When either the finger or foot is pressed upon a key connected with _k_, the outer end of the back-fall _gg_ is pulled down, which opens the pallet _p_. The compressed air in _a_ then rushes through the groove _bb_ into the bellows _cc_, which rises and lifts with it all the action attached to it by _l_. As the top of the bellows _cc_ rises, it lifts up the throttle-valve _d_ (regulated by the wire _m_) which prevents the ingress of any more compressed air by _bb_. But the action of the key on _gg_, which opened the pallet _p_, also allowed the double-acting waste-valve _e_ to close, and the tape _f_ hangs loose. The compressed air, therefore, as it is admitted through _bb_ cannot escape, but on the other hand when the key releases the outer end of _g_, and lets it rise up again, the tape _f_ becomes tightened and opens the waste-valve, the bellows _cc_ then drops into its closed position. [Illustration: Fig. 3. The Pneumatic Lever] The organ touch could now be made as light as that of a pianoforte, much lighter than ever before. This epoch-making invention, introduced in 1832, rendered possible extraordinary developments. It was at first strangely ignored and opposed. The English organ-builders refused to take it up. Barker was at length driven to France, where, in the person of Aristide Cavaille-Coll, he found a more far-seeing man. After Cavaille-Coll had fully demonstrated the practical value of Barker's invention, Willis and others joined in its development, and they contemporaneously overcame all difficulties and brought the pneumatic action into general favor. This process, of course, took time, and up to about fifty years ago pneumatic action was found only in a few organs of large calibre. The recent revolution in organ building and in organ tone, of which this book treats, was founded upon the pneumatic and electro-pneumatic actions invented by Barker.[2] It is safe to say that the art of organ building has advanced more during the last fifty years than in any previous three centuries. We are literally correct in saying that a veritable revolution has already been effected--and the end is not yet. As leaders in this revolutionary movement, three names stand out with startling prominence--Henry Willis, Aristide Cavaille-Coll and Robert Hope-Jones. Others have made contributions to detail (notably Hilborne L. Roosevelt), but it is due to the genius, the inventions and the work of those three great men that the modern organ stands where it does to-day. We propose: 1. To enumerate and describe the inventions and improvements that have so entirely transformed the instrument; 2. To trace the progress of the revolution in our own country; and, 3. To describe the chief actors in the drama.