steam, its effect in clearing the tunnel is very marked. This effect is said, however, to be rather impeded than assisted by the presence of the shafts, and I found that the persons in charge of the tunnel, taking this circumstance, and the water admitted into the tunnel by the shafts, into consideration, would be pleased to have them closed altogether, and to depend for ventilation upon a natural current from end to end being caused by difference of temperature, or prevailing winds, and by the artificial current produced by the passage of quick trains.” “Its most dangerous enemy is frost. In winter enormous icicles are sometimes formed by the gradual accretion, and if not removed would be very dangerous.” Of the Sapperton tunnel on the Birmingham and Gloucester section of the Midland railway, 1,760 yards long,[140] Mr. Storrow says, “the grade is very steep, 1 in 70, or 75½ feet to the mile. Hence, in running up the tunnel a great deal of power is required, and an assistant locomotive is kept constantly in use. This, of course, creates a large additional development of smoke and steam, so that after the passage of a single heavy freight train up, the smoke would fill the tunnel for hours, and be quite offensive. Formerly coke was used on English railways, but latterly coal (which, as we know, produces a much greater volume of smoke) is now universally used. All agree that this change has been very injurious in tunnels. At the Sapperton tunnel the assistant engine is usually run down the tunnel immediately after the train has passed up, and thus assists in clearing away the smoke, but a quick passenger train running down the steep incline at great speed is found to be far more effectual, and is indeed the only effectual ventilation.”[141] “The inspector of the road, who accompanied me, thought the opening in the shaft of some use, and said the men wanted it. The superintendent thought it of no use whatever, and that it was something of a nuisance from the water which dripped from it. He was of opinion that after a tunnel was constructed, it would he better to close all shafts, and to trust to quick trains for ventilation. As the tunnel now is, the men continue in it all day, whenever necessary, though they do not like it, but if two freight trains should follow each other without the smoke being cleared away, it would be very difficult for them to work, and if four trains followed each other, it would be impossible.” In confirmation of the view that tunnels are better for ventilation without shafts, than with them, Mr. Storrow gives the particulars of interesting conversations he had with Mr. Brotherhood and Mr. Brassey upon the subject. Both say that numerous shafts are unnecessary for ventilation. They make eddies and currents, and interfere with each other. Mr. Brassey particularly says, that the passage of a train at quick speed is the best ventilator.