With respect to the subject of tunneling thus incidentally introduced, in subaqueous work of this kind, I have already alluded to that which is done by "cut and cover," but where the influx of water is a source of great difficulty, as it was in the old Thames tunnel (though in this case for water one should read silt or mud), I do not know that anything has been devised so ingenious as the Thames tunnel shield; improvement has, however, been made by the application of compressed air.
In the instance of the Hudson River tunnel, the work was done in the manner proposed so long ago as the year 1830 by Lord Cochrane (Earl Dundonald) in that specification of his, No. 6,018, wherein he discloses, not merely the crude idea, but the very details needed for compressed air cylinder-sinking and tunneling, included air-locks and hydraulically-sealed modes for the introduction and extraction of materials. I may, perhaps, be permitted to mention that some few years ago I devised for a tunnel through the water-bearing chalk a mode of excavation by the use of compressed air to hold back the water, and combined with the employment of a tunneling machine. This work, I regret to say, was not carried out. But there are, happily, cases of subaqueous tunneling where the water can be dealt with by ordinary pumping power, more or less extensive, and where the material is capable of being cut by a tunneling machine. This was so in the Mersey tunnel, and would be in the Channel tunnel. In the Mersey tunnel, and in the experimental work of the Channel tunnel, Colonel Beaumont and Major English's tunneling machine has done most admirable work. In the 7 foot 4 inch diameter heading, in the new red sandstone of the Mersey tunnel, a speed of as much as 10 yards forward in twenty-four hours has been averaged, while a maximum of 11-2/3 yards has been attained; while in the 7 foot heading for the Channel tunnel, in the gray chalk, a maximum speed of as much as 24 yards forward in the twenty-four hours has been attained on the English side; and with the later machine put to work at the French end, a maximum speed of as much as 27-1/3 yards forward in the twenty-four hours has been effected. In ordinary land tunneling since 1862 there has been great progress, by the substitution of dynamite and preparations of a similar nature for gunpowder, and by the improvements in the rock-drills worked by compressed air, which are used in making the holes into which the explosive is charged. For boring for water, and for many other purposes, the diamond drill has proved of great service, and most certainly its advent should be welcomed by the geologist, as it has enabled specimens of the stratum passed through to be taken in the natural, unbroken condition, exhibiting not only the material and the very structure of the rock, but the direction and the angle of the dip of the beds.