quiet grief, so many of the renowned and the humble followed his remains from the village-church to the rural graveyard, wore so pensive a fitness to the eye as the simple bridge over Sleepy-Hollow Creek, near to which Ichabod Crane encountered the headless horseman,—not only as typical of his genius, which thus gave a local charm to the scene, but because the country-people, in their heartfelt wish to do him honor, had hung wreaths of laurel upon the rude planks.

Fragments, as well as entire roadways and arches of natural bridges, are more numerous in rocky, mountainous, and volcanic regions than is generally supposed; the action of the water in excavating cliffs, the segments of caverns, the, accidental shapes of geological formations, often result in structures so adapted for the use and like the shape of bridges as to appear of artificial origin. In the States of Alabama and Kentucky, especially, we have notable instances of these remarkable freaks of Nature: there is one in Walker County, of the former State, which, as a local curiosity, is unsurpassed; and one in the romantic County of Christian, in the latter State, makes a span of seventy feet with an altitude of thirty; while the vicinity of the famous Alabaster Mountain of Arkansas boasts a very curious and interesting formation of this species. Two of these natural bridges are of such vast proportions and symmetrical structure that they rank among the wonders of the world, and have long been the goals of pilgrimage, the shrines of travel. Their structure would hint the requisites, and their forms the lines of beauty, desirable in architectural prototypes. Across Cedar Creek, in Rockbridge County, Virginia, a beautiful and gigantic arch, thrown by elemental forces and shaped by time, extends. It is a stratified arch, whence you gaze down two hundred feet upon the flowing water; its sides are rock, nearly perpendicular. Popular conjecture reasonably deems it the fragmentary arch of an immense limestone cave; its loftiness imparts an aspect of lightness, although at the centre it is nearly fifty feet thick, and so massive is the whole that over it passes a public road, so that by keeping in the middle one might cross unaware of the marvel. To realize its height it must be viewed from beneath; from the side of the creek it has a Gothic aspect; its immense walls, clad with forest-trees, its dizzy elevation, buttress-like masses, and aërial symmetry make this sublime arch one of those objects which impress the imagination with grace and grandeur all the more impressive because the mysterious work of Nature,—eloquent of the ages, and instinct with the latent forces of the universe. Equally remarkable, but in a diverse style, is the Giant's Causeway, whose innumerable black stone columns rise from two to four hundred feet above the water's edge in the County of Antrim, on the north coast of Ireland. These basaltic pillars are for the most part pentagonal, whose five sides are closely united, not in one conglomerate mass, but, articulated so aptly that to be traced the ball and socket must be disjointed.

The effect of statuary upon bridges is memorable: the Imperial statues which line that of Berlin form an impressive array; and whoever has seen the figures on the Bridge of Sant' Angelo at Home, when illuminated on a Carnival night, or the statues upon Santa Trinità at Florence, bathed in moonlight, and their outline distinctly revealed against sky and water, cannot but realize how harmoniously sculpture may illustrate and heighten the architecture of the bridge. More quaint than appropriate is pictorial embellishment; a beautiful Madonna or local saint placed midway or at either end of a bridge, especially one of mediæval form and fashion, seems appropriate; but elaborate painting, such as one sees at Lucerne, strikes us as more curious than desirable. The bridge which divides the town and crosses the Reuss is covered, yet most of the pictures are weather-stained; as no vehicles are allowed, foot-passengers can examine them at ease. They are in triangular frames, ten feet apart, but few have any technical merit. One series illustrates Swiss history; and the Kapellbrücke has the pictorial life of the Saint of the town; while the Mile Bridge exhibits a quaint and rough copy of the famous "Dance of Death."

In Switzerland what fearful ravines and foaming cascades do bridges cross! sometimes so aërial, and overhanging such precipices, as to justify to the imagination the name superstitiously bestowed on more than one, of the Devil's Bridge; while from few is a more lovely effect of near water seen than the "arrowy Rhone," as we gaze down upon its "blue rushing" beneath the bridge at Geneva. Perhaps the varied pictorial effects of bridges, at least in a city, are nowhere more striking than at Venice, whose five hundred, with their mellow tint and association with palatial architecture and streets of water, especially when revealed by the soft and radiant hues of an Italian sunset, present outlines, shapes, colors, and contrasts so harmonious and beautiful as to warm and haunt the imagination while they charm the eye. It is remarkable, as an artistic fact, how graciously these structures adapt themselves to such diverse scenes,—equally, though variously, picturesque amid the sturdy foliage and wild gorges of the Alps, the bustle, fog, and mast-forest of the Thames, and the crystal atmosphere, Byzantine edifices, and silent canals of Venice.

