International Correspondence Schools · Scranton, Pa. : The Colliery engineer co. · leaf 343
mine work,compressed air is used instead ofsteam, which loses much of itspressure through condensation.The use of compressed air greatlypromotes ventilation. Percus-sion drills are under a pressureof from 60 to 70 lb. per squareinch. In one hour one will drilla hole froni 2 to 2^ inches indiameter… … 15()1. Drill-bits are of different shapes, beingvaried to suit the work to be done. For uniform hard rock,the bit is cross-shaped, with the arms of equal length and atright angles to each other. For seamy rock, the arms ofthe bits are of equal length, but cross each other X fashion.For soft rock,…
Railroad engineering
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