Halfway across the span he paused, and sinking into the shadow of an
iron girder, began watching the surface of the water.

He was, in fact, attempting to understand those murky depths. From his
room he had detected a strange light. Either reflected on the water or
shining up through it, this light appeared a pale yellow glow, such as
he had often seen given off by the jelly fish in the Pacific. That there
was no such jelly fish to be found in fresh water he knew quite well.
And he had never in his life noticed that glow in the river.

Now, as he surveyed the surroundings, he realized that the light could
not have been reflected from any illumination in street or building. The
glow from the water had appeared close to the wall of the empty building
through which his four assailants of the night before had made good
their escape.

As he stood there, slouching in the shadows, Johnny gave a great start;
the light had appeared again. Beyond question it was beneath the water,
not shining upon it. From this vantage point the light seemed stronger.
It appeared for a few seconds, then disappeared again. Johnny scratched
his head. What could it mean? For some time he stood in a brown study,
then he laughed silently to himself.

"Probably phosphorescent substances being sent out from the drainpipe of
a factory or chemical laboratory," he decided.

At that instant he was all alert. His hand closed on his automatic. A
stealthy footfall had sounded on the bridge.

"Oh! It's you," he whispered a moment later.