Whoever has truly felt the aërial perspective of Turner has attained a delicate sense of the pictorial significance of the bridge; for, as we look through his floating mists, we descry, amid Nature's most evanescent phenomena, the span, the arch, the connecting lines or masses whereby this familiar image seems to identify itself not less with Nature than with Art. Among the drawings which Arctic voyagers have brought home, many a bridge of ice, enormous and symmetrical, seems to tempt adventurous feet and to reflect a like form of fleecy cloud-land; daguerreotyped by the frost in miniature, the same structures may be traced on the window-pane; printed on the fossil and the strata of rock, in the veins of bark and the lips of shells, or floating in sunbeams, an identical design appears; and, on a summer morning, as the eye carefully roams over a lawn, how often do the most perfect little suspension-bridges hang from spear to spear of herbage, their filmy span embossed with glittering dew-drops!

INTERNAL STRUCTURE AND PROGRESSION OF THE GLACIER

It is not my intention, in these articles, to discuss a general theory of the glaciers upon physical and mechanical principles. My special studies, always limited to Natural History, have but indifferently fitted me for such a task, and quite recently the subject has been admirably treated from this point of view by Dr. Tyndall, in his charming volume entitled "Glaciers of the Alps." I have worked upon the glaciers as an amateur, devoting my summer vacations, with friends desirous of sharing my leisure, to excursions in the Alps, for the sake of relaxation from the closer application of my professional studies, and have considered them especially in their connection with geological phenomena, with a view of obtaining, by means of a thorough acquaintance with glaciers as they exist now, some insight into the glacial phenomena of past times, the distribution of drift, the transportation of boulders, etc. It was, however, impossible to treat one series of facts without some reference to the other; but such explanations as I have given of the mechanism of the glacier, in connection with its structure, are presented in the language of the unprofessional observer, without any attempt at the technicalities of the physicist. I do not wonder, therefore, that those who have looked upon the glacier chiefly with reference to the physical and mechanical principles involved in its structure and movement should have found my Natural Philosophy defective. I am satisfied with their agreement as to my correct observation of the facts, and am the less inclined to quarrel with the doubts thrown on my theory since I see that the most eminent physicists of the day do not differ from me more sharply than they do from each other. The facts will eventually test all our theories, and they form, after all, the only impartial jury to which we can appeal. In the mean while, I am not sorry that just at this moment, when recent investigations and publications have aroused new interest in the glaciers, the course of these articles brings me naturally to a discussion of the subject in its bearing upon geological questions. I shall, however, address myself especially, as I have done throughout these papers, to my unprofessional readers, who, while they admire the glaciers, may also wish to form a general idea of their structure and mode of action, as well as to know something of the important part they have played in the later geological history of our earth. It would, indeed, be out of place, were I to undertake here a discussion of the different views entertained by the various students who have investigated the glacier itself, among whom Dr. Tyndall is especially distinguished, or those of the more theoretical writers, among whom Mr. Hopkins occupies a prominent position.

Removed, as I am, from all possibility of renewing my own observations, begun in 1836 and ended in 1845, I will take this opportunity to call the attention of those particularly interested in the matter to one essential point with reference to which all other observers differ from me. I mean the stratification of the glacier, which I do not believe to be rightly understood, even at this moment. It may seem presumptuous to dissent absolutely from the statements of one who has seen so much and so well as Dr. Tyndall, on a question for the solution of which, from the physicist's point of view, his special studies have been a far better preparation than mine; and yet I feel confident that I was correct in describing the stratification of the glacier as a fundamental feature of its structure, and the so-called dirt-bands as the margins of the snow-strata successively deposited, and in no way originating in the ice-cascades. I shall endeavor to make this plain to my readers in the course of the present article. I believe, also, that renewed observations will satisfy dissenting observers that there really exists a net-work of capillary fissures extending throughout the whole glacier, constantly closing and reopening, and constituting the channels by means of which water filtrates into its mass. This infiltration, also, has been denied, in consequence of the failure of some experiments in which an attempt was made to introduce colored fluids into the glacier. To this I can only answer, that I succeeded completely, myself, in the self-same experiments which a later investigator found impracticable, and that I see no reason why the failure of the latter attempt should cast a doubt upon the former. The explanation of the difference in the result may, perhaps, be found in the fact, that, as a sponge gorged with water can admit no more fluid than it already contains, so the glacier, under certain circumstances, and especially at noonday in summer, may be so soaked with water that all attempts to pour colored fluids into it would necessarily fail. I have stated, in my work upon glaciers, that my infiltration-experiments were chiefly made at night; and I chose that time, because I knew the glacier would most readily admit an additional supply of liquid from without when the water formed during the day at its surface and rushing over it in myriad rills had ceased to flow.

