TUNNEL, in civil engineering, an arched passage formed under. ground to conduct a canal or road on a lower level than the natural surface. The derivation of this word, which, in the sense above given, is unnoticed by most lexicographers, is rather uncertain. Richardson places it among the derivatives of tun, and defines it as " any inclosure, inclosed way or passage ;" as a eliiinuey.tunnel. or passage for smoke, in which sense the word tumid or funned is used by Spenser and other early Engli‘l writers ; a passage for liquor, in which sense, as well as in that last mentioned, it is convertible with funnel; or a net shaped like a tunnel for liquids, wide at the mouth, and diminishing to a point. Ile also observes that " Tooke thinks tan and its diminutive tunnd (Anglo-Saxon, nend, tend) are the past participles of the [Anglo Saxon] verb tyn-an, to enclose, to encompass.' Long tunnels are usually made through hills in order to avoid the inconvenience and loss of power occasioned by conducting a canal, road, or railway over elevated ground, and the enormous expense of such an open excavation as would be necessary in order to preserve the re quisite level. Those of less extent are frequently constructed to avoid the opposition of landowners, or to afford uninterrupted passage under a road, canal, or river. Many tunnels of the latter character differ in no material point from bridges; but in the case of oblique crossings a tunnel is distinguished from an oblique or skew bridge by its faces being at right angles with the direction of the lower passage, instead of being parallel with the direction of the upper passage. Of this character are the tunnels under the Hampstead Road, between Euston Square and Camden Town, and the Kensall Green tunnel, under the Harrow Road, both on the line of the London and North-Western rail way ; and on the West London railway, passing under the Paddington canal at Wormholt Scrubbs. The Thames tunnel is the most re markable example of tunnelling under a river, and, although far less extensive than many other tunnels, it is, from the. almost insuperable difficulties of its situation, perhaps the most astonishing work of the kind ever executed. Another class of tunnels are those made under towns, in order to form a canal or railway communication with points which are inaccessible by an open passage, except at great expense. The Regent's canal, for example, passes under Islington, London, by a tunnel three-quarters of a mile long ; and the Liverpool and Man chester railway is conducted from the station at Edge Hill, on the outskirts of Liverpool, to the docks at Wapping, for goods traffic, and to Lime Street, in the centre of the town, for passenger traffic, by two tunnels, each of which is about a mile and a quarter long.
The construction of tunnels is by no means of recent origin, although it is only of late that they have become common. The outlet for the drainage of the lake Copais in Bceotia, [Moms, iu GEoo. Dry.) is one of the oldest monuments of the labour of man. The great tunnel in Samoa. which was seven stadia, or 4200 Greek feet, in length, was driven through a mountain 900 feet high, fur the purpose of serving as the bed of a channel to convey water from a natural source to the city of Samoa (Herod., iii. 60.) The Posilipo near Naples, which is at least as old as the beginning of the reign of Tiberius, is a tunnel three-quarters of a mile long. Tho tunnel which was made at an early period in the Roman republic for the partial drainage of the Alban Lake is above a mile in length. Of ancient works of this character there is a remarkable example in the subterranean canal from lake Fucinus, or the lake of Celano. to the river Siris, originally formed by the emperor Claudius, and cleared out some years since by order of the Neapolitan government. This extensive tunnel. which is about three miles long, thirty feet high and twenty-eight wide at the entrance, and nowhere leas than twenty feet high, passes in part through solid rock, and is lined in other places with masonry ; and it appears to have been constructed in a planner resembling that now usually followed, the excavation having been carried on by several parties or gangs simulta neously, by means of vertical shafts, and inclined passages or galleries from the aides of the mountain. A minute account of the tunnel,
as it appeared during the cicaring-out, is given in the thirty-eighth volume of ' Blackwood's Edinburgh Magazine' (p. 657), in a paper entitled ' Eight Days in the Abruzzi.' The object of this tunnel is to carry off the superfluous waters of the lake ; but in more recent times similar works have been executed for navigable canals. Them are generally in England of small transverse dimensions, being calculated for the passage of single boats, and very often without towing-paths, in which case the boats are either hauled through by a rope or chain, worked by a steam-engine. or propelled by men lying on their backs on the deck, or on projecting boards provided for the purpose, and thrusting against the sides or roof of the tunnel with their feet. This dangerous practice has occasioned much loss of life, and is also objec tionable on account of its tediousness, as boats are often detained for a long time at one end of the tunnel while a boat is coming from the opposite end. In the evidence before tho House of Lords on the Great Western Railway bill, in 1835, it was stated that great delays were experienced at the Islington tunnel when any accidental derange ment prevented the steam-engine and chain from working ; because, although boats were occasionally " legged " through in as little as seventeen minutes, the ordinary time required for working a light barge through the tunnel, by two men, was half an hour, and for a loaded barge three-quarters of an hour, or frequently an hour. In such cases boats arriving in the opposite direction had to wait at the mouth of the tunnel, until, frequently, as many as half a dozen were collected, which, when their turn arrived, passed through in a train. At some of the longer tunnels this inconvenience was even greater. At the Harecastle tunnel, on the Trent and Mersey, or Grand Trunk canal. two hours were formerly required to effect a passage of little more than a mile and a half.. " This place is so frequented," observes the Baron Dupin, in his' Commercial Power of Great Britain; " that at the moment when the passage begins. a file of boats a mile long is often seen." To prevent confusion, those going towards Liverpool were allowed to pass in the morning only. and those in the contrary direction in the evening. This tunnel, which was formed by Brindley, and was one of the earliest works of the kind executed in this country, was commenced about the year 1766. It is 2830 yards long, 12 feet wide, and 9 feet high, and is in some parts as much as 70 yards beneath the surface. It is lined with a semicircular brick arch, and was completed for the small sum of 3/. 10s. 8d. per yard. Increased traffic upon the canal having rendered it necessary either to construct a new tunnel or to enlarge the old one, the former alternative was adopted ; and in 1822 Telford was engaged to superintend the work. The new tunnel, which runs parallel with that of Brindley, is 2926 yards long, 14 feet wide, and 16 feet high ; and, notwithstanding its greater dimensions, it was executed in less than three years : the original tunnel occupied eleven years. The new tunnel has an iron towing-path, so supported as to allow the water to play freely beneath it, which gives the advantage, so far as the play of the waves is con cerned, of a waterway of the full width of the tunnel. It is perfectly straight, and the light can be seen from end to end; and so agreeable is the travelling through it, that one of the bargemen said, after passing it, that he wished it extended all the way to Manchester.