“I think,” said he, “you started from Lower Kennington Lane?”

“Yes, from this point,” I replied, indicating the spot with a pencil.

“Then,” said Thorndyke, “if we swing the chart round twenty degrees to correct the deviation of the compass, we can compare it with the ordnance map.”

He set off with the protractor an angle of twenty degrees from the north and south line and turned the chart round to that extent. After closely scrutinizing the map and the chart and comparing the one with the other, he said: