XXVII. Note on the distributions of temperature and vapour pressure around a horizontal wet cylinder

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Abstract
Experiments are described in which unventilated thermocouple psychrometers are used to determine the temperature and vapour pressure distributions above, below, and to one side of the wet surface of a horizontal cylinder. The presence of a convection current passing over the surface in a downwards direction is revealed when the surface is cooled to a temperature well below that of the air. When the surface temperature is raised, a temperature is reached at which very little movement of the air occurs. Under the conditions of the present experiment this occurred when the surface temperature was just over 1° C. below the temperature of the surroundings. With further increase in surface temperature the convection currents change to an upward direction. It is shown that the rate of evaporation falls to a minimum value in the region of little air-movement, and, on the assumption that the whole of the moisture loss takes placeby diffusion, the experimental data for the vapour pressure gradients near the wet surface are seen to lead to values of the correct order of magnitude for the coefficient of diffusion of water vapour into air.

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