Sift the meals, flour, salt and baking powder together 3 times.
Add the soda to the molasses and sour cream, blending well. Add the water or milk.
Stir in the dry ingredients.
Fill generously buttered cans ¾ full. Butter inside lids and tie securely. Place on trivet or rack in a heavy kettle to which boiling water to a depth of 1" has been added. Cover kettle and steam for 3 hours. Add additional boiling water occasionally if necessary.