"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."