They are listing various measurements; cables and chains have special meanings when measuring certain things. "A gnat's whisker" is an additional, humorous, measurement added on at the end to indicate there is just a tiny, tiny bit more way to go - approximately the width of a gnat's whisker. (Sometimes "a cat's whisker" is used.) In any case, think of how extremely fine an insect's whisker would be; that's why it's used to indicate a miniscule amount.
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