A few years ago Mr. Barlow, a very practical engineer, came forward to meet this crying want, and offered, at a cost of
£16,000, in less than a year, to bore a subway through the bed of the Thames. The idea was not a new one. As early as
1799 an attempt had been made to construct a tunnel under the Thames between Gravesend and Tilbury; and in 1804 a similar work was actually begun between Rotherhithe and Limehouse, which, after proceeding 1,000 feet, broke in; fifty-four engineers of the day deciding that such a work not only would never commercially pay, but was also impracticable.
Brunel's scheme of the Thames Tunnel cost half a million of money, and took twenty-one years' labour to complete.
Mr. Barlow's tunnel, from Tower Hill to Tooley Street, was of course looked upon as chimerical, Mr. Barlow, with less ambition and genius, but more common sense and thriftiness than his great predecessor, took good care to remember that the crown of Brunel's arches, in some places, came within four feet of the river water. In the Tower subway the average distance preserved is thirty feet, and in no place is there less than eighteen feet of sound London clay between the arch and the tideway. The cardinal principle of Mr. Barlow was to sink deep into the London clay, which is as impervious to water as stone, and in which no pumping would be required.
The works were begun on February 16, 1869, by breaking ground for the shaft on the north side of the river ; in February, 1870, numerous visitors were conveyed from one shaft-head to the other. The tunnel commences, as we have said, at Tower Hill, where a hoarding encloses a small square of ground, not larger than an ordinary sitting-room, for which, however, the Government made the Company pay at the rate of about £240,000 an acre. In the centre of this is a little circular shaft, about fourteen feet diameter and sixty feet deep, and at the end of this, facing south, a clean, bright, vaulted chamber, which serves as a waiting-room. At the end of this chamber is the tunnel, a tube of iron not unlike the adit of a mine, which, in its darkness and silence, heightened by the knowledge that this grim-looking road runs down deeply below the bed of the river, gives it at first sight anything but an inviting appearance. The length of the whole tunnel is about 1,340 feet, or as nearly as possible about a quarter of a mile. From Tower Hill it runs in a south-west direction, and, passing under Barclay's brewery, emerges under a shaft similar to that at entering, but only fifty feet deep, and out of this the passengers will come within a few yards of Tooley Street, close to the railway station. From the Tower Hill shaft to the centre of the river the tunnel makes a dip of about one in thirty. From this point it rises again at the same incline to what we may call the Tooley Street station. The method of constructing the tunnel, we need hardly remark, from its excessive cheapness, was simple in the extreme. It has been built in 18-inch lengths of cast-iron tubing, perfectly circular , each 18-inch circle being built up of three segments, with a key-piece at the top, which, fitting in like a wedge, holds the rest with the rigidity of a solid casting. The cast-iron shield used for excavation was less than two and a half tons weight. In front of the shield, which was slightly concave, was an aperture about two feet square, closed with a sliding iron water-tight door, and at the back of the shield were iron sockets, into which screw-jacks fitted, and, when worked by hand, forced the shield forward. The mode of advance was this. When a shaft on Tower Hill had been bored to a sufficient depth below the London clay, the shield was lowered and placed in its required position. The water-tight door we have spoken of as in centre was then opened. Through this aperture sufficient clay, just of the consistency of hard cheese, was cut away by hand till a chamber was made large enough for a man, who entered and worked till there was room for two, and these soon made a circular space exactly the size of the shield and about two feet deep. This done, the miners came out, and with their screw-jacks forced the shield forward into the space which they had cut, but with the long telescope-like cap of the shield still over them. Under cover of this an 18-inch ring was quickly put in and bolted together; and while this was doing, the clay was being excavated from the front of the shield as before. Thus every eight hours, night and day, Sundays and week days, the shield went forward eighteen inches, and eighteen inches length of iron was added to the tube, which so advanced at the rate of 5 feet 4 inches every twenty-four hours.
