For the filling, combine the sugar, cornstarch, and salt in a nonreactive saucepan. Gradually add the water and lemon juice, stirring to make a smooth mixture. Over medium-high heat, bring the mixture to a boil, stirring constantly. Boil 1 minute, still stirring, as it turns thick and clear, then remove from the heat. Pour half the hot- mixture into the yolks, stirring constantly, then pour the mixture back into the pan, stirring to keep the eggs from scrambling. Cook over low heat, stirring, a few minutes more. Stir in the butter and lemon zest. Pour the lemon mixture into the baked pie shell and smooth with a spatula. Preheat the oven to 350F. To make the meringue, beat the egg whites in a large clean bowl with the cream of tartar. When they make soft peaks, gradually add the sugar and continue until the meringue becomes stiff and glossy and the sugar crystals are dissolved (rub a little between your fingers to feel). Spoon the meringue over the lemon filling, making sure the edges are covered and sealed (otherwise the filling will bubble out). Smooth the meringue, then make peaks (or whatever design you wish) with the back of a spoon. Bake until the surface is lightly browned, about 12 minutes. Cool before serving; refrigerate if not serving right away. NOTES : This is the old American favorite without any gussying up. Recipe by: A Feast Of Fruits Posted to recipelu-digest Volume 01 Number 591 by "Marie Smith" on Jan 24, 1998