1. The physical theory of evaporation in still air and in moving air is outlined.

2. An apparatus is described in which controlled conditions of temperature, humidity and wind velocity are obtained.

3. The effects of these three factors upon the rate of evaporation from simple physical systems have been investigated.

4. Sources of error attaching to experiments carried out in still air are enumerated.

5. The law of saturation deficiency is shown to be theoretically untrue.

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