Mix sugar, corn syrup, water and salt in saucepan.Heat slowly until sugar is dissolved:then boil gently, without stirring to 240 deg.F. on a candy thermometer (firm ball stage) . While syrup is boiling, beat egg whites until stiff. Beating constantly, slowly pour about 1/3 of the hot syrup sover egg whites. Put remaining syrup back to cook to 265 deg.F. (hard ball stage) Gradually beat this syrup into egg whites: Add vanilla and continue beating until mixture will hold its shape. ( If weather is damp and candy refuses to hold its shape, beat in one teaspoon of powdered sugar at this stage.) Add walnuts. Drop from teaspoon onto waxed paper; or pour into a buttered 9-inch square pan and cut into squares when cool. Makes about `1 1/2 pounds of candy.