In a saucepan combine the sugar and cornstarch. Add the water and cook over medium heat, stirring consistency until thickened. Bring the mixture to a boil, and boil for 1 minute. Remove the pan from the heat. In a medium bowl beat the egg yolks lightly. Pour a small amount of the hot liquid into the egg yolks, stirring constantly. Return the yolk mixture to the saucepan, stirring constantly. Bring the mixture to a boil, and boil until thickened. Blend in the butter, lemon juice, and lemon rind. Pour the filling into the pie shell. In a large bowl beat the egg whites with the cream of tartar and 6 tablespoons of sugar until stiff. Spread the meringue over the pie filling, making sure the meringue touches the edges of the pie. Bake at 400 F. for 8 minutes, or until the meringiue is light brown. Let the pie cool before serving.