The paper analyses heat flow by natural convection from a fin of uniform conductivity and cross-sectional area, fixed at one end to a hot wall and immersed in a cooler fluid. A resistance network analogue is used in conjunction with a new type of iterative analogue computer to determine the temperature distributions and fin efficiencies for a variety of fin proportions, on the assumption that heat loss is proportional to the local temperature to the power 5/4. Results are compared with the case of linear heat loss.