Over medium heat, saute bacon in large Dutch oven til lightly brown. Drain off fat, leaving just enough to saute the onions and celery.
Add onion and celery to bacon, saute until onions are soft (about 5 min). Add clam juice, water and potatoes. Bring to a boil. Simmer over low heat until potatoes are tender (20-25 min). Add clams, stir in thyme and continue to simmer.
Heat the cream separately, almost to boiling, then pour it slowly into the clam mixture. Add salt and pepper to taste and stir in butter. Sprinkle with parsley before serving.