As we have already said, a number of shafts passing vertically down the pier effect a chimney kind of a communication between the air-chamber and the upper world. In the central and widest of these was a winding stair-case, which was lengthened as the pier reached downward, and was used for people to pass up and down. The smaller shafts, which also passed. down the pier perpendicularly, contained the pipes which serve to introduce the compressed air, the hose for moistening the sand, the pump which removes it, machines for the introduction of materials, and a telegraphic arrangement by means of which the workmen from beneath, "where all things hideous are," are able to correspond every moment with "those that breathe in the rosy light."

The entrance into the caisson itself was effected by means of an air-lock at the bottom of the winding stair-case-a lock which, like the caisson, is constructed of thick iron, and is an integral part of it. As soon as the chamber was entered, which was capable of holding six or eight persons, the current of air admitted rushed round with such impetuosity that even strong organizations entering this kingdom of darkness and night for the first time could not disembarrass themselves of a certain feeling of uneasiness. The iron door that led to the outer world pressed firmer against its frame, by the force of the air streaming in, than could be done by a lock or any other contrivance. The stop-cock through which the air streamed in was not closed until the atmosphere in the air-lock had reached the same density as that in the main part of the caisson. As soon as this was the case the door leading into the caisson opened of itself, and we were ready to enter this subterraneous workshop, where even the clearest voice loses its sound, and where, deep under the echo of human speech-yea, deep under the water's undermost depths-busy workmen pave the way for the sinking pier.

For a while one felt perfectly comfortable in this underworld -a world such as no mythology and no superstition ever dreamed of. The transition. indeed, became apparent by pain in the ears, bleeding at the nose, or a feeling of suffocation; but these inconveniences and seeming dangers, inevitable upon such a visit to hell, were insignificant in comparison with the interest which it offered. It was undertaken by hundreds and hundreds of visitors, including many ladies, and none returned from that depth without carrying along with them one of the most remarkable reminiscences of their whole life. Shrouded in a mantle of vapor labor the workmen there, loosening the sand; dim flicker the flames of the lamps, and the air had such a strange density and moisture that one wandered about almost as if he were in a dream. For a short time all this was extremely interesting and delightful, but it was not long before the wish to escape again from this strange situation gained the upper hand over the charm which it exercised. Gladly did the visitor, after a quarter of an hour, re-enter the air-lock, with an unfeigned feeling of relief, to watch the air beginning to escape from this chamber. At once the door behind him leading from the caisson closed by the denser air, and fastened as firmly as if there was a mountain behind it. The compressed element escaped whistling from the air-lock; the air within was more and more equalized with the air without;

a few minutes, and they were of equal density; then the door, no longer pressed against its frame by the dense atmosphere, opened to the winding stairs, and the visitor came forth taking a long breath, and, to use Schiller's words, once more "greets the heavenly light" which shone from far above down the shaft.

THE BRIDGE WHEN COMPLETED.

At present both the piers may be considered as finished. The east pier has been resting with its caisson on the rock since the 28th of February, and the filling of the chambers was then rapidly accomplished. Its western companion had then only three feet more to sink, and this it might have done in a very short time, but the supply of granite failed to arrive in time, and so interrupted the building itself. It is laid down in the plan that the portion of the piers above water, and exposed to the action of the air, shall be built of the strongest granite, while the parts extending from the rock to a certain point under the lowest water shall be built from limestone blocks from Grafton quarry, in Illinois. When the expected granite arrived, the construction of the piers above the surface of the water made rapid progress, and in a few weeks they will have reached the prescribed height of fifty feet above the water level. Their total height, or, if you prefer it, their total depth, will then, as stated above, be 194 and 165 feet respectively-the east pier being the highest, because the rock on the Illinois side of the river lies deeper than it does on the Missouri side. The hexagonal foundation of the piers is 82 feet in length; their weight amounts to from 28,000 to 33,000 tons. No less solid and massive is the construction of the abutments. In their case, likewise, they had to go down to the rocks. Upon the Missouri side of the river this presented little difficulty, which, however, will be made up for on the Illinois. side, on account of the nature of the American bottom. On this side the works are already advancing, inside a gigantic coffer-dam, towards the surface. On the other side they are just being begun. We know, however, that in the character of this work a beginning is the beginning of a certain, and particularly of an early, termination. It will therefore not be long before the Illinois abutment will rapidly follow its vis-a-vis and the two piers.

