Y'all should read 'John Henry Days' by Colson Whitehead. Although fictional, he has certainly done his homework researchng the John Henry myth, and describing the technique of 'steel drivin''. One other thing to add to all you brains out there when 'yer figgyurin'out yer' hammerin' math: Don't forget that those handles flexed, making that ol' head snap like a whip. Steel drivers used to 'bone' (much the same as old time baseball players used to do to their bats) the handles of their hammers to supposedly make them last longer. I can't say that I can see that it would really work, but it adds to the lore of a trade ofold.......
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