Mix the sugar, cream of tartar, and baking powder in a small pan.
Add the water and stir until it boils.
Beat the egg white and salt until stiff and beat in the sugar syrup a spoonful at a time.
When half has been added, add the lemon juice and the rest of the syrup.
Put the basin over boiling water and beat hard until it reaches the desired consistency.
If it becomes too thick add a little boiling water and continue to beat over the hot water.
Unlike Royal Icing, this does not set hard.