The chief engineer of the line is doubtful as to whether there ought not to be a shaft. At times the current of cold air through the tunnel is so strong, that it would be an inconvenience to the trains if not checked by the curtains. They also have the effect of keeping up a temperature in the tunnel sufficient to prevent the water that drips on to the rails becoming frozen in winter. Great trouble and inconvenience were experienced from this cause in the first instance. The freezing of water, lodged on the rails, has scarcely ever happened during the last few years. The time that a train will take to pass through the tunnel of the Alps must now be considered. If it had been constructed throughout on the level, or with every favourable gradients, and that a train would be permitted to run throughout at express speed, it might be conveyed from entrance to exit in about twelve minutes. This gives a speed of say thirty-eight miles an hour. But with a gradient, the average of which is 1 in 45, anything approaching this rate of speed would be simply impossible. Length, however, is not the only element that has to be taken into account. M. Auguste Perdonnet, in his “_Traité Elémentaire des Chemins de Fer_” (Paris 1858-60), says with great truth: “Les fortes pentes sont plus nuisibles dans les souterrains que dans toute autre partie, d’un chemin de fer. L’humidité empechant la boue, qui impregne les rails, de secher, l’ascension de fortes rampes y devient tres penible. Il faut, donc, s’appliquer a les eviter plus encore dans les tunnels qu’a ciel ouvert.” No matter how stoutly the tunnel of the Alps may be lined, and no matter how impenetrably the tunnel may act as a barrier against infiltration of water, it never can be free from moisture; not the moisture of a downfall of rain, which washes the rails and “makes things pleasant,” but it will be one in which will be conbined, unchemically and therefore loosely, the steam of the engines condensed into water, minute particles of grease, and smoke; all three will, _inter alios locos_, find a resting place upon the rails, and they will thus create on them a thick, clammy, _pasty_ moisture, which will sensibly diminish the engine’s adhesion, and act as a most serious impediment to its progress. It will therefore be impossible with the foregoing elements, and with a gradient of 1 in 45, to calculate on a higher rate of speed from the Modane entrance to the centre than ten miles an hour, or six minutes to the mile—twenty-two minutes; and our belief is that to maintain even this rate of speed it will be necessary to have a fiercely burning fire in the fire-box and tubes, together with a plentiful fall of sand from the sand boxes on to the rails. As soon as the train has arrived at the centre, which is also the summit of the tunnel, the engine will get relieved, and with the gradient almost level the second half of the tunnel can, as a simple question of speed, be run at the highest possible rate.