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Journal of Experimental Biology 41,771-781 (1964)
Published by Company of Biologists 1964

The Evaporation of Water from Helix Aspersa : II. Measurement of Air Flow and the Diffusion of Water Vapour

JOHN MACHIN ¹

¹ Department of Zoology, Queen Mary College, University of London; Department of Zoology, University of Toronto, Toronto 5, Canada.

1. Air flow and the water-vapour gradient over a freely evaporating surface is described, and the concept of boundary-layer thickness which expresses such a gradient is introduced.

2. A direct method of estimating boundary-layer thickness by means of a hot-wire anemometer is described.

3. Comparison of observed and effective boundary-layer thickness, calculated from evaporation data, have suggested that the gradient of water vapour is steeper than aerodynamic measurements indicate.

4. An empirical relationship between aerodynamic and vapour boundary layers has been obtained for a two-dimensional evaporator and for the snail, Helix aspersa, and a new evaporation formula derived.

5. Evidence for supposing that aerodynamic and vapour boundary layers do not coincide unless the evaporating surface is greater than 22 cm. In length is presented.

6. Possible errors in the estimation of boundary-layer thickness when air flow is interrupted by the snail's shell are considered.

7. The importance of differences in air flow produced by placing the animal in different positions relative to the wind is discussed.