[Verginius Rufus]

The "h" in "history" should be pronounced because the word is accented on the antepenult. But in "historian" and "historical" the first syllable is not accented so "an historian" or "an historical" would be correct.

Actually, it's my impression that some people pronounce an unaccented "h" and some don't...so "a historian" might not be incorrect.

It also works in the middle of a word--"h" in "vehicle" silent, but "h" in "vehicular" pronounced.

[rolling_stone]

methinks you are wrong
unsounded is different than unaccented.

http://owl.english.purdue.edu/owl/resource/591/01/

[rabidralph]

I took an editing class a few years back and they said that the “h” in history, and its variations, should always be pronounced and that the addition of the “an” was incorrect. It’s interesting, nonetheless.