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Foot-warmers are metal tanks, about the size of a pillow, that warm the feet of passengers in cold weather.
They are naphthalene-filled, naphthalene having the characteristic that it melts at a comfortable temperature for feet. When cold, the naphthalene solidifies, while when heated it melts. The solidification of the warmer from the melted to solid state releases a lot of heat energy at a constant temperature.
The foot-warmer tank has a handle at one end, as it would be a little too hot to carry around otherwise.