July 19, 1900] 



NATURE 



281 



THE GREAT ALPINE TUNNELS} 

 "T^HE subject for this evening's discourse is that of the three 

 -*■ great tunnels through the Alps — viz. the Mont Cenis, the 

 St. Gothard, and that which is now in course of construction, 

 the Simplon. 



Bui before dealing with the details of these particular works, 

 it will be desirable to consider what tunnelling is, and also 

 some of the more remarkable instances of it in bygone days. 



One great drawback in connection with the subject — 

 so far as a discourse is concerned — is its unsuitability for 

 the photographic art. Unlike a battleship, or a splendid 

 bridge, or a grand block of buildings which can be made into 

 fine views and pictures, the work of the mole is hardly adapted 

 to the sensitive plate. I therefore propose to make use of the 

 " language of the pencil," and to make a few rough sketches on 

 the blackboard : by these means I trust I may be able to ex- 

 plain some of the difficulties which have to be encountered, and 

 also show how a tunnel is constructed. The child's definition 

 of drawing, " first you think, and then you draw a line round 

 your think," will come to our aid. 



The art of tunnelling dates back to very remote ages, and 

 there are records of such works which were constructed 500 to 

 600 years before the Christian era. 



An interesting account is given by one of your most distin- 

 guished members, in an article in the '* Encyclopaedia 

 Britannica," of the tunnel under the river Euphrates at Babylon. 

 This city, similar in some respects to London, lay half on one 

 side and half on the other side of the river. High walls, pene- 



Yig.l. 



Cross Sectkot 



oPthe 



Aqueduct 



oF EUPALINOS. 



In the IslancC 



of 



SAM OS 





trated by occasional gates, surrounded the city, and lined each of 



the banks of the river. These gates (of which a pair of the 



great hinges can be seen in the British Museum) were closed at 



night and during war ; and a tunnel was constructed below the 



bed of the river by means of what is technically known as the 



"cut-and-cover" system. In those days the Greathead shield 



was unknown, and consequently the river had to be diverted, so 



t the excavation could be made in the dry bed and cut open 



daylight, the arch being built, the ground restored, and the 



allowed to resume its former course. The tunnel is said 



have been 15 feet in width, and 12 feet in height, built of 



k. 



Herodotus gives an account of the diversion of the river 



a great excavation or artificial lake forty miles square, and 



tes that the besieging enemy, so soon as the water was drawn 



entered into the city by the river bed. It is believed that 



same excavation was made use of for the construction of the 



nel. It is, however, desirable to state that doubts have been 



own on the subject, and it is possible that it may have to be 



relegated to mythology. 



The next instance of a tunnel is that referred to by Herodotus 

 in the Island of Samos (" Herodotus," iii. p. 60) (see Fig. i), and 

 L^^^ is satisfactory to know that although very considerable doubts 

 ^^^H^ere expressed as to the accuracy of his statements, recent in- 

 ^^^•estigations prove that he was exactly correct. The description 

 ^^Hiven by him, when expressed in English words and figures, is as 



^^^k1 a discourse delivered at the Royal Institution on Friday, May 25, by 

 ^^^■nncis Fox, Mem. Inst. C.E. I 



^tk NO 1603, VOL. 62] 



follows : "They have a mountain which is 910 feet in height ; 

 entirely through this they have made a passage, the length of 

 which is 1416 yards. It is, moreover, 8 feet high, and as many 

 wide. By the side of this there is also an artificial canal, which 

 in like manner goes quite through the mountain ; and though 

 only 3 feet in breadth, is 30 feet deep. This, by the means of 

 pipes, conveys to the city the waters of a copious spring." 



The commentators on this passage say that Herodotus must 

 have made a mistake, but the Rev. H. F. Tozer, in his book 

 "The Islands of the Mgt&n" p. 167, gives the results of a 

 personal visit. 



He says the tunnel is 7 to 8 feet in width ; that two-thirds of 

 its width are occupied by a footpath, the other third being a water- 

 course, 30 feet deep at one end. He and other writers consider 

 that insufficient allowance was made for the fall of the water, 

 and that the water channel had to be deepened. To describe it 

 in more modern language, the resident engineer evidently made 

 a mistake in his levels, necessitating a much deeper excavation 

 than was at first anticipated. 



Another, and if possible a more interesting, instance of tun- 

 nelling is that described in the Proceedings of the Palestine 

 Exploration Society, in connection with the Pool of Siloam, 

 made by Hezekiah, B.C. 710, 2 Kings xx. 20 (" Palestine 

 Exploration," 1882, p. 178). See Fig. 2. 



About 710 B.C. a tunnel was driven from the spring to the 

 well — by actual tunnelling — the work being commenced at the 

 two ends, and by shafts, and the workmen met in the middle. 

 The tunnel was only 2 feet in width and 3 feet in height, ex- 

 cept at the probable point of meeting, where the height is 4 feet 

 6 inches. The length is 1708 feet, and there is a fall of i foot 

 in this distance. About the middle of its course there are 

 apparently two false cuts, as if a wrong direction had been 



Fig. 2. 



Plcuv of Tunnel front SprCrtg toPool 

 of SiZoafrty. 



(Not to scale.) 



taken : but possibly these were intentional, and provided passing, 

 places for the workmen and material. 



On the soffit of the tunnel is carved an inscription, of which 

 the following is a translation : — 



" Behold the excavation. Now this had been the history of 

 the excavation. While the workmen were still lifting up the 

 pick, each towards his neighbour, and while 3 cubits (4 feet 6 

 inches) still remained to cut through, each heard the voice of the 

 other, who called to his neighbour, since there was an excess of 

 rock on the right hand and on the left. And on the day of the 

 excavation the workmen struck each to meet his neighbour pick 

 against pick, and there flowed the waters from the spring to the 

 pool for a thousand two hundred culiits (1820 feet), and a 

 hundred cubits (151 feet) was the height of the rock over the 

 head of the workmen." 



A Roman engineer gives an account of a tunnel which was 

 being driven under his directions for an aqueduct. And as he 

 was only able to visit the work occasionally, he describes how 

 on one of his visits he found the two headings had missed each 

 other, and he says that had his visit been deferred much longer 

 there would have been two tunnels. 



The accurate meeting of the headings or driftways of a tunnel 

 can only be attained by the exercise of great care, both as 

 regards direction as well as level. 



We need not go very far to find instances of such an error as 

 inaccurate meeting, but there is one well-known case on an im- 

 portant main line in the Midland Counties where the engineers 

 failed to meet, and to this day reverse curves exist in the tunnel 

 to overcome the difficulty. 



To attain this accurate meeting fine wires are hung down the 



