This thesis presents the results of an experimental study of the heat-transfer ability of a finned surface as a function of the spacing between fins. The results are expressed as heat transfer per unit length of finned section rather than the heat transfer per fin. Circumferential steel fins, seven and three-quarter inches in diameter, were used for the investigation. Data was taken for fin spacings ranging from zero to one and one-half inches. The experimental heat-transfer rate to atmospheric air was determined by measuring the volume of saturated steam which condensed in the test section over a measured time interval. Temperature readings were also taken on the outer radius of the fin surface using a radiation pyrometer. An analytical study of the overall heat transfer per unit length of the test section was broken down into four parts. These were heat transfer from 1) the outer edge of the fins, 2) the pipe through the middle of the fins, 3) lateral fin surfaces by radiation, and 4) the lateral fin surfaces by convection. The values for the first three parts were calculated from theoretical equations and the difference between the sum of the first three parts and the total measured heat transfer resulted in an evaluation of the convection heat transfer from the lateral fin surfaces. The results of the experiment show the optimum fin spacing for this particular test section to be five-sixteenths of an inch. The shape of the curve showing the heat-transfer rate per unit length of finned section as a function of fin spacing shows that there is a sharp maximum and that the value of fin spacing is very critical in order to obtain the maximum heat transfer from the test section --Abstract, page ii-iii