A sugar thermometer is essential for this kind of icing.
Put the water in a saucepan, rinsing it up the sides. Add the sugar and cream of tartar and dissolve over low heat. When dissolved bring to boiling point (240° F.) and boil without stirring for 3 minutes or until a thread forms from a spoon (255–260° F.). Leave to cool slightly and beat the egg white stiffly. Pour the sugar syrup in a thin, slow stream over the egg white. Beat until it starts to thicken. Pour quickly over cake.
©1975 The Estate of Elizabeth Ayrton