Past tense Spelling

[David Metevia]

In the August, 2008 issue of Smithsonian magazine there is an article
about a kidnapping and murder that occurred in Chicago in 1924.  The
story has a picture of the Chicago Daily News headline: KILL BOY
KIDNAPED FOR RANSOM.  What caught my eye was the spelling of past tense
of _kidnap_.  How do the standard past tense formations change over
time?  In my mind I think of the past tense of words like kidnap that
end with a short vowel that the final consonant would be doubled -
kidnapped, rapped, admitted, and so on.  Does anyone know of resources
on this topic that track spelling "rule" changes?

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