A cylindrical metal can, blackened on the outside, 0.10m high and 0.05m in diameter, contains liquid 4He at its normal boiling point of 4.22K, at which its heat of vaporization is 20.4 kJ jkg. Completely surrounding the helium can are walls maintained at the temperature of liquid nitrogen (77.35 K), and the intervening space is continuously evacuated to a very low pressure. How much helium is lost per hour?