“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.”