"That cannot be denied," was his reply. "This neighbourhood opens up a wide field of investigation to the inquisitive gentry from the Fatherland. Knowledge of the secrets of the defences of the Firth of Forth would be of the utmost advantage to Germany in the event of an invasion. The local submarine defences and corps of submarine miners have been done away with, yet the entrance of the estuary is commanded by strong batteries upon the island of Inchkeith, opposite Leith; the Forth Bridge is defended by masked batteries at Dalmeny at the one end and at Carlingnose at the other, while upon Inchgarvie, the rock beneath the centre of the bridge, is a powerful battery of six-inch guns. The true strength of these defences, and the existence of others, are, of course, kept an absolute secret, but Germany is equally anxious to learn them, as she is to know exactly what our plans are regarding this new naval base and its fortifications." "But if the new base were established, might not the Forth Bridge be blown into the water by the enemy, and our fleet bottled up by the wreckage?" I ventured to remark. "That's just the point, Jack," my friend said; "whether the Rosyth works are carried out or not, the Germans would, without doubt, use their best endeavours to blow up the bridge; first in order to cut direct communication between north and south, and secondly, to prevent British ships using St. Margaret's Hope as a haven of refuge." "And even in face of the document discovered upon the auctioneer's clerk, the Government deny the activity of spies!" "Yes," said Ray in a hard voice. "A week ago I was up here, and examined the safe in the offices we've passed. I was only laughed at for my pains. I must admit, of course, that no document has ever been missing, and that the safe has not been tampered with in any way." "A complete mystery." "One which, my dear Jack, we must solve," he said, as we retraced our steps back to North Queensferry station, where we luckily caught a train back to Edinburgh. Next morning we travelled again to Dalmeny, and in the grey mist hired a boat at the slippery landing-stage opposite the Hawes Inn. Refusing the assistance of the boatman, Ray took off his coat and commenced to row to the opposite shore. His action surprised me, as we could easily have gone over by the steam ferry. It was high tide, and by degrees as we got into mid-stream he allowed the boat to drift towards one of the sets of four circular caissons in which the foundations of the gigantic bridge, with its bewildering masses of ironwork, were set. Against one of them the boat drifted, and he placed his hand upon the masonry to prevent a collision. As he did so, his keen eyes discerned something which caused him to pull back and examine it more closely. As he did so, a train rumbled high above us.