The top surface of a lake is covered with a very thin coating of ice, which remains at the constant temperature of 10^oF. The water under it is at 32^oF. Assuming that the heat which is liberated when the water freezes flows up through the ice by conduction only and that the temperature variation across the ice layer is linear, find the time required to form a layer of ice one-inch thick.

Properties of ice: Density, = 57 lb/ft³

Latent heat of fusion, = 144 BTU/lb

Thermal conductivity, k = 1.5 BTU/h-ft-^oF