Self-playing musical instruments have been known for more than forty years, but it is within the past twenty-five years that devices have been invented for controlling tones by pneumatic or electrical appliances to produce expressions. Examples of the later of these three kinds of musical instruments may be found in the United States patents of Zimmermann in 1882, Tanaka, 1890, and Gally, 1879. The science of _acoustics_ and its practical applications have greatly advanced, chiefly due to the researches of Helmholtz, referred to above. When the nature and laws of the waves of sound became fully known a great field of inventions was opened. Then came the telephone, phonograph, graphophone and gramophone. The telephone depends upon a combination of electricity and the waves of the human voice. The phonograph and its modifications depend alone on sound waves--the recording of the waves from one vibrating membrane and their exact reproduction on another vibrating membrane. The acoustic properties of churches and other buildings were improved by the adaptation of banks of fine wires to prevent the re-echoing of sounds. _Auricular tubes_ adapted to be applied to the ears and concealed by the hair, and other forms of aural instruments, were devised. The _Megaphone_ of Edison appeared, consisting of two large funnels having elastic conducting tubes from their apices to the aural orifice. Conversation in moderate tones has been heard and understood by their use at a distance of one and a half miles. The megaphone has been found very useful in speaking to large outdoor crowds.