“Sudden Well
PeTe 30
R: Voice, D: Mom, T: Part
This spell creates a shaft 10 feet square and 100 feet deep. The shaft is not reinforced, so it collapses swiftly after it is created.
(Base 3, +2 Voice, +1 Part, +4 size)”
How is that spell built?
→ earth matter? then the individual is 10 cubic pace (Ars magica p152) of stuff. I’m not a pro in those units since we only use meters anyway (metric system is easier for us so we just put a “pace = meter and foot = 1/3m” rule) but i’m assuming the pace is like 3 feet. So that can’t be the case with a +4 size which seem rather more than what the spell do 10x10x100 feet.
→ stone matter: where is the +1 level to affect stone (Perdo guidelines text, p155 ArM5)? but the size modifier do match the spell description since for stone it’s “a single cubic pace” (Ars magica p152)
From my calculation that spell should be level 35 to assume the stone modifier.
My understanding is that the spell only works where the soil is deep, it stops when it hits solid rock.
As for loose stones and rocks in the target volume, it is unclear if they fall to the bottom of the well once the soil is removed, or if they are removed at the same time time as the soil, for being part of the soil.
I don’t think it is entirely clear where the distinction between coarse grained sand and small pebbles is particle size-wise.When does the pebbles in a volume of soil become distinct from plain soil, when using Terram?
Your calculations seem to be correct.
Considering the “it collapses swiftly after it is created” part of the spell, it is probably dirt/earth that it is supposed to affect.
It will work just fine as written, but as you note the size modifier is larger than needed if the spell creates a hole in dirt/earth. Which isn’t an error as such, but suboptimal. (Spells can do less than the guidelines and parameters allow.)
Edit:
Oh, and there is errata for it:
Sudden Well (p. 31): Change the level to PeTe 25, and change to +3 size in the calculation.