The organ is an instrument with keyboards and pipes. It plays with the effect of the "wind" produced with constant pressure by the bellows, which is distributed to the pipes by the "ventilators", valves set inside the wind chest (the chest on which the pipes are set), which are opened by pressing the keys.
There is a connection between the keys and the ventilators. When this connection is formed of simple rigid tie rods made of metal or wood, we have a "mechanical" organ- an organ with "mechanical transmission". With this system of transmission, which is also the simplest in terms of construction and consequently the safest and most functional, the organist communicates directly with his instrument even to the point of being able to influence the "pronunciation" of the pipes by varying the way in which he presses the keys. It is clear that this cannot happen in organs with "pneumatic" or "electric" transmission, in which the means that provides for the opening of the ventilators when the keys are pressed is made up of complicated devices activated by strong air pressures or electrically.
Albert Schweitzer left us a highly analytic and detailed evaluation of these facts. Over half a century ago he stated: “… Our pneumatic device is precision without life. It is limited to entrusting the effort to air pressure; it is completely lacking in the vital, elastic element present in the mechanical lever... all the performer's efforts must be directed to masking the lack of life in this precision. No organist now wants a mechanical transmission. Yet how many could play well and clearly on their old mechanical transmission and now fiddle about on the new transmission, which they are so proud of, with a lack of precision that they are not aware of. With mechanical transmission the fingers can feel precisely, from the point of resistance of the key, when the sound is formed. And a pressed key reacts upwards, under one's finger, and as soon as the finger hints at leaving it, the key comes up again with all its inertia and pushes the finger up with it. The force of the key co-operates with the will. Only with mechanical transmission does one find one self in true, vivid contact with one's instrument. With pneumatic transmission one communicates with the instrument by telegraph!"
The qualification of "mechanical" is today universally accepted as a prerequisite for defining an organ as "artistic". Personally, l have no hesitation in saying that only if it is mechanical can the organ be considered a musical "instrument", in the sense of a "means" for making music.