Read it through once

Bestowing something half-way between a smile and a frown upon his two companions, and again beckoning them to follow him, the man hastened across the apartment, which was of considerable extent, but low in the roof. He was preparing to ascend a steep staircase, or rather ladder, leading to another floor of warehouses above: when a bright flash of lightning streamed down the aperture, and a peal of thunder followed, which shook the crazy building to its centre.