Although there has been a number of experimental determinations of the thermal conductivity of air, its value even to two figures is still uncertain. It is true that the values hitherto obtained are of the same order as that deduced from the kinetic theory by Clausius, Maxwell, and others, but they differ among themselves to the extent of 20 per cent. There have been only two methods of experiment. Stefan, Kundt and Warburg, and Winkelmann observed the rate of cooling of a thermometer in an envelope of rarefied air, the outer surface of which was kept at a constant temperature. Schleier-macher and Schwarze measured the rate at which heat passed from an electrically heated wire stretched along the axis to the surface of a cylinder maintained at a lower constant temperature. Repetitions of these two methods have not cleared away the uncertainty as to what is the correct value of the thermal conductivity of air. Under these circumstances it might be interesting to give an account of another method for determining this quantity. It will be noticed that in this method, which was suggested by Prof. Poynting, the conductivity is measured at ordinary pressures, there being no need to take precautions against convection currents by the reduction of the pressure as is necessary in the other methods.