Pour about 1/2 inch of water into the bottom of sauce pan or double boiler and bring to a boil over medium heat. Make sure that when the top portion of the double boiler is added that the water will not touch the bottom of the bowl.
In a large heatproof bowl, off the heat, combine sugar, cream of tartar, salt, water, and egg whites. Using a hand mixer beat at medium speed for 1 minute. Place over the saucepan of boiling water. Using the hand mixer beat on high speed for 7 minutes. Remove from heat, and fold in vanilla.