A Historical Lesson

I have seen the words "historic" and "historical" used with both "an" and "a," as in "an historic ocassion." Here is lesson XXI from Illustrated Lessons in Our Language by G.P Quackenbos, 1882.

...Whether an or a is to be used, depends on the sound with which the following word commences....
An is used before words commencing with a and i, and most words commencing with e, o, u, and h not sounded. A is used before all other words.
A must be used when the following word commences with the sound of u in unit; as, a unit, a ewe, a eulogy, a humour.
A must be used before one; as, a one-horse wagon.
A must be used before words commencing with h sounded; as, a hat, a hen.

That would also include a historic ocassion and a hysterical person.
The confusion stems from usage seen in the King James Bible, when all words having a French origin that began with h were pronounced with the h silent, such as "an hotel." This is no longer correct, even in England. So drop the n before historical.
(Source: Harper Dictionary of Contemporary Usage, 2nd ed. New York: Harper & Row, 1985: p. 1.)

(Illustrated Lessons in Our Language by G.P Quackenbos, is Available on CD at Lady's Maid Books.)