In the centre of _a b_, Fig. 26, is a depression _x_ sunk in steel, smoothly polished and intended to receive the end _p′_ of the axis. I place the end _p′_ in this depression, and, holding the axis upright, bring down upon its upper end _p_ a steel cap, finely polished within, which holds the axis at the top, the pressure both at top and bottom being so gentle, and the polish of the touching surfaces so perfect, that the disk can rotate with an exceedingly small amount of friction. At _c_, Fig. 27, is the cap which fits on to the upper end of the axis _p p′_. In this figure the disk _d e_ is shown covering the top of the cylinder C. You may neglect for the present the wheel-work of the figure. Turning the disk _d e_ slowly round, its perforations may be caused to coincide or not coincide with those of the cylinder underneath. As the disk turns, its orifices come alternately over the perforations of the cylinder and over the spaces between the perforations. Hence it is plain that if air were urged into C, and if the disk could be caused to rotate at the same time, we should accomplish our object, and carve into puffs the streams of air. In this beautiful instrument the disk is caused to rotate by the very air currents which it renders intermittent. This is done by the simple device of causing the perforations to pass _obliquely_ through the top of the cylinder C, and also obliquely, but oppositely inclined, through the rotating disk _d e_. The air is thus caused to issue from C, not vertically, but in side currents, which impinge against the disk and drive it round. In this way, by its passage through the siren, the air is molded into sonorous waves. Another moment will make you acquainted with the recording portion of the instrument. At the upper part of the steel axis _p p′_, Fig. 27, is a screw _s_, working into a pair of toothed wheels (seen when the back of the instrument is turned toward you). As the disk and its axis turn, these wheels rotate. In front you simply see two graduated dials, Fig. 28, each furnished with an index like the hand of a clock. These indexes record the number of revolutions executed by the disk in any given time. By pushing the button _a_ or _b_ the wheel-work is thrown into or out of action, thus starting or suspending, in a moment, the process of recording. Finally, by the pins _m_, _n_, _o_, _p_, Fig. 27, any series of orifices in the top of the cylinder C can be opened or closed at pleasure. By pressing _m_, one series is opened; by pressing _n_, another. By pressing two keys, two series of orifices are opened; by pressing three keys, three series; and by pressing all the keys, puffs are caused to issue from the four series simultaneously. The perfect instrument is now before you, and your knowledge of it is complete. [Illustration: FIG. 28.]