Another variation from the ordinary process occurs in those tunnels which are formed by means of an open cutting, and subsequently covered in. Such are called open tunnels, and are sometimes preferred where the object of the tunnel is to avoid the permanent severance of lands rather than to penetrate ground too elevated for an open cutting. The short tunnel on the London and North-Western railway, at Kensill Green, and parts of the Underground railway, in London, were formed in this way. In such cases the sides of the cutting are made nearly vertical, and supported by timbers until the brickwork ix executed.
While the projects for some of the earliest English railways were before parliament, much discussion took place relative to the ventila tion and lighting of tunnels, and to the effect which they might have upon the health of persone riding through them. Most of the objec tions raised against tunnels during the period referred to are now exploded, and some of them appear not a little ridiculous. It was urged by their opponents that the (lamp cold air common to all sub terraneous excavations would prove highly detrimental to health ; that the noxious gases emitted from the locomotive engines would accumu late and render the air irrespirable ; and that the sudden transitions from light to darkness, and rim rent& would be very injurious to the sight. The discomfort arising from these evils, so far as they really exist, and from the deafening noise of tunnel-travelling, are amply sufficient to give a preference to an open cutting, when such a line is obtainable at moderato expense, but they by no means bear out the predictions of the alarmists. Some interesting experiments made in the tunnel on the Leeds and Selby railway are recorded in a paper by Mr. Walker ' On Ventilating and Lighting Tunnels,' in the ' Transac tions of the Institution of Civil Engineers,' vol. i. p. 95. While the tunnel alluded to was in progress, it was determined to leave the working shafts open, to promote ventilation and to admit light. The former object was sufficiently attained to prevent any serious incon venience to passengers, but, so far as the trains are concerned, little benefit is derived from the light admitted by them, although attempts were made to diffuse it by means of tin reflectors. The experiment succeeded so far as to enable a person to read the larger print in a newspaper advertisement in any part of the tunnel ; but it is stated that, owing to the rough and dirty state of the walls and the obliquity of the rays thrown upon them, the rays reflected from them were too feeble to be useful in a case of such sudden transition from the light of day as that experienced by persons passing through with a train.
Reflectors would also be rendered useless during the passage of a train by the quantity of steam emitted by the engine. In order to settle the queation as to the supposed unhealthiness of tunnels, in February, 1837, Dr. Paris, Dr. Watson, Mr. W. Lawrence, lecturer on anatomy and surgery, Mr. It. Phillips, lecturer on chemistry, and Mr. Lucas, surgeon, were requested to visit and report upon the Primrose Hill tunneL Although the ventilation was then imperfect, owing to the western extremity of the tunnel being unfinished, and the steam was allowed to escape from the engine for a space of twenty minutes, during which it remained stationary in the tunnel, those gentlemen reported that for so many feet above their heads the atmosphere remained clear, and apparently unaffected by steam or effInvia of any kind, and that neither damp nor cold was perceptible. They further express their opinion "that the dangers incurred in passing through well-constructed tunnels are no greater than those incurred in ordinary travelling upon an open railway or upon a turnpike-road ; and that the apprehensions which have been expressed that such tunnels are likely to prove detrimental to the health or inconvenient to the feel ings of those who go through them are perfectly groundless." • These opinions are fully corroborated by the observations of Drs. Davy, Williamson, and Reid, upon the Leeds and Selby railway tunnel, which, as well as the report on the Primrose Hill tunnel, were given in evidence before the Select Committee of the House of Commons on the London and Brighton railways, in 1837. The objection arising from darkness is obviated on the London and North-Western and many other railways by the use of lamps in the roofs of the carriages, which afford an agreeable though small degree of light in the interior ; but in a few cases the tunnels themselves are lighted by gas-lamps attached to the side walls.