From an old tea merchant in London I received my instructions for making a perfect cup of tea.
First rinse out the teapot with cold water and then fill it with boiling water, and let stand while you bring the water intended for the tea to a boil. Just before the water boils, turn out the water in the teapot and wipe dry. Then add the tea leaves and pour on the freshly boiled water. Cover the pot with a tea cosy or wrap in a towel and let stand exactly seven minutes.
The tea is now ready to drink. This will give you a delicious drink of ambrosia that will delight the heart of true lovers of a good cup of tea.
The use of a cosy for the teapot is to hold the heat in the pot and thus prevent quick cooling. Use one level teaspoon of tea to each one-half pint of water. Measure the water before boiling. The water must be poured on the tea immediately upon reaching the boiling point. After boiling for two minutes or longer the water quickly loses its natural gases.
Mrs. Wilson's Cook Book (1920).