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.