Wash and cut in squares one pound of any white fish; pare, cut in dice three medium-sized potatoes; chop fine one large onion; put in the bottom of a kettle a layer of the potatoes; then a layer of fish, then tomatoes; add a tablespoonful of onion, half a teaspoonful of powdered thyme, a saltspoonful of salt, a dash of pepper, half a teaspoonful of celery seed, and so continue until the materials are all used. Have the last layer potatoes. Pour over a pint of boiling water, cover the kettle and cook over a moderate fire, without stirring, for twenty minutes. Meantime heat a pint of milk in a double boiler; take the chowder from the fire, and cover the top with crushed water-crackers. Pour over the milk, add a tablespoonful of butter cut in bits, and serve.