August 31, 1882] 



NATURE 



443 



densed, and cnmbustion is aided by the natural draught of the 

 chimney alone, instead of being urged by a forced blast as on 

 open portions of the line. Whether a hot-water, a compressed 

 air, or a compressed gas locomotive could be contrived to meet 

 the exigencies of metropolitan traffic is a question which, I 

 think, might be usefully discussed at the present or some future 

 meeting of the Association. 



A reference to the underground railway naturally suggests the 

 wider que tion of tunnels in general. The construction of 

 tunnels was not one of the novelties presenting it-elf to railway 

 engineers, for many miles of tunnel had been driven by canal 

 engineers before a single mile of passenger railway had been 

 built in this or any other country. To foreign engineers belongs 

 the honour of having boldly conceived and ably accomplished 

 tunnel works of a magnitude which would have appalled a canal 

 engineer. I need only refer to the Mont Cenis Tunnel, over jh 

 miles in length, commenced in 1S57 and finished in 1S70 ; the 

 St. Gcthard Tunnel, g\ miles in length, commenced in 1S72 

 and finished in 1882 ; and the Hoosac Tunnel, 4^ miles in 

 length, commenced in 1854 and finished in 1875. In all cases 

 rock of the hardest character had to be pierced, and it is need- 

 less to remark that without the aid of the machinist in devising 

 and manufacturing compressed air machinery and rock-boring 

 plant the railway engineer could not have accomplished his 

 task. Intermediate shafts are not attainable in tunnels driven 

 through great mountain ranges, so all the work has to be done 

 at two faces. In the case of the Mont Cenis Tunnel the mean 

 rate of progress was 257 feet and the maximum 400 feet per 

 month. In the St. Gothard Tunnel the mean rate was 429 

 feet and the maximum 810 feet. In the Hoosac Tunnel the 

 average rate was 150 feet per month. 



Tunnels under broad navigable rivers and estuaries have been 

 a subject of discussion by engine rs for at lea-t a century, but 

 the only one at present completed is the unfortunate and costly 

 Thames Tunnel. Two important works of the class are, however, 

 now well in hand, the Severn Tunnel at Portskewet, and the 

 Mersey Tunnel at Liverpool. Having reference to this fact, it 

 will be interesting to quote the following passage from a letter 

 addressed to the press by a Mr. Thomas Deakin on March 6, 

 1835, that is to say, more than forty-seven years ago. Mr. Deakin 

 writes : ' The Great Western Railroad from London to Bristol 

 will be accomplished no doubt, and why not continue it under 

 the Severn mouth, near Chepstow, Monmouthshire, through 

 Glamorganshire, and to Milford Haven in Pembrokeshire? °It 

 would then traverse the coal-field of South Wales throughout its 

 whole extent— a tract of country possessing al-o inexhaustible 

 stores of iron-stone. A tunnel was once so proposed to be formed 

 under the Mersey at Liverpool, and had it not been for the failure 

 of the Thames Tunnel would most probably have been carried 

 into effect." It is not a little singular that the two tunnels thus 

 foreshadowed by Mr. Dakin should both be in hand at the present 

 moment. 



Undoubtedly the numerous accidents which occurred during 

 the construction of the Thames Tunnel, together with its enormous 

 cost of about 1,500/. per lineal yard, and the eighteen years 

 occupied in its construction, destroyed the chance of any other 

 projected subaqueous tunnel for many subsequent years. One 

 lesson enforced by the Thames Tunnel was the necessity of 

 leaving a reasonable thickness of ground between the water and 

 the tunnel. In the Severn Tunnel the minimum thickness is 40 

 feet and in the Mersey Tunnel 22 feet. The w idth of river at 

 the point of crossing of the former tunnel is 2 j miles, and the 

 maximum depth of rails below high water, 163 feet. In the case 

 of the Mersey Tunnel the width is nearly three-quarters of a mile, 

 and the depth 144 feet. The Thames Tunnel, as almost every- 

 one knows, was carried on by means of a special contrivance 

 termed by Brunei a "shield." No special appliances have been 

 adopted in either of the Severn or the Mersey Tunnels. Both are 

 driven in the ordinary way, but of course enormous pumping 

 power is required and has been provided. 



When no special appliances are used in the construction of a 

 subaqueous tunnel, it will be clear to all that an unknown risk is 

 encountered. All may go well, and the engineer will then justly 

 receive congratulations from everyone for his boldness and suc- 

 cess. But, on the other hand, something may go wrong, even 

 rt the last moment, and I fear the engineer then would be abused 

 no less roundly by the unthinking public for his temerity and 

 consequent failure. It would be a " Majuha Hill " incident over 

 igain, and if the accident caused much lo;s of life the engineer 

 probably would envy the fate of the brave but ill-starred 



General Colley, who at least fell with the victims of his 

 rashness. 