The clay was so completely water-proof, that water had to be sent down to the workmen in cans to mix with the cement. No traces of fresh-water shells were found; but very large clay-stones and a great many sharks' teeth and marine shells. So perfect were Mr. Barlow's calculations, that the two opposite tunnels met within a quarter of an inch. The small interval between the iron and the clay was filled with blue lias cement, which coats the tube and protects it from oxidisation. The gain to the East-end of London by this successful and cleverly executed undertaking is enormous, and the intercourse between the north and south banks of the Thames is greatly facilitated ; and the conception has been seized upon by Mr. Bateman as the basis of his well-known suggestion for a submarine tube to carry a railway from England to France. The Thames tube is 7 feet in clear internal diameter, and it originally carried a railway of 2 feet 6 inches gauge. On this railway formerly ran an omnibus capable of conveying twelve passengers. The omnibus was constructed of iron ; it was light, but very strong, and ran upon eight wheels, and was connected with a rope of steel wire by means of a gripe that could be at any time tightened or relaxed at pleasure, and at each end of the tunnel this wire ran over a drum worked by means of a stationary engine.
If the carriage was stopped in the centre of the tunnel, the beat of the paddles of the steamers above could be heard, and even the hammering on board ships. In time there will be subways at Gravesend, Woolwich, and Greenwich. The next to be formed, however, is one from St. George's Church in the Borough to Cannon Street. The Tower subway is now only used for foot-passengers, at a charge of one halfpenny.
Walter Thornbury, Old And New London, c1880
As I was thinking of these things I disappeared from the world indeed, going down a lighted spiral staircase which buries itself in the earth on the right bank of the Thames, opposite the Tower. I went down and down between two dingy walls until I found myself at the round opening of the gigantic iron tube, which seems to undulate like a great intestine in the enormous belly of the river. The inside of this tube presents the appearance of a subterranean corridor, of which the end is invisible. It is lighted by a row of lights as far as you can see, which shed a veiled light, like sepulchral lamps; the atmosphere is foggy; you go along considerable stretches without meeting a soul; the walls sweat like those of an aqueduct; the floor moves under your feet like the deck of a vessel; the steps and voices of the people coming the other way give forth a cavernous sound, and are heard before you see the people, and they at a distance seem like great shadows; there is, in short, a sort of something mysterious, which without alarming causes in your heart a vague sense of disquiet. When then you have reached the middle and no longer see the end in either direction, and feel the silence of a catacomb, and know not how much farther you must go, and reflect that in the water beneath, in the obscure depths of the river, is where suicides meet death, and that over your head vessels are passing, and that if a crack should open in the wall you would not even have the time to recommend your soul to God, in that moment how lovely seems the sun !
I believe I had come a good part of a mile when I reached the opposite opening on the left bank of the Thames; I went up a staircase, the mate of the other, and came out in front of the Tower of London.
Edmondo de Amicis Jottings about London (trans), 1883
Tower Subway.—A curious feat of engineering skill, in the shape of an iron tube seven feet in diameter driven through the bed of the Thames between Great Tower-hill and Vine-street. The original intention was to have passengers drawn backwards and forwards in a small tram omnibus. This, however, was found unremunerative, and the rails having been taken up the tunnel has since been open as a footway. Unfortunately, however, after subtracting from its diameter the amount necessary to afford a sufficient width of platform, there is not much head-room left, and it is not advisable for any but the very briefest of Her Majesty’s lieges to attempt the passage in high-heeled boots, or with a hat to which he attaches any particular value. It has, however, one admirable quality, that of having cost remarkably little in construction. NEAREST Railway Stations, Aldgate (Metrop.) and Cannon-street (S.E.); Omnibus Routes, Aldgate High-street and Fenchurch-street; Cab Rank, Great Tower-street.
Charles Dickens (Jr.), Dickens's Dictionary of London, 1879
John Fletcher Porter, London Pictorially Described, 
see also A.R.Bennett in London and Londoners - click here