This paper describes an experimental study of transient natural convection heating and cooling of a 30 cm diameter horizontal cylinder filled with water or water-glycerol solutions. The Rayleigh number range covered by these experiments is 8 × 107 -3 × 109. The flow regime is turbulent. The overall Nusselt numbers measured when the cylinder is filled only partially with water agree with the other measurements and low-Ra correlations, provided the average heat flux is based on the wetted portion of the cylinder surface. Qualitative descriptions of the turbulent circulation during heating experiments are also reported. © 1988.