3-021-353 - The Print Collector/Heritage Images

Section of the Alps showing the course of the railway tunnel, 1869. 'In the actual execution of the work at Modane and Bardonnèche the first thing to be done was, of course, to fix the precise horizontal line for the axis of the tunnel, and to mark its position visibly from the outside. Certain points were to be indicated by signals, across the summit of the mountain, through which a vertical plane containing the axis of the projected tunnel would pass. The two engineers, Signor Copello and Signor Borelli...had to erect a signal-tower on the very top of the Grand Vallon, 11,000 ft. above the level of the sea, almost equidistant from the two extremities of the tunnel...A line was then drawn, up one slope of the mountain, to the summit, and down the other, which was marked by nineteen flags or other signals; the horizontal base-line subtending the vertical plane thus described was to be the axis of the tunnel. The length being ascertained by triangulation, it was determined that this line should be divided into two equal sections, one of which should have an ascending gradient of 1 in 45½ and the other a descent of 1 in 2000...The [diagram] will show the line of the tunnel and the outline of the mountain range through which it is bored'. From "Illustrated London News", 1869.