Rovinski spent the night in that building. He had with him a dark lantern, and he made the most thorough examination of the machinery at the mouth of the shaft. He was a man of great mechanical ability and an expert in applied electricity. He understood that machinery, with all its complicated arrangements and appliances, as well as if he had built it himself. In fact, while examining it, he thought of some very valuable improvements which might have been made in it. 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. He disconnected one of the engines, for he sneeringly said to himself that the other one was more than sufficient to lower and raise the car. He charged and arranged all the batteries and put in perfect working order the mechanism by which Clewe had established a connection between the car and the engines, using one of the chains as a conductor, so that he could himself check or start the engines if an emergency should render it necessary. Then Rovinski, bounding around like a wild animal in a cage, took out a key he had brought with him, opened the trap-door, lifted it back, and gazed down. He could see a beautifully cut well, but that was all. But no matter how deep it was, he intended to go down to the bottom of it. He started the engine and lowered the car to the ground. Then he looked up at a grating which hung above it and determined to make use of this protection. He could not lower it in the ordinary way after he had entered the car, but in fifteen minutes he had arranged a pulley and rope by which, after the car had gone below the surface, he could lower the grating to its place. He got in, started down into the dark hole, stopped the engine, lowered the grating, went down a little farther, and turned on the electric lights.