"Simply by the position of this board. It revolves by the same magnetic influence that causes the compass card always to point to the magnetic-north, only here we have made corrections for the variation of the compass. The only drawback is that objects to the south are shown inverted on the board, but with a little practice one soon gets accustomed to this. It is like the inverted image on the ground glass of a camera, and to an experienced photographer that state of affairs presents no difficulty. Here we are: right abeam on the starboard hand you can see Bull Point, the ordnance depot. When were you last up the river Tamar, might I ask?" "Quite two years ago." "And it is more than seven since last I saw Saltash Bridge. There have been vast changes, and I suppose had you been at the periscope all the way up the Hamoaze you would have noticed a difference in the aspect of the place." "I see that they’ve covered in the magazines with earth," said Hythe. "That’s for protection against attack from aerial craft." "And it is the same in Devonport and Keyham Yards. There’s no doubt the British nation has developed an air-invasion panic. For instance: years ago all the building slips in the Royal Dockyards were roofed in. I can remember the huge sloping roofs with their skylights arranged after the manner of the ports of an old three-decker man-of-war. Early in the present century these roofs were removed, and the slips were left entirely open and uncovered. Now what do I see? Every dock, every slip, every Government workshop of importance is housed in with a bomb-proof steel roof. And with one touch of my little finger I could do more damage, and without being seen, to that place yonder than the combined aerial fleets of Germany and Austria. But see, we are in sight of Saltash Bridge. Just below that structure I mean to bring up for the night. I regret, Mr. Hythe, I cannot give you an opportunity to stretch your legs upon Devon or Cornish soil, but I trust that you will realize that circumstances prevent my so doing." The "Aphrodite" sank to the bed of the river between the lofty bridge and the chains of the floating-bridge plying between the village of Saltash and the left bank of the Tamar. Here in fifty-six feet of water she was in absolute safety, and since there was no occasion to recharge her accumulators, she had chosen a berth where she was partially protected from the full force of the tide by water-pipes that are laid across the bed of the river by the side of the Albert Bridge.