Preheat oven to 325°F, and lightly grease a 10-inch tube pan.
Measure the flour, sugar, baking powder, and salt in a large bowl, and toss together to combine thoroughly.
Separate the seven eggs, adding the yolks to a small mixing bowl, and the whites to a larger one.
Add water, oil, lemon juice and zest, and vanilla to the eggs yolks in the smaller bowl. Beat together until the mixture is smooth.
Add the egg yolk mixture to the flour mixture, and beat the batter until it is well combined.
Add the cream of tartar to the egg whites. Beat the egg whites with a mixer or a whisk until they are stiff, but not dry.
Fold the egg whites into the batter gently by adding a quarter of it at a time.
Spoon the batter gently into the prepared tube pan. Using a clean butter knife, run through the middle of the cake all the way around to remove any large air pockets.
Bake on the lowest rack of your oven for about 50 to 55 minutes. The top of the cake should spring back slightly when you tap it.
When it is done, remove the pan from the oven, and invert it immediately and place on a rack – but do not remove from the pan. Allow it to cool for about 1 hour.
Once cooled, run a sharp knife around the sides and center of the pan to release the cake.