From BiblePortal Wikipedia

King James Dictionary [1]

Whereby, adv. where and by.

1. By which.

You take my life, when you do take the means whereby I live.

2. By what, interrogatively.

Whereby shall I know this?  Luke 1 .

Webster's Dictionary [2]

(1): ( adv.) By what; how; - used interrogatively.

(2): ( adv.) By which; - used relatively.