In a kettle cover the veal knuckles with 12 cups of the water, bring the water to a boil, and skim the froth. Add the remaining 1/2 cup water, bring the stock to a simmer, and skim any froth. Add the onion, the leeks, the carrot, the celery, the salt, and the cheesecloth bag and simmer the stock, skimming the froth, for 4 hours. Add the giblets and simmer the stock, skimming the froth and adding boiling water if necessary to keep the ingredients barely covered, for 2 hours more. Strain the stock through a fine sieve set over a bowl, pressing hard on the solids, and let it cool. Chill the stock and remove the fat. The stock may be frozen. Makes about 6 cups. SOUP 29