[Note 8: This exposition of the mechanical principle of tone-production is intended to be graphic, rather than strictly technical. For the sake of simplicity, that portion of the expiratory energy expended in friction against the throat walls, tongue, cheeks, etc., is disregarded, as well as that expended in propelling the air out of the mouth, in displacing the same quantity of external air, etc.] The mechanical operation of tone-production comprises the following transformations of energy: First, the energy exerted in the contraction of the expiratory muscles is converted into energy of condensation or elasticity of the air in the lungs and trachea. Second, this energy of condensation of the air is converted into energy of motion of the vocal cords. In other words, the expiratory energy is transformed into energy of motion. One objection, at first sight very serious, may be offered against this statement: the amount of strength exerted in the contractions of the breath muscles seems many times greater than is accounted for in the motion of the vocal cords. The movements of the vocal cords are so slight as to be observable only with the aid of a specially devised apparatus, the stroboscope. Can all the expiratory force expended in tone-production show such a small result? This apparent objection is found to be groundless in view of the application in this operation of Pascal's law. As this topic was fully treated in Chapter II of Part II, no further explanation is required here. The erroneous idea of vocal mechanics involved in the doctrine of breath-control is now fully exposed. Tone can be produced only when the expired air exerts a pressure on the vocal cords. There is no necessity for any conscious or voluntary check on the expiration. The energy of the expiration is expended in setting the vocal cords in motion. No energy of condensation is left in the expired air the instant it has passed the vocal cords. Beyond that point there is no expiratory pressure. In one sense it is true that the expiration is "controlled" in tone-production. But this control is strictly an automatic action. The vocal cords are adjusted, by the appropriate muscular contractions, to move in response to the air pressure exerted against them. This action involves, as a necessary consequence, the holding back by the vocal cords of the out-rushing air. So long as the vocal cords remain in the position for producing tone, they also control the expiration. In this sense breath-control is an inseparable feature of tone-production. All that need be known of the mechanics of the voice is therefore perfectly plain. The vocal cords are set in motion by the pressure against them of the expired breath. This operation is in accordance with Pascal's law and the law of the conservation of energy.