Aid in writing this volume is acknowledged in the course of its chapters. The author’s grateful thanks are rendered also to Dr. L. A. Fischer, of the Bureau of Standards at Washington, who has revised the paragraphs describing the work of the Bureau; to Mr. C. R. Mann of the Ryerson Physical Laboratory, University of Chicago, who corrected the paragraphs on the interferometer; to Mr. Walter A. Mitchell, formerly of Columbia University, New York, who revised most of the chapters on measurement. Mr. Thomas E. Fant, Head of the Department of Construction and Repair at the Navy Yard, Washington, D. C., gave the picture of the model basin here reproduced. Mr. Walter Hough of the National Museum, Washington, D. C., contributed a photograph of the Pomo basket also reproduced here. Mr. John Van Vleck and Mr. Henry G. Stott of New York, Mr. George R. Prowse and Mr. Edson L. Pease of Montreal, have furnished drawings and photographs for illustrations of unusual interest. Mr. George F. C. Smillie, of the Bureau of Engraving, Washington, D. C., Mr. Percival E. Fansler, Mr. Ernest Ingersoll, and Mr. Ashley P. Peck, of New York, have read in proof parts of the chapters which follow. Their corrections and suggestions have been indispensable. Professor Bradley Stoughton, of the School of Mines, Columbia University, New York, has been good enough to contribute a brief list of books on steel, supplementing the chapter on that theme written with his revision. Had it been feasible, other chapters would have been supplemented in like manner by other teachers of mark. In 1902 the American Library Association published an annotated guide to the literature of American history, engaging forty critics and scholars of distinction, with Mr. J. N. Larned as editor. It is hoped that at no distant day guides on the same helpful plan will be issued in the field of science, duly supplemented and revised from time to time. * * * * * In the present volume the author has endeavored to include in his survey the main facts to the close of May, 1906. NEW YORK, September, 1906. INVENTORS AT WORK CHAPTER I INTRODUCTORY