Group +5 orders of magnitude is 1 million. 2000 paces across is over 3 million. I'm not seeing how you get 200 javelins per square pace rather than 1/3. If we halve the diameter and stick on another size magnitude we can get a dozen javelins per man and two dozen per horse.
Remember that Group gives you the equivalent of 10 base Individuals, not 10 items!
Actually, I was quite conservative. I wrote 2000, when I should have written about 5000. That's an area of less than 250025003.2=20 000 000 square paces -- less than 19.7 million if you use a slightly better approximation for pi.
A pace is 3 feet, i.e. 36 inches. A cubic pace (a base Individual of Herbam) is then 363636 cubic inches.
A javelin has a radius of 1 quarter of an inch. The area of its section is then less than 3.2/16 = 0.2 square inches. Multiplied by a length of 5 paces = 60 inches, we get a volume of slightly less than 12 cubic inches. Then, with a base individual of Herbam, you get slightly more than 363636/12=3888 javelins - in fact, more than 3960 with a slightly better approximation for pi. With 1 million base Individuals, you are actually creating more than 3.96 billion javelins. Spread over 19.7 million square paces, you are getting over 200 javelins per square pace.
This sounds so ridiculous 
But the numbers are correct.
Quantity has it's own quality.
Though I'ld give it diameter concentration to allow for the javelins to fall from a great enough height to do any real damage.
Actually, Momentary duration is up to a round (check e.g. Crest of the Earth Wave), enough for a javelin to fall up to two hundred paces. And a javelin (or a slingstone for that matter) - dropped even a few dozen paces strikes stronger than when thrown by a strong man.
Though I've never been quite sure exactly how long a a round is I've always viewed most spells resolving in a relatively small fraction of that time. Since they are now resolved as part of combat and not at the end of it. After all a formulaic spell might be cast and resolved before anyone else can act. Even if it isn't fast cast.
Crest of the earth wave I think is a special case because the movement speed of the spell is specified so it might not actually reach it's targets till after they have had a chance to act. In fact since the speed of the wave is listed as 50 paces per round a reader might infer this momentary effect lasts even longer then one round making it a very special case.
How long the spell lasts is open to interpretation either way. Although shafts .5 inches wide and 5 feet long would not be as dangerous as you think. Straight thin wooden shafts have a relatively low terminal velocity and tend to absorb the force of impact by bending and deflecting rather than penetrating. I would suggest the javalins be a little shorter and fatter with a pointed teardrop shape. The center of mass somewhere between a quarter and a third of the way up from the bottom. This would improve their drag coefficient and limit deflection.
Hermetic Magic cannot affect a target it can't detect , even if the mage knows it is there.
You would need Arcane Connection to affect liquid in sealed or non-transparent containers.
I like the javelin rain 
For a less blatant use of magic, make everything made of plant products (or regular plants) in a certain area venomous to the touch. Quite crippling and you do not have the church blattering about the heathens raining death on the rightful cristians.
For blatant use, simply attacking the army with an awakened forest of murderous trees might be enough as well.
Cheers,
Xavi
Or do the same spell as CrTe and make them Iron.
But we're trying to do it with Herbam. We have Terram covered with plenty of variants on the theme of mass burial. Anyway, for a Terram rain of death a rain of fist-head sized flints is more efficient than iron javelins by 3 magniudes - stone rather than metal, natural form rather than manufactured and, the more massive base size of stone compared to metal.
Why can I not help but think of a version of this called "the call of Bacchus".
Oh i LIKE that one.
200 per sq pace might be overdoing it though? Thats roughly 14*14, simplified its about one every 6-7cm...
This one gets even nastier if you make it CrTe and let it be metal javelins, yeah the area size is reduced but even strong armour and the best of shields wont work well against it.
Yup, can do it with water as well. Or still CrAq but make it all ICE. :mrgreen:
"loosing control" does not equate spasm. It means inability to control, its like cutting off nerves, not like having them go raving. Yes the dance part IS cosmetic, but the spasm part is NOT.
And riding, well i can do that for 8 hours straight and still walk away in better shape than the horse, and if you get badly tired by sitting on a trotting horse for just a half hour, then you´re not sitting well enough.
Hmm, what would the Base be on that one?
And i think your size calculation may be way off. And i think contact poison would raise magnitude a bit extra.
Overall though, i like the spell concept.
True, but even if troops have good armour you will still probably cause at least 1+ light wounds on the vast majority of people in the targeted area. So against well armoured opponents it may not be a "kill all" spell, but it would certainly cut them up enough to no longer be an effective fighting force.
And against UNarmoured foes, its a slaughter.
Buggers... I KNEW there was something else about it i was missing!
Anyways, lots of interesting spell versions!
