Beat egg whites until foamy. Add salt and cream of tartar and beat until whites stand in peaks. Beat in cup sugar gradually. Beat egg yolks with remaining cup sugar, water, vanilla, lemon juice and rind until thick and light colored. Fold the yolk mixture into the egg whites and gradually fold in flour. Pour into 2 ungreased 9-inch round layer cake pans. Bake in slow oven (325 F.) 30 to 35 minutes. Cool on rack before removing pans. Put one layer of cake on a plate, cover with sherbet, top with a second layer of cake and garnish with sweetened whipped cream. Yield: 10-12 servings.