The present volume on Mechanical Flight consists, as the title-page indicates, of two parts. The first, dealing with the long and notable series of early experiments with small models, was written almost entirely by Secretary Langley with the assistance of Mr. E. C. Huffaker and Mr. G. L. Fowler in 1897. Such chapters as were not complete have been finished by the writer and are easily noted as they are written in the third person. It has been subjected only to such revision as it would have received had Mr. Langley lived to supervise this publication, and has therefore the highest value as an historical record.