midnight
Other gangs came along , carrying strange screw-doors of iron , and a multitude of devices new to the eyes of miners . Everything had been measured and prepared above-ground . It remained only to throw the material together , according to a prearranged plan . By midnight , all was ready . Three `` sand hogs , '' with a gallant young doctor who had volunteered , prepared to enter . A steady throbbing sound told that machinery connected with an outlet pipe -- solidly embedded in the cement -- had been set in motion . The newly made walls threatened to bulge inwards , and the signal was given to stop .