While we admit a number of causes as affecting the motion of a glacier, namely, the natural tendency of heavy bodies to slide down a sloping surface, the pressure to which the mass is subjected forcing it onward, the infiltration of moisture, its freezing and consequent expansion,—we must also remember that these various causes, by which the accumulated masses of snow and ice are brought down from higher to lower levels, are not all acting at all times with the same intensity, nor is their action always the same at every point of the moving mass. While the bulk of snow and ice moves from higher to lower levels, the whole mass of the snow, in consequence of its own downward tendency, is also under a strong vertical pressure, arising from its own incumbent weight, and that pressure is, of course, greater at its bottom than at its centre or surface. It is therefore plain, that, inasmuch as the snow can be compressed by its own weight, it will be more compact at the bottom of such an accumulation than at its surface, this cause acting most powerfully at the upper part of a glacier, where the snow has not yet been transformed into a more solid icy mass. To these two agencies, the downward tendency and the vertical pressure, must be added the pressure from behind, which is most-effective where the mass is largest and the amount of motion in a given time greatest. In the glacier, the mass is, of course, largest in the centre, where the trough which holds it is deepest, and least on the margins, where the trough slopes upward and becomes more shallow. Consequently, the middle of a glacier always advances more rapidly than the sides. Were the slope of the ground over which it passes, combined with the pressure to which the mass is subjected, the whole secret of the onward progress of a glacier, it is evident that the rate of advance would be gradually accelerated, reaching its maximum at its lower extremity, and losing its impetus by degrees on the higher levels nearer the point where the descent begins. This, however, is not the case. The glacier of the Aar, for instance, is about ten miles in length; its rate of annual motion is greatest near the point of junction of the two great branches by which it is formed, diminishing farther down, and reaching a minimum at its lower extremity. But in the upper regions, near their origin, the progress of these branches is again gradually less. Let us see whether the next cause of displacement, the infiltration of moisture, may not in some measure explain this retardation, at least of the lower part of the glacier. This agency, like that of the compression of the snow by its own weight and the pressure from behind, is most effective where the accumulation is largest. In the centre, where the body of the mass is greatest, it will imbibe the most moisture. But here a modifying influence comes in, not sufficiently considered by the investigators of glacial structure. We have already seen that snow and ice at different degrees of compactness are not equally permeable to moisture. Above the line at which the annual winter snow melts, there is, of course, little moisture; but below that point, as soon as the temperature rises in summer sufficiently to melt the surface, the water easily penetrates the mass, passing through it more readily where the snow is lightest and least compact,—in short, where it has not begun its transformation into ice. A summer's day sends countless rills of water trickling through such a mass of snow. If the snow be loose and porous throughout, the water will pass through its whole thickness, accumulating at the bottom, so that the lower portion of the mass will be damper, more completely soaked with water, than the upper part; if, on the contrary, in consequence of the process previously described, alternate melting and freezing combined with pressure, the mass has assumed the character of icy snow, it does not admit moisture so readily, and still farther down, where the snow is actually transformed into pure compact ice, the amount of surface-water admitted into its structure will, of course, be greatly diminished. There may, however, be conditions under which even the looser snow is comparatively impervious to water; as, for instance, when rain falls upon a snow-field which has been long under a low temperature, and an ice-crust is formed upon its surface, preventing the water from penetrating below. Admitting, as I believe we must, that the water thus introduced into the snow and ice is one of the most powerful agents to which its motion is due, we must suppose that it has a twofold influence, since its action when fluid and when frozen would be different. When fluid, it would contribute to the advance of the mass in proportion to its quantity; but when frozen, its expansion would produce a displacement corresponding to the greater volume of ice as compared with water; add to this that while trickling through the mass it will loosen and displace the particles of already consolidated ice. I have already said that I did not intend to trespass on the ground of the physicist, and I will not enter here upon any discussion as to the probable action of the laws of hydrostatic pressure and dilatation in this connection. I will only state, that, so far as my own observation goes, the movement of the glacier is most rapid where the greatest amount of moisture is introduced into the mass, and that I believe there must be a direct relation between these two facts. If I am right in this, then the motion, so far as it is connected with infiltrated moisture or with the dilatation caused by the freezing of that moisture, will, of course, be most rapid where the glacier is most easily penetrated by water, namely, in the region of the névé and in the upper portion of the glacier-troughs, where the névé begins to be transformed into more or less porous ice. This cause also accounts, in part at least, for another singular fact in the motion of the glacier: that, in its higher levels, where its character is more porous and the water entering at the surface sinks readily to the bottom, there the bottom seems to move more rapidly than the superficial parts of the mass, whereas at the lower end of the glacier, in the region of the compact ice, where the infiltration of the water at the bottom is at its minimum, while the disintegrating influences at the surface admit of infiltration to a certain limited depth, there the motion is greater near the surface than toward the bottom. But, under all circumstances, it is plain that the various causes producing motion, gravitation, pressure, infiltration of water, frost, will combine to propel the mass at a greater rate along its axis than near its margins. For details concerning the facts of the case, I would refer to my work entitled "Système Glaciaire."

