One thought rose instinctively at once in the young man's mind. They
must first see if any one was injured in the other compartments, or
among the debris of the broken carriage; and then they must make
for the open mouth of the tunnel, through which the light of day
still gleamed bright behind them.

He peered in hastily at the other three windows. Not a soul in any
one of the remaining compartments! It was a very empty train, he
had noticed himself, when he had got in at Tilgate; the one solitary
occupant of the front compartment of their carriage, a fat old
lady with a big black bag, had bundled out at Chetwood. They were
alone in the tunnel--at this end of the train at least; their sole
duty now was to make haste and save themselves.

He gazed overhead. The tunnel was bricked in with an arch on top.
The way through in front was blocked, of course, by the fallen mass
of water-logged sandstone. He glanced back towards the open mouth.
A curious circumstance, half-way down to the opening, attracted at
once his keen and practised eye.

Strange to say, the roof at one spot was not a true arc of a
circle. It bulged slightly downwards, in a flattened arch, as if
some superincumbent weight were pressing hard upon it. Great heavens,
what was this? Another trouble in store! He looked again, still
more earnestly, and started with horror.

In the twinkling of an eye, his reason told him, beyond the shadow
of a doubt, what was happening at the bulge. A second fall was
just about to take place close by them. Clearly there were TWO
weak points m the roof of the tunnel. One had already given way in