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And then followed a discussion on "struts," roof timbers and tie-rods, Jack describing in a modest, impersonal way the various methods used by the members of the staff with which he was connected, Morris, as usual, becoming so absorbed in the warding off of "cave-ins" that for the moment he forgot the table, his hostess and everybody about him, a situation which, while it delighted Peter, who was bursting with pride over Jack, was beginning to wear upon Miss Felicia, who was entirely indifferent as to whether the top covering of MacFarlane's underground hole fell in or not.
In many mines, also, a roof of coal a foot or more in thickness must be left because the material above the coal is not solid enough to prevent cave-ins. When the mine is abandoned and closed these pillars and roofings remain untouched, because removing them constitutes one of the greatest dangers to life, and is one of the frequent causes of mine accidents.
It is the unknown that awes, and when she first stepped from the cage and peered down the long, low tunnel through which a tramway ran she caught her breath rather quickly. She had an active imagination, and she conjured cave-ins, explosions, and all the other mine horrors she had read about. Their host had spared no expense to make the occasion a gala one.
"No sir, I wouldn't go back in that there mine for five thousand dollars, I'm out, an' I stays out," while another added, "'Twouldn't be of no use, sir; mos' likely he was catched in some o' them cave-ins; he stopped to give us all warnin' an' he was about the last one to start."
There, a city in ruins, flung to the ground by convulsions in the earth. These views were varied continuously by the sun's oblique rays, or were completely swallowed up by gray mists in the middle of blizzards. Then explosions, cave-ins, and great iceberg somersaults would occur all around us, altering the scenery like the changing landscape in a diorama.
Excavations of large waterways then becomes a comparatively easy task. We have no high mountains on Mars; in fact, none exceeding 3,000 feet in altitude. Owing to the difference in gravity the angle of repose on Mars is nearly acute as against 45 degrees on your Earth, which permits of almost perpendicular walls to the canals and lessens the danger of landslides and cave-ins.
A few were left behind, to care for the camp in case of emergencies, to keep the roofs as free from snow as possible and to avoid cave-ins. The rest filed outside, one by one, awkwardly testing the bindings of their snowshoes, and awaiting the command.
She looked down what the map showed was the chute of Hacker Tow Head, and then the current carried her almost to the bank at the head of Buffalo Island. Here there was a stretch of caving bank; the earth, undercut by the river current, was lumping off in chunks and slices. Her boat bobbed and danced in the waves from the cave-ins, and the rocking pleased her fancy.
Margaret observed that the timbered sides of the tunnel leaned inward slightly and that the roof was heavily cross-timbered. "It looks safe," she thought aloud. "It's safe enough," returned Dunke carelessly. "The place for cave-ins is at the head of the workings, before we get drifts timbered." "Are we going into any of those places?"
"Gneiss rock for eleven hundred feet and then some mica schist that we have had to shore up every time we move our drills," answered Jack quietly. "Any cave-ins?" Morris was leaning forward now, his eyes riveted on the boy's. What information he wanted he felt sure he now could get. "Not yet, but plenty of water. "And when is he going to get through?" "That depends on what is ahead of us.
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