standing every effort to attain the opposite shore, they were driven away by the frequent bursts of sand and water. The engineer having afterwards sounded the ground from above, reported that he had no doubt the two fractures communicated underneath; and therefore admitted that it was quite impracticable to go further except by means of a coffer-dam or caissons. On the 30th of March, 1809, the directors offered a reward for the most approved plan of completing the archway. Fifty-four plans having been obtained by this announcement, they were referred to the opinion of scientific men. These gentlemen reported that they were unanimously of opinion, that an archway, of any useful size, was impracticable under the Thames by an underground excavation on any of the plans that had come before them; observing, at the same time, that they did not pretend to assign limits to the ingenuity of other men. A further trial was made by a third engineer, who operated from above the river, but it proved equally fruitless. Thus ended, in 1809, all the exertions and the efforts made during nearly seven years, for the purpose of accomplishing an archway under the bed of the Thames; at the end of which period not so much as a drain had been completed, nor had the miners succeeded in working in any of those strata wherein the excavation for the archway must eventually have been effected.

Several years afterwards, Mr. Brunel was prevailed upon by one of the most active promoters of the archway enterprise (Mr. J. Wyatt) to turn his attention to the subject; and, being furnished with the documents connected with the first attempt, he devised his plan with the impression that both the excavation and the structure might be made on a full scale at once.

Before proceeding to an exposition of the plan adopted by Mr Brunel, and of the means by which he has carried it into execu. tion, we have to state that the structure of the Thames Tunnel, as represented in the annexed view, is thirty-eight feet in width, and twenty-two feet six inches in height, externally; and that a length of six hundred feet, in the style of a double arcade, has been made, though one arch only is open to public inspection. The excavation, therefore, made under the Thames for this struc ture presents a sectional surface of eight hundred and fifty feet, which is equal to sixty times the area of the drift. At high water, the head of the river is about seventy-five feet above the foot of the excavation, and consequently three times the height of that room. These circumstances, independently of the nature of the ground, are sufficient to place the work of the Thames Tunnel among the boldest enterprises in the art of engineering.

Notwithstanding that the first attempt had contributed to dis

courage all idea of success, there were still sufficient evidences to indicate that by beginning in the stratum of dry firm sand, and keeping close under the stratum of clay forming the bottom of the river, there was space enough to effect the object, although the nature of the intervening ground had been ascertained to be very loose in many places. All the information obtained from the miner's report concurred with the opinions of geologists in pointing out that the most eligible line for the Tunnel was to keep as near to the bottom of the river as the security of the work would permit. The first idea of the plan which appeared to the engineer best calculated for making an excavation fit for the object under so overwhelming a head of water, was suggested by the sight of a piece of a keel of a ship which had been eroded by the operation of the worm called the terido. From this he conceived it practicable, as his specification describes it, to make a circular opening of sufficient capacity at once. However, of the two modes which he described, he gave the preference to that of proceeding by forming, simultaneously, several contiguous excavations by means of an apparatus which has been denominated the shield. This shield, upon the whole, partakes of the character of a powerful coffer-dam, applied in a horizontal instead of the vertical direction. It consists of twelve parallel frames lying close to each other, like so many volumes in a bookcase. Each frame, being nearly twenty-two feet in height, is divided into three stories: the whole presents therefore thirty-six openings or cells. It is from these cells that the miners, operating by small quantities at a time, like so many teridos, are able to dig the ground in front, while others at the back bring up the brick structure. For locomotive action each frame is provided with two substantial legs resting on equally substantial shoes, (not unlike snow-shoes;) these legs are provided with joints, that fit the frames for a pacing movement. The shield has been pushed forward six hundred feet of its assigned career; and has left behind a substantial structure in the form of a double arcade.

With regard to the external form of the structure, and the mode adopted for its execution, it must be obvious to persons acquainted with such matters, that the most substantial form, and the best calculated at the same time to prevent, as far as practicable, any derangement in alluvial strata of various degrees of density, is the square form, as corresponding with that mode of building which is technically called underjoining and underlaying. Thus, in fact, the bed of the river, with its contents, has been underlayed to receive the superstructure.

An indispensable requisite in a work or this nature was, that it

should be made proof against the greatest disasters that were to be apprehended, notwithstanding every precaution that could be taken. Mr. Brunel's plan was considered by his grace the Duke of Wellington, by Dr. Wallaston, and by those engineers and scientific men who had the opportunity of examining the designs, and of hearing the description given by the engineer, as being well calculated to accomplish the contemplated object, although some apprehensions were raised at the time as to what might result from so formidable an occurrence as an irruption of the river, considering the extent of the devastation it might cause in the ground and among the works. The engineer afforded such explanations as allayed, in some degree, those apprehensions which, it must be admitted, he has since completely dispelled by undeniable facts.

It was under these auspices that the plan was brought before the public in 1823, and that in the month of February, 1824, subscriptions were obtained to a large amount to carry it into effect, notwithstanding the novelty of the scheme, and its risks.

The company having been incorporated in 1824 by an act of parliament, the work was begun in March, 1825. A shaft fifty feet diameter was constructed, destined to form ultimately the descent for the footways. This structure was in the first instance laid upon piles, and raised to the height of forty-two feet, includ ing a cast-iron rim, intended to act as a cutter. A steam engine of thirty-horse power was mounted on the top of this structure. In this state, the piles being removed, this tower was brought to rest upon the edge of the cast-iron rim. It is easy to comprehend, that, by clearing the ground inside, the whole must have descended. In this manner a structure, weighing about twee hundred tons, was lowered to the depth of forty feet, through a stratum twenty-six feet deep, consisting of gravel and sand full of water, wherein the drift-makers had met with almost insurmountable obstacles. It is to be remarked, that for this, and for the whole operation of the Tunnel, the engineer did not employ a larger steam engine than had been required in the operations of the drift-way. As the body of the Tunnel was to be opened at the depth of forty feet, the shaft was continued to sixty-four feet, by underlaying, leaving the space in the side open for the hori zontal work. A well, or cistern, twenty-five feet diameter, was further made at the bottom of this shaft, for draining the ground; but in sinking it a quicksand suddenly burst upon the work. This event confirmed the report of the drift-makers, and of the geologists, as to the existence of a dangerous bed of sand at about eighty to eighty-five feet from the level of high water. The shield destined