THE CHANNEL TUNNEL.

Globe, Volume VI, Issue 620, 14 June 1876, Page 3

THE CHANNEL TUNNEL.

A London exchange says 'The Society of Engineers on the 20th instdiscussed theformidable engineering question of the ventilation of the Channel tunnel. The president remarked that it would be necessary to renew the air once an hour, and that in order to do this it would be needful lo keep up a current through the whole bore of the tunnel at the velocity of twenty miles per hour. It might have been added that such a current would present a very appreciable resistance to the movement of the trains, and that the formulae according to which the of the atmosphere is now calculated and which make it proportionate to the square of the velocity of the t'ain would probably require much correction for additional resistance in so small an area as the cross section of the tunnel would present. At all events, there would be a difference in resistance, according to the motion of a train with or against that of the ventilation, equal to the total atmospheric resistance at a speed of forty miles per hour. But that the question win ever go so far as to render necessary the discus sion of the mode ol ventilation of the finished tunnel may well be thought

problematical. The fathers of engineering science in this country professed to recognise but one engineering difficulty—that of finance. This, though a true statement in the rough, is not absolutely true. The limits of physical possibility lie closer to us than we always remember ; and the moment we attempt to vary our level, to rise much above or sink much below the ordinary barometric pressure of the atmosphere, we run our beads against these limits. The rapidly increasing pressure of the water limits the operations of the diver. The rarefaction of the air causes mountain sickness at heights which our muscular power enables us readily to attain ; and the thickness of the shell of air above which human life is extinguished is well known to the scientific balloonist. The Royal Commission on Coal gravely assumed the possibility of extracting that mineral from a depth where the temperature of the earth is such that it would raise that of the air in a shaft or adit to blood-heat ; and added the naif suggestion that perhaps the air might there be cooled by artificial means! These are instances of definite physical difficulties which no money can overcome.

Perhaps no one is in a position to say that the construction of a tunnel under the Straits of Dover would be opposed by insuperable physical difficulties. But it must be admitted that the scheme involves an attempt to penetrate an unknown region, in which such difficulties are extremely likely to occur. And, apart from these grave obstacles —physical, financial, and strategic—which beset the main question, there are practical troubles besetting the preliminary steps which are of no trifling magnitude. Thus it is well known to all persons familiar with the construction of tunnels that it would be necessary to attempt such a work as the tunnelling of the Channel either from one end alone, or by means of running a small pioneer tunnel, or heading, through from end to end before the main enterprise was commenced. This is the method now generally adopted; It has much to recommend it in many ways; but its chief importance is, that it secures the accuracy of the direction of the main work. It is obvious however that the construction of a little headway of from 4ft to 6ft high by 3ft or 4ft wide for the distance of fifteen miles, at a depth of hundreds of feet below the bottom of the Channel, is in itself a task of no small magnitude. To construct two such driftways and to run them with such accuracy of level and of direction that they should meet half.way in to double the difficu’ty. But allowing that it is within the power of science so to ascertain both direction and level as to bring the two sets of miners within hearing of the pickaxes of one another, this only meets the theoretic part of the difficulty. The physical, difficulties still remain. The driftway will have to be ventilated; the materia] excavated will have to be removed ; above all, the driftway will have to be drained.

It is here that we run most closely to the verge of physical impossibility. The question of physical possibility is allowed by the advocates of the tunnel to depend on the existence for the whole distance to be traversed of an unbroken stratum of the lower chalk. If the formation of the Channel has been the result of one of those great faults or convulsions which are so familiar to the geologist, thia hope is illusory. In what other way this great rift may have originated is not very clear. But, apart from any doubt of that kind, and supposing that the most sanguine hopes of the advocates of the tunnel are realised, what is the probability of the reasonable dryness of the driftway 7 What is the ground for supposing that the long culvert under the floor of the Channel would be anything but a perennial spring 7 As to the percolation of water through the upper chalk it is needless to speak. Not only do the flint bands give free passage to water, but there is good reason to believe that there is a circulation of some sort through the body of the chalk itself. At all events there are fissures enough, if such be not the case, to produce the same results. But the lower chalk is a more homogeneous rock, less traversed by fissures. It is so compact and fine-grained as to form a very excellent building stone for interior work.