There are available in the literature many data for the transfer of heat from a tube to an enclosed fluid flowing with steady, unidirectional motion. Knowledge with respect to the applicability of such steady flow data to the analysis of heat transfer from a pipe to a fluid, the velocity of which is a periodic function of time, is necessary for the solution of certain types of heat-transfer problems. An outstanding example of a system in which such periodic heat transfer occurs is the internal-combustion engine under detonating and nondetonating conditions. The experiments (1) described in this paper are the beginning of a long-range program on the subject of heat transfer in the cylinders of internal-combustion engines and, although the results of the tests are not immediately applicable to engine analysis, some light is shed on the phenomenon.