We will next consider the stratification of the glacier. I have stated in my introductory remarks, that I consider this to be one of its primary and fundamental features, and I confess, that, after a careful examination of the results obtained by my successors in the field of glacial phenomena, I still believe that the original stratification of the mass of snow from which the glacier arises gives us the key to many facts of its internal structure. The ultimate features resulting from this connection are so exceedingly intricate and entangled that their relation is not easily explained. Nevertheless, I trust my readers will follow me in this Alpine excursion, where I shall try to smooth the asperities of the road for them as much as possible.

Imparted to it, at the very beginning of its formation, by the manner in which snow accumulates, and retained through all its transformations, the stratification of a glacier, however distorted, and at times almost obliterated, remains, notwithstanding, as distinct to one who is acquainted with all its phases, as is the stratified character of metamorphic rocks to the skilful geologist, even though they may be readily mistaken for plutonic masses by the common observer. Indeed, even those secondary features, as the dirt-bands, for instance, which we shall see to be intimately connected with snow-strata, and which eventually become so prominent as to be mistaken for the cause of the lines of stratification, do nevertheless tend, when properly understood, to make the evidence of stratification more permanent, and to point out its primitive lines.

On the plains, in our latitude, we rarely have the accumulated layers of several successive snow-storms preserved one above another. We can, therefore, hardly imagine with what distinctness the sequence of such beds is marked in the upper Alpine regions. The first cause of this distinction between the layers is the quality of the snow when it falls, then the immediate changes it undergoes after its deposit, then the falling of mist or rain upon it, and lastly and most efficient of all, the accumulation of dust upon its surface. One who has not felt the violence of a storm in the high mountains, and seen the clouds of dust and sand carried along with the gusts of wind passing over a mountain-ridge and sweeping through the valley beyond, can hardly conceive that not only the superficial aspect of a glacier, but its internal structure also, can be materially affected by such a cause. Not only are dust and sand thus transported in large quantities to the higher mountain-regions, but leaves are frequently found strewn upon the upper glacier, and even pine-cones, and maple-seeds flying upward on their spread wings, are scattered thousands of feet above and many miles beyond the forests where they grew.

This accumulation of sand and dust goes on all the year round, but the amount accumulated over one and the same surface is greatest during the summer, when the largest expanse of rocky wall is bare of snow and its loose soil dried by the heat so as to be easily dislodged. This summer deposit of loose inorganic materials, light enough to be transported by the wind, forms the main line of division between the snow of one year and the next, though only that of the last year is visible for its whole extent. Those of the preceding years, as we shall see hereafter, exhibit only their edges cropping out lower down one beyond another, being brought successively to lower levels by the onward motion of the glacier.

Other observers of the glacier, Professor Forbes and Dr. Tyndall, have noticed only the edges of these seams, and called them dirt-bands. Looking upon them as merely superficial phenomena, they have given explanations of their appearance which I hold to be quite untenable. Indeed, to consider these successive lines of dirt on the glacier as limited only to its surface, and to explain them from that point of view, is much as if a geologist were to consider the lines presented by the strata on a cut through a sedimentary mass of rock as representing their whole extent, and to explain them as a superficial deposit due to external causes.

A few more details may help to make this statement clearer to my readers. Let us imagine that a fresh layer of snow has fallen in these mountain-regions, and that a deposit of dirt has been scattered over its surface, which, if any moisture arises from the melting of the snow or from the falling of rain or mist, will become more closely compacted with it. The next snow-storm deposits a fresh bed of snow, separated from the one below it by the sheet of dust just described, and this bed may, in its turn, receive a like deposit. For greater ease and simplicity of explanation, I speak here as if each successive snow-layer were thus indicated; of course this is not literally true, because snow-storms in the winter may follow each other so fast that there is no time for such a collection of foreign materials upon each newly formed surface. But whenever such a fresh snow-bed, or accumulation of beds, remains with its surface exposed for some time, such a deposit of dirt will inevitably be found upon it. This process may go on till we have a number of successive snow-layers divided from each other by thin sheets of dust. Of course, such seams, marking the stratification of snow, are as permanent and indelible as the seams of coarser materials alternating with the finest mud in a sedimentary rock.