Several persons have assisted in various ways in the preparation of this book. The author takes pleasure in acknowledging such service, and in extending to each one his appreciation. Mr. Roger B. Saylor, of the Barringer High School of Newark, N. J read all the manuscript and offered many helpful suggestions. The remarkable photo graphs Oi sound-waves were furnished by Professor Arthur L. Foley of Indiana University. Mr. Carl 0. Voegelin, of the Central High School of Newark, N. J read all the galley proofs and furnished many helpful suggestions. The photo graphs illustrating vibrations produced by the singing voice were taken by Dr. Dayton C. Miller of the Case School of Applied Science. Generous assistance in reading galley proofs was given by Mr. Silas A. Lottridge of East Orange High School. The photograph Of the first power-driven aeroplane was supplied by Mr. Orville Wright. The au thor is indebted to Mr. Walter White, his associate in the South Side High School, for suggestions pertaining to the mathematical side of physics, and to Mr. Carl J. Hunkins of the same school for helpful criticism, particularly in the later chapters.