"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:
"By mere inspection it seems fairly easy to identify the thoroughfares that correspond to the lines of the chart. Take the part that is near your destination. At nine twenty-one you passed under a bridge, going westward. That would seem to be Glasshouse Street. Then you turned south, apparently along the Albert Embankment, where you heard the tug's whistle. Then you heard a passenger train start on your left; that would be Vauxhall Station. Next you turned round due east and passed under a large railway bridge, which suggests the bridge that carries the Station over Upper Kennington Lane. If that is so, your house should be on the south side of Upper Kennington Lane, some three hundred yards from the bridge. But we may as well test our inferences by one or two measurements."
"How can you do that if you don't know the exact scale of the chart?"
"I will show you," said Thorndyke. "We shall establish the true scale and that will form part of the proof."
He rapidly constructed on the upper blank part of the paper, a proportional diagram consisting of two intersecting lines with a single cross-line.
"This long line," he explained, "is the distance from Stillbury's house to the Vauxhall railway bridge as it appears on the chart; the shorter cross-line is the same distance taken from the ordnance map. If our inference is correct and the chart is reasonably accurate, all the other distances will show a similar proportion. Let us try some of them. Take the distance from Vauxhall bridge to the Glasshouse Street bridge."