These four piers will form the substructure which now approaches its termination with rapid strides. Upon the masses thereof, which are put together to last for an eternity, the bridge itself will rest, which is destined to facilitate the proudest inland commerce over the proudest of streams. They will carry three arches, which, as was already remarked, will measure-those extending from the abutments to the piers 500 feet each, and the span of the principal arch between the two piers 520 feet. The possibility of erecting such long spans, considering the enormous weight which they will have to bear, was at first strongly doubted, and still more strongly contested. Captain Eads, however, sustained on the one side by his calculations, on the other by the example of the arched bridge at Kulinburg, in Holland, which spans the Leck with a span of 500 feet, as well as by the plans of the English bridge-engineer

Telford, which were made in the beginning of this century, was enabled to invalidate and set aside all these objections. Cast-steel is selected as the material of these arches. Each of them will be double, that is to say, will consist of two concentric arches 12 feet apart, and joined together by a network of the most massive steel braces. Such double arches will be stretched four in each span, running parallel with each other from pier to pier. Upon their iron necks will be laid the real bridge in two stories. The lower of these stories is intended for the railways; the upper belongs to vehicles and foot passengers. Being fifty feet wide, both will afford space enough to satisfy the demands of the liveliest traffic. Meanwhile, underneath, the largest steamers, even when the water is at its highest, may dash along; and while over them the East and West exchange their riches, they may, unimpeded, perform the exchange between the North and the South. St. Louis, however, will not only have the boldest arch bridge in the world, but it will also have the first structure of the kind built of steel, the true noble metal of our times. Let us leave to Europe her Krupp and her arsenal full of cast-steel cannon-the one steel bridge over the Mississippi casts into the shade all that equivocal wealth of the old world.

It remains to say a few words in regard to the shore structures, or, more properly, to the approaches to the bridge. The street leading directly to the bridge-Washington avenue-is one of the broadest and finest in St. Louis. Like the whole of the St. Louis shore, it slopes rapidly when it approaches the river. It will be sufficient, therefore, to prolong the bridge, which rises about fifty feet above the shore, a comparatively short distance—three blocks-1,049 feet into the city, in order that its level may equal that of Washington avenue. A viaduct of five arches, of twenty-seven feet span each, under which the traffic of the cross streets below may be carried on unobstructedly, will form the continuation of the bridge, and of course will be of the same height and breadth. At the end of it the high level road will pass into Washington avenue, which still continues to rise, whereas the low level road, with its railways, will run into a tunnel, 4,800 feet in length, which passes under a large portion of the city, and terminates at the spot where the great St. Louis Central Railroad Depot will be erected-where at present the Pacific railroad crosses Eleventh street. The tunnel will be fifteen feet wide and seventeen feet high. By means of soundings and borings it has been ascertained that there are only layers of clay to be tunneled through, and therefore the latter portion of the enterprise will offer no particular difficulties. With the approach to the bridge over the flat marshy ground on the Illinois shore, the company itself has nothing whatever to do. Dykes and trestles, branching off according to the convenience of the different railroad companies to north, south, or east, will complete the connection with the bridge. The upper carriage-way will be carried out upon solid constructions as far as Fourth street in East St. Louis, from which point the Missouri traffic will divide up in all directions.

And now, what will this gigantic work-measuring from the Illinois abutment to Washington avenue, in St. Louis, 2,230 feet-cost? We put down the estimates for the different parts, as well as for the whole structure':

Of this capital, three millions ($1,200,000 in St. Louis, the rest in New York) have already been subscribed, and the outlay up to the present moment is $1,700,000. At the same time the financial management has hitherto been so successful, and the different contracts made so advantageously, that the progress of the bridge will certainly not be interrupted by any pecuniary difficulties. No less certain is it that advantage will be taken of the work as soon as it is completed. The data which have been made and collected with extreme care in regard to this point by one of the directors, Dr. William Taussig-who must be considered one of the most energetic promoters and patrons of the great national enterprise-lead to the following results:

