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The Food and Agriculture Organization of the United Nations would like to express sincere thanks to the technical author, Mr. Peter L. Fraenkel, and to the editor, Mr. Neil Burton, Intermediate Technology Group, United Kingdom, for the preparation of the publication.

The publishers would also like to thank the following especially for the use of material from which illustrations have been drawn:

B. Y. S. Napel; Grundfos; Khan, H. R. and the Institute of Civil Engineers, London; Lysen, E. and C W D, The Netherlands; National Aero­nautical Laboratory, Bangalore, India; Smithsonian Institute; and the UN Economic and Social Commission for Asia and the Pacific.