National Geographic : 1944 May
Bituminous Coal Institute Just as a Mower's Flashing Blade Clips Hay, So Does a Cutting Machine MowaBituminous Wall In preparation for blasting, the machine saws an undercut. To shield the roof, which appears soft, from the shock ofthe explosions, theblade will pivot and saw an overcut. Similarly, vertical cuts are possible. Ties and track will be dragged as a unit when the room isworked out(page 583).