From 1890 Abel

What Abel describes as a steamer we today call a double boiler.

A double boiler is used to slowly cook or melt foods. The lower 2/3 of a small pot with a lid fits inside a larger pot that holds boiling water. The food to be cooked was placed inside the smaller pot. This structure keeps the cooking food away from direct heat.

The heat comes from the steam of the boiling water, and food cooks more slowly and is less apt to stick to the pot.