What determines whose Global spell sticks?

[ceremony]

You can dispel a global enchantment with another global enchantment. I had always thought this was based on gem input into the spell.

However, in the case Master Shake cited, I cast my Gift of Health first, and got the message that I had successfully cast it. Two Messages down the list was another "dire portent" informing me that another Gift of Health had overwhelmed mine.

The question is: what determines this dispel chance? My spell had 51 gems and was cast first, and his only had 41 and was cast afterward. Both were cast by mages with the same Nature skill.

Apparently, there is no requirement to use more gems than the spell you are dispelling. Is there always a small base probability that a global enchantment will dispel another, and using more gems simply increases this chance? If so, what is this base chance and what is the per-gem improvement to it? If not, what is this mechanism based on?

EDIT: I should make it clear that while I say my spell was cast "first," I am only inferring this from the fact that my spel report was listed first in the message list. Both were cast on the same turn. So I guess another question I have is whether this would have occurred differently had his spell been cast on the next turn rather than the same turn - are globals treated differently for dispel purposes when cast on the same turn rather than different turns?

[ February 23, 2004, 20:13: Message edited by: ceremony ]