The · rate of evaporation from water surfaces varies with the temperature of the water, the \" . . elocity of the wind at the water surface, and the dryness of the air. Consequently, the rate of evaporation from rivers, lakes, canals, or resen·oirs varies widely in different localities and for the same locality in different seasons. The method adopted for measuring· the evaporation from a body of water consists in measuring the lo:ss of water from a pan which is so placed that the contained

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... r bas, as nearly as possible, the same temperature ~md exposure as that of the water which it is intended to represent. If this could be perf~ct1y accomplished, the observation would give directly the rate of evaporatit)n desired; but since the inclosing of the water in the pan tends to change its temperature and ex. posure, the observation is an indirect one. And in order to know how closely it represents the evaporation from the natural water surface, the observations must iuclude the water temperatures and the wind force from which the difference of temperature and exposure may be estimated. DESCRIPTION OF THE EVAPORATING PAN.