1. The construction and use of a wind-tunnel apparatus is described in which measurements of evaporation under controlled conditions of temperature, humidity and air flow can be made.

2. Two mathematical formulae, applicable to evaporation in relatively low wind speeds, are described. It is suggested that a promising approach to evaporation from moist-skinned animals is provided by the application of Leighly's formula:

E = K(p0-pd)c(v/x)n,

where the rate of evaporation (E) is expressed in terms of the vapour pressure at the evaporating surface (p0) and in the ambient air (pd), the wind speed (v) and the length of the evaporating surface parallel to the wind (x). The constant, K, is calculated independently and the terms n and c are left for empirical determination.

3. Values of n and c for different types of evaporating surface are given together with the method used in their calculation. Those relating to flat evaporators and to the snail, Helix aspersa, are shown to differ significantly.

4. In general n increases and c decreases as the amount of air disturbance caused by the snail increases.

5. The fact that n for flat surfaces is in good agreement with previously established theory is taken as evidence that Leighly's formula may be validly applied.

6. The combined determination of n and c is introduced as a convenient assessment of a complex form in terms of air flow and evaporation.

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