The only danger involved in this part of the work lay in the fact that between this lining and the tail end of the shield was always a space of an inch to an inch and a half which was left unprotected. This small opening would, under ordinary circumstances, be insignificant, but in some instances where the mud covering at the top was very soft and not very thick, there was danger of the compressed air from within, pushing at the rate of several thousand pounds to the square inch, blowing it away and leaving the aperture open to the direct action of the water above. This was not anticipated, of course, not even thought of. The shield was going rapidly forward and it was predicted by Henderson and Laverty at intervals that the tunnel would surely go through within the year. Some time the following winter, however, when the shield was doing such excellent work, it encountered a rock which turned its cutting edge and, in addition, necessitated the drilling out of the rock in front. A bulkhead had to be built, once sufficient stone had been cut away, to permit the repairing of the edge. This took exactly fifteen days. In the meantime, at the back of the shield, at the little crevice described, compressed air, two thousand pounds to the square inch, was pushing away at the mud outside, gradually hollowing out a cup-like depression eighty-five feet long (Mr. Henderson had soundings taken afterwards), which extended backward along the top of the completed tunnel toward the shore. There was then nothing but water overhead. It was at this time that the engineers, listening to the river, which, raked by the outpouring of air from below, was rolling gravel and stones above the tunnel top and pounding on it like a drum, learned that such was the case. It was easy enough to fix it temporarily by stuffing the crevice with bags, but one of these days when the shield was repaired it would have to be moved forward to permit the insertion of a new ring of plates, and then, what? At once McGlathery scented trouble. It was the wretched river again (water), up to its old tricks with him. He was seriously disturbed, and went to pray before St. Columba, but incidentally, when he was on duty, he hovered about this particular opening like a wasp. He wanted to know what was doing there every three minutes in the day, and he talked to the night foreman about it, as well as Laverty and Mr. Henderson. Mr. Henderson, at Laverty’s and McGlathery’s request, came down and surveyed it and meditated upon it. “When the time comes to move the shield,” he said, “you’ll just have to keep plenty of bags stuffed around that opening, everywhere, except where the men are putting in the plates. We’ll have extra air pressure that day, all we can stand, and I think that’ll fix everything all right. Have plenty of men here to keep those bags in position, but don’t let ’em know there’s anything wrong, and we’ll be all right. Let me know when you’re ready to start, and I’ll come down.”