In many cases of tunnels under estuaries, special appliances 

 could be used which would obviate all risk and make the suc- 

 cessful completion of the work a mathematical certainty. A 

 tunnel under the Humber, about I J mile in length, projected by 

 myself in 1873, the Bill for which was subsequently passed by 

 the Commons and thrown out by the Lords, was a case in point. 

 The bed of the Humber is of very fine jilt, and I proposed to 

 build the tunnel in lengths of 160 feet, under the protection of 

 rectangular iron caissons 160 feet long by 42 wide, sunk by the 

 pneumatic process. As the pressure of the air in the caissons 

 would always be slightly in excess of that due to the head of 

 water in the river, no interruption from influx of water could 

 ever occur, and the operation of building the tunnel in lergths 

 inside this huge diving-bell would be as certain and free from 

 risk as the every-day work of sinking a bridge-pier by the 

 pneumatic process. 



A tunnel over a mile in length, now in progress under the 

 Hudson River at New York, is being driven through a silty 

 stratum by the aid of compressed air, and with a certain amount 

 of success, as only some twenty men have been drowned up to 

 the proent lime. The principle upon which the compressed air 

 is used is, however, a false one, since it is merely forced into the 

 tunnel with a view to uphold the ground by its pressure, like so 

 much timbering, and not to keep out the water on the principle 

 of a diving-bell. It is clear, therefore, that the completion of 

 the Hudson River Tunnel, if the present system be persevered in, 

 is purely a matter of conjecture, and all we can do is to hope for 

 the best. The same remark applies, of course, to the Severn 

 Tunnel and the Mersey Tunnel, although in those cases the 

 character of the ground is such that the contingencies are small 

 in comparison with those encountered in the construction of the 

 Thames Tunnel and the Hudson River Tunnel. Nevertheless, 

 as I have already observed, unless special appliances of the 

 nature of the pneumatic process be used, a subaqueous tunnel, 

 whether it be the Channel Tunnel itself or one but a few yards 

 in length, must necessarily present an unknown risk. The pro- 

 totype of all these tunnels is the one commenced at Rotherhithe 

 in 1809, which was successfully driven a distance of 900 feet 

 under the Thames, and failed when within a little more than 100 

 feet of the opposite shore. A tunnel about li mile in length 

 was commenced about ten years ago under the Detroit River in 

 America, but was abandoned in a similar manner. So far 

 good fortune has attended both the Severn and Mersey 

 Tunnels, and there is, I am glad to say, every chance of its 

 continuing. 



That the series of mishaps with the Thames Tunnel, and the 

 consequent postponement of all other projects for subaqueous 

 tunnels, were due to errors in design and want of foresight on 

 the part of the engineer, is patent to everyone now, and was 

 foreseen by at least one acute contemporary of Brunei himself. 

 Only a few months ago, when turning over the leaves of an old 

 periodical, I became aware of a fact that a scheme, identical in 

 all its main features with my Humber Tunnel project, had been 

 suggested for adoption in the case of the Thames Tunnel, in lieu 

 of the plan proposed by Brunei. Writing in December, 1823, 

 or fifty-nine years ago, the author of the project, a working 

 smith of the name of Johnson, says : " I propose to construcc 

 the Thames Tunnel without coffer-dams by making it in parts 28 

 feet in length, each part having the ends temporarily stopped up, 

 and being constructed on the same principle as th. diving-bell. 

 The men dig from the inside round the edge as if sinking a well, 

 and throw the earth towards a dredger, the buckets of which 

 work some feet below the bottom of the excavation. Each length 

 will be suspended between two vessels, and be conveyed to the 

 place where it is let down." A description of the mode of con- 

 necting the several lengths is given, and I may add that the 

 tunnel blocks had a sloping face to tend to bring the faces of the 

 joints together, a plan since adopted with the huge concrete 

 blocks at Kurrachee and other harbours. There is not a flaw in 

 the design from beginning to end, as modern experience in the 

 sinking of numerous bridge-piers on precisely the same plan has 

 amply demonstrated. It is beyond all doubt that if the design 

 of this working smith had been adopted in lieu of that tendered 

 by Brunei, the Thames Tunnel would have been completed in a 

 couple of years, instead of eighteen years, and at a cost of about 

 300/. per yard instead of 1,500/. 



If another tuunel be constructed under [the Thames, which is 

 far from improbable, as the requirements of below-bridge traffic 