At least thirteen railroads will have their terminus on the Illinois shore of the Mississippi in East St. Louis. And at least eleven railways will soon leave St. Louis itself, cutting the State of Missouri in all directions. Of only three of all these have we any statistical reports, and these relate only to the freight traffic of the year 1867. They show that during that year 767,400 tons of freight were carried over these lines. The most modest estimate of the traffic of twelve railways, which will be the total number finished and in operation before the completion of the bridge, cannot place it below a million of tons. The contracts already made with the different railway companies, and those still to be negotiated, secure to the Bridge Company an average tariff of 65 cents a ton, which would yield a yearly revenue from freight alone of $550,500. The remaining traffic (horse-cars, coal carts, farmers' wagons, and other freight conveyances, along with cattle transport), according to present estimates, may be reckoned at $129,647, and passengers on the railways $112,000, so that altogether the total revenue would amount to $892,147. From this sum $10,000 must be subtracted for annual incidental expenses, and there will remain over a sum equal to eight and a half per cent. on a capital of ten millions.

It is expected that the bridge will be inaugurated in the last days of next year. However, if we may draw a conclusion from the past favors of fortune upon the work, the latter part of the summer of 1871 will see the first train of cars pass over the steel and granite structures of this unrivaled bridge. Then it will not only be a source of pride to every Missourian in particular and every American in general, but its massive and yet magnificently elegant forms will be a source of astonishment to the ordinary spectator and of admiring appreciation to the professional engineer. Then likewise will be brilliantly verified the words with which the architect closed the report which he laid before the company in the spring of 1868, and which are as follows:

"It is safe in stating that rarely has an enterprise been inaugurated which appeals so strongly to the support of our citizens of all classes, which promises

so much to add to the welfare and prosperity of the city, and which offers such a safe and remunerative return for the labor and capital invested in it."

At the present time the west pier is sunk to the rock, and the air-chambers of both piers, and the shafts in them, have been filled up with concrete; and the masonry has been carried up to about six feet above low-water lines. The caisson for the east abutment is being built at Carondelet, and will be launched about August 10th of this year.

The west abutment has also been built up to about twelve feet above low. water, and by February 1st of next year all the masonry of the piers will be ready for the superstructure. The contract for the superstructure has been. awarded to the Kingston Bridge Company, of Pittsburg, Pa., and that com- . pany is now working in the most urgent manner to fill their contract, which obliges them to furnish and raise the superstructure of the bridge within seventeen months. A notable feature of this contract consists in the fact that it has been let at prices below those estimated by the Chief Engineer.

This constitutes a brief outline description of the great St. Louis Railway and Passenger Bridge, which is now in process of construction.

A very brief classification of the approved bridges of the day, and an allusion to specimens of the various kinds, will, perhaps, enable the casual reader to receive a better impression of the magnitude of the St. Louis bridge. There are four prominent styles of bridges, which are generally adopted by the engineering profession when they aim to erect something that will endure to remote generations-the tubular, the suspension, the lattice, and the archall constructed of iron, in one or more of its forms. The tubular, invented by Robert Stephenson, although materially aided by Fairbairn, will always, we think, be regarded as one of the great ideas of the nineteenth century. It is a straight, hollow, rectangular tube. The Britannia bridge is the grandest specimen; for its longest span or reach, between supports, is 459 feet. But long as it is, it was lifted in one piece 100 feet high, to its present postion. The Victoria bridge has no span of equal length, nor was it elevated in the same way.

The suspension, in its crude forms, is of ancient date. It is found in all lands, but until later years it has never received the indorsement of engineers as the reliable support of railway trains; and in this respect it can hardly be said to have thoroughly disarmed sound criticism, when we claim we are building something that is truly permanent. It possesses some qualities that will always render it popular. It can be constructed more easily in many positions. A much greater span can be obtained than by any other known method, and the cost is comparatively less. Perhaps this last feature can be understood when we remember that the Niagara bridge, with a span of 821 feet, was built for less than the yearly interest on the sum expended on the Britannia bridge. Its general construction is well known. In Europe, the prominent specimens are the Menai, by Telford, with a span of 580 feet, and the Freyburg, in Switzerland, with a span of 870 feet. In this country, Ellet and Roebling have identified themselves with the Wheeling, Niagara, Cincinnati, and other bridges. Ellet constructed the Wheeling bridge, 1,000 feet span, which failed to with