My first instinct is to totally disagree. I believe that with a spell affecting an area (Room, Circle, Structure, Boundary), every legitimate target within the area is affected, regardless of whether it's in a sealed container or not. (What would it even mean for Hermetic Magic to detect a target? Are there little Gremlins of Hermes that go around ticking boxes on magic clipboards based on what they see?)
Do you have any RAW evidence that supports your interpretation?
Consider the following: Wind of Mundane Silence destroys all effects in a room, detected or not. Rooster's Clow (Bjornaer Target: Sound version of Demon's Eternal Oblivion) effects all demon's who hear the call, whether the magus hears them or not.
If the target is Room, then the magus must be able to detect the room, not everything within it that the spell may or may not effect.
However, if the target is Group (+ multiple size), then the target is not "all within a certain area" but "X number of discrete items, defined by the caster." The magus must be able to actively detect the group, and so must be able to detect the water he would want to change in some way.
I have to agree to the objection: this spell does not work unless an intellego spell is first used to detect all water within range: sight.
A version using Columbae Ring Magic would work, however.
I'm trying to formulate a bloodthirsty berserker bramble spell wherein the caster creates one big, extra vicious bramble whose canes whip about seeking victims and every time they wound a creature they take root in that creature doing ongoing, increasing wounds and each such rooting acts as a new centre of explosive growth. As long as there is a sufficient density of victims there is an onrushing wave of thorns faster than a man can run. Once the plant has run out of blood, growth slows right down so it just looks like a giant, bone filled bramble bank unless you step too close.
I don't know how to cost that effect tho'.
I've always been partial to removing the armies's tools. I once had this enchanted into a Bow on a smaller level, with 24 uses per day, but was able to scale it up to fit the 10,000 man limit. This was mainly for any medieval army decked out in chainmail, scale, plate, etc. with swords and shields. Both spells are also pretty much right out of the book, modified for size.
Field of Rusted Decay Lvl 40 (version of Rusted Decay of the Ten-Score Years, ArM5 p155)
Range: Sight Duration: Momentary Target: Group
The metal within the Boundary becomes brittle and rusted and will fall apart upon any movement or stress. Everything also loses its sharpness/edge.
(Base 4, +3 Sight, +2 Group, +3 Size)
Similarly, if you're facing a field of archers or engineers with portable bridges, trebuchets, etc.
Field of Rotted Wood Lvl 40 (version of Curse of the Rotted Wood, ArM5 p137)
Range: Sight Duration: Momentary Target: Group
Non-living wooden objects in the boudaries rot and fall apart. The wood slows crumbles apart, depending on how large the item is.
(Base 4, +3 Sight, +2 Group, +3 Size)
Hmmm... I guess an army of barbarians in nothing but leather or animal skins wielding bone and stone axes would get around the above two spells. I think the PeAn version has to be Lvl 45 since it's a slightly higher base for processed goods.
Boundary target makes both of these rituals...
dang it - knew I was forgetting something in my Serf's Parma. Editting the original post so it reflects the Group + Size idea and as Lvl 40 formulaic spells then.
I'm sure I've seen the creation of ice with Creo Aquam somewhere, but can't find a single example. Could you give me a pointer?
Actually, "Spasms of the uncontrolled hand" does exactly that, with the same guideline.
Well, I I'm quite amazed that after 8 hours straight trotting either you or the horse are able to walk at all That's certainly a very impressive feat, for both.
And no, I do not get badly tired sitting on a trotting horse for half an hour, but I do feel some fatigue - in Ars Magica terms, I've probably lost at least one fatigue level. I'd say trotting is about as fatiguing for a rider as walking uphill (on a fairly steep hill), or climbing stairs -- and not at a leisurely pace. For a horse, it seems about as jogging for a human. Do it for 8 hours? It can be done, but it's no small feat.
But the level 40 versions are formulaic.
Bloodthirsty Berserker Bramble Cr(MuRe)He 35
R: touch, D: diameter, T: individual
This spell creates one big (many stems up to ten paces long), extra vicious bramble whose canes whip about seeking victims and every time they wound a creature (damage +15) they take root in that creature doing ongoing, increasing wounds (ie a light wound in the first round after being caught, then a medium wound next round etc.) and each such rooting acts as a new centre of explosive growth (40 paces per round). As long as there is a sufficient density of victims there is an onrushing wave of thorns faster than a man can run. Once the plant has run out of blood, growth slows right down so it just looks like a giant, bone filled bramble bank unless you step too close. The plant does not attack the caster but the caster may well find herself surrounded by thorns.
Sister Mahult ex Jerbiton's version grows luscious blackberries - it's not her sigil, she just likes blackeberries.
Base 1 (make plant), +1 touch, +1 diameter, +1 muto (for bloodthirstiness and sawing through mail), +1 rego (hunt everything except the caster) +4 size, +1 (rapid ongoing growth)