Book excerpt: An experimental study examines the transient natural convective heat transfer in a liquid filled horizontal cylinder with a constant uniform heat flux. The transient and quasi-steady state response of the local and averaged Nusselt number is correlated with a modified Rayleigh number. The transient response is mapped against the stretch time scale represented by the product of the Fourier modulus and modified Rayleigh number. Internal fluid temperature probe measurements and flow visualization reveal boundary layer type flow along the side walls with increased temperature fluctuations near the wall. Unstable mixing zones occur neat the top and bottom of the cylinder which may be three dimensional.