An observer on the street, looking at a military band, notices certain movements of one member of the organization which result in what he termed the sound of the drum; but a deaf man by his side, though he sees the movements, hears nothing. This, being analyzed, means that the movements of the drummer's arm, conveyed through the drumstick to the membrane of the drum, give rise to movements in it which set up corresponding movements of the air within the drum, which again cause movements of the body of the instrument, the whole causing movements of the external air; and here the purely physical process ends. The movements other than muscular ones are not readily observed, but experiments not only prove that they exist, but demonstrate their nature, even to their exact rate of occurrence, their size, etc. These movements are termed _vibrations_, and, as has been indicated previously, they are the sole physical cause of sound. But that the latter is not due wholly to a physical origin is evident from the fact that sound for the deaf does not exist. It must, therefore, be a personal, a subjective experience, and as the sleeping, unconscious person does not necessarily hear a sound, the process is not wholly a corporeal or physiological process; it is finally an experience of the mind, the consciousness, and so is psychological as well as physiological. The fact that sound has a physical basis in the vibrations of bodies, either solid, liquid, or gaseous, may be brought home to one in various ways. Concussion or shaking of some kind is essential to start these vibrations. The air is made up of its particles, and one being moved sets up, inevitably, movements in neighboring particles on all sides, hence vibrations travel in all directions; which explains why a sound in the street may be heard by those in every part of the street not too distant, and also in the upper rooms of the houses and below in the basements. This is an important fact for the singer or speaker to bear in mind. His purpose must be to set up vibrations that will travel with great perfection and rapidity in all directions. The following experiments of a simple kind will serve to convince those who may not have given much attention to the subject that sound is due to movements of some object, which we term the sounding body, strictly that which starts the vibrations by its own movements or vibrations.