THE CHANNEL TUNNEL. 409 



it will be best to pass. Before attempting to do so due consideration 

 should be given to the following questions : — 



1. What are the military requii-ements with regard to the tunnel ? 



2. Where must the tunnel end inland in order to give the best 



accommodation to existing railways ? 



3. What are the most favourable strata for the tunnel works ? 



4. On what line will any error in our geological calculations be of 



least moment? 



5. "Which is the shortest practicable submarine line ? 



The problem, in so far as it depends on surface features of the land 

 and geological conditions, is very different in the two countries, being 

 simpler in France. The points where the tunnel can pass under the 

 shore-line in France are limited by geological conditions to the part of 

 the coast which lies between the Ferme Mouron on the east (where the 

 boring was made in 1867) and Sangatte on the west, a distance of about 

 2 miles. The country, for some way inland throughout this distance, is 

 low and flat, and, probably looking to military requirements, or to com- 

 munications with existing railways alone, it is not very material, within 

 these limits, where the tunnel first reaches the coast. The point may be 

 determined wholly by engineering and geological requirements. 



The case is very different and much more complicated on the English 

 coast. Geological conditions will admit of the tunnel passing under the 

 shore-line anywhere between St. Margaret's Bay on the east and Shake- 

 speare Cliff on the west, a distance of 4h miles. With the exception of 

 the valley of the river Dour, which enters the sea at Dover, the land 

 along this part of the coast is high, bounded by chalk cliffs from 200 to 

 600 feet high. 



Such being the nature of the coast, there are only three places where 

 the exit from the tunnel can be made — in the valley of the Dour near 

 Dover, inland in the chalk escarpment to the north of Folkestone, and in 

 the landslip between Abbot's Cliff and Folkestone. In the first case the 

 tunnel would pass under the shore-line to the cast of Dover, and in the 

 two latter cases to the west of Dover. At the last-mentioned place, 

 between Abbot's Cliff and Folkestone, the tunnel on the South Eastern 

 Railway has been partially destroyed by a landslip, and the line was 

 blocked for some weeks near Abbot's Cliff by a fall of the chalk in 1876. 

 This part of the South Eastern Railway is liable to be obstructed at any 

 time by larger landslips. For this reason it would be out of the question 

 to place the tunnel mouth on such treacherous ground, even if the place 

 satisfied other requirements, such as facilities for the defence of the 

 tunnel, and for communication with existing railways. Similarly, if the 

 tunnel exit were in the chalk escarpment to the north of Folkestone, it 

 would be near no fortified place, nor could junctions be conveniently 

 made with existing railways. Moreover, this position would necessitate 

 at least 6 miles of land tunnel, in addition to a longer sea tunnel, than 

 from any point of the shore further to the east. The valley of the Dour 

 alone remains to beconsidei-ed. The tunnel mouth can there be placed at 

 a moderate height above high-water level, which means a shorter length 

 of tunnel and better gradients between the tunnel mouth and the lowest 

 point of the line beneath the sea ; it can be placed either within or with- 

 out the fortifications, as the military authorities may require ; and, as 

 the existing railways terminate at Dover, they can with ease be con- 

 nected by short railways with the tunnel line. It remains to be seen 



