For the river, I just meant temporary dams at either end to keep from losing what's in it; once there's more world it can be linked into a longer watercourse or whatever. For the gas bag, as long as it's not intersecting something solid anywhere should be fine, it's just a test before expending the effort to make one big enough for the whole thing. I still don't have a good way to put it in 1940s-IC terms, but basically what I'm after is a force field or Maxwell's Demon to keep the air from just expanding away into the nothing around us, at best something like the usual concept of a dimensional wall or a classical Crystal Sphere to be the outer edge of our new pocket universe. That would require being expandable or at least permeable to non-gasses and gas contained in something else, the permeable version would make for a rather odd sort of world if it couldn't be removed later, though. In any case, the analogy of a zeppelin cell was just approaching the idea IC.
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"Anko, what you do in your free time is your own choice. Use it wisely. And if you do not use it wisely, make sure you thoroughly enjoy whatever unwise thing you are doing." - HymnOfRagnorok as Orochimaru at SpaceBattles
woot Med. Eng., verb, 1st & 3rd pers. prsnt. sg. know, knows
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"Anko, what you do in your free time is your own choice. Use it wisely. And if you do not use it wisely, make sure you thoroughly enjoy whatever unwise thing you are doing." - HymnOfRagnorok as Orochimaru at SpaceBattles
woot Med. Eng., verb, 1st & 3rd pers. prsnt. sg. know, knows