. Shield and compressed air tunneling . Fio. 67.—Typos of shield jacks, (o) and (ft) piston type, (c) single acting typo,(rf) and (e) .single acting with auxiliary plunger. the jacks are spaced uniloiniiy around the circumference, butin some eases more jacks have b(>en placed below the horizontaldiameter than above, because greater force is generally neededat the bottom than at the top, owing to the tendency of the shieldto dive. 15G SHIELD TUNNELING

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Keywords: ., bookcentury1900, bookdecade1920, bookidshieldcompre, bookyear1922