He knew that it was an apparatus for lowering the car to a great depth, and, climbing into the car, he examined everything it contained.

Coming down, he noticed the grating, and he knew what it was for.

He looked over the engines and calculated the strength of the chains on the windlasses.

He took an impression of the lock of the trap-door, and when he went away in the very early hours of the morning he understood the apparatus which was intended to lower the car as well as any person who had managed it.

He knew nothing about the shaft under the great door, but this he intended to investigate as thoroughly as he had investigated the machinery.
The next night he entered the building very soon after Cunningham had gone his rounds, and he immediately set to work to prepare for his descent into the shaft.