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."

[Illustration: The Track Chart, showing the route followed by Weiss's
carriage.

A.--Starting-point in Lower Kennington Lane.

B.--Position of Mr. Weiss's house. The dotted lines connecting the
bridges indicate probable railway lines.]

He made the two measurements carefully, and, as the point of the
dividers came down almost precisely in the correct place on the diagram,
he looked up at me.

"Considering the roughness of the method by which the chart was made, I
think that is pretty conclusive, though, if you look at the various
arches that you passed under and see how nearly they appear to follow
the position of the South-Western Railway line, you hardly need further
proof. But I will take a few more proportional measurements for the
satisfaction of proving the case by scientific methods before we proceed