1. In a heavy 5- to 6-quart casserole, melt the butter over moderate heat. When the foam begins to subside, add the onions and celery and, stirring frequently, cook for about 5 minutes, until the vegetables are soft but not brown.
2. Stir in the peas and water, then bring to a boil over high heat, meanwhile skimming off the foam and scum that rise to the surface. Add the potatoes, marjoram, 2 teaspoons of the salt and the black pepper, reduce the heat to low, and simmer partially covered for 1 hour.
3. Meanwhile, combine the ground pork, sage, the remaining 1½ teaspoons of salt and the white pepper in a bowl. Knead vigorously with both hands, then beat with a wooden spoon until the mixture is smooth.
4. Moistening your hands in cold water occasionally, pinch off about 1 tablespoon of the pork mixture at a time and shape each piece into a ball about 1 inch in diameter.
5. When the soup has cooked its allotted time, drop in the pork balls and return the soup to a simmer. Cover the pot partially and continue to simmer for 30 minutes.
6. To test the pork for doneness, lift one of the balls out of the water with a slotted spoon and pierce it deeply with the point of a small sharp knife.
7. If the liquid that trickles out is clear yellow, the pork is done even though the meat itself appears somewhat pink; however, if the liquid is pink, simmer the soup for 5 to 10 minutes longer. Taste for seasoning and serve the soup at once.
