Nearly two years ago the writer received an invitation from the American Society of Civil Engineers to submit a paper upon Evaporation at its last annual convention. Having already experimented upon the subject for several years and accumula.ted a certain mass of data, the invitation was accepted, under the supposition that it would not prove a very difficult task. On reviewing the whole subject, however, it became evident that a more extensive series of experiments would have to be entered upon than any previously undertaken, to throw light upon certain points which will appear in the course of this paper. These were at once begun and have been prosecuted diligently ever since. The theoretical side of the question was surrounded with so many difficulties, that the writer found it necessary to experiment in many branches of meteorological work in order to arrive at correct conclusions. The larger part of these experiments being more in the nature of preparatory work, are not. alluded to at all in this paper.