I want to work out how long it takes for them to cover the earth given travel at around 60 mph, random paths, and doubling in numbers every few seconds. My feeling is that this ends up working like the destruction of Jupiter in 2010: Odyssey 2 - you get so many that the mass collapses as a black hole - but my maths really isn't good enough; do they spread out and blanket the world first, or does the implosion happen while they're still over the UK? Obviously it depends on things like their mass (20 to 150lb depending on your source), replication rate, etc., and how many times they double before replication errors creep in. Let's assume that it's Voldemort casting the spell and aiming to maximise muggle casualties, so spelled to target cars, trucks, trains, aircraft etc.
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