tyamada
Brigg:
I'm not splitting hairs, he is saying that a side feed air brush is a
bottom(siphon) feed, when the bottom feed bottle is used and is a
gravity feed when the cup adapter is used (where the cup is above the
airbrush).
He gave an example of the Renegade Spirit which comes with a bottle
and a color cup.
http://www.badgerairbrush.com/Renegade_Series.asp
It's like the Badger 360 that is convertible from a bottom(siphon) or
gravity feed.
We're
almost saying the same thing. Isn't language fun? 
I believe what is happening here is we are getting our mords wixed up! 
Ken is actually
calling it a gravity/artesian well attachment, and with a jar adapter
it turns it into a true bottom fed airbrush. I've noticed that Ken likes to use the words bottom fed, instead of siphon fed, I'll come back to this.
In the case of a side feed airbrush the color
cup attaches to the side, and the tube is completely horizontal. As we
all know, gravity only works in one direction, and that is down. Gravity is considered an unstoppable force, but it actually quite weak, and it can't push paint sideways. The way
a side feed works is by utilizing the principle of liquids seeking
their own level. Air pressure is pushing down on the surface of the
paint in the color cup and that is what is feeding the paint to the
airbrush. An artesian well works by pressure from the ground above the well water and pushes the water to the surface. Hence Ken's description. But air pressure alone can't account for the paint getting into the air flow.
ALL airbrushes are, to a certain extent, siphon airbrushes, and this is why I think Ken eschews the word siphon to describe a bottom feed airbrush. It is the siphon created by the air supply that pulls the paint into the air flow where it atomizes.This is how the Badger 360 can be both gravity and bottom feed. If Badger made a side feed cup that fit the 360, it could also work as a side feed.
Gravity fed airbrushes drip the paint into the air flow, and don't use much of the air pressure to move the paint along. Too low, and the air brush can't atomize the paint properly and some of the pressure is required to keep the paint flowing smoothly, Run out of paint, and you'll notice that the airbrush will vacuum what paint there is out of the cup. You can even hear a little sucking sound as it evacuates the last bits of paint, leaving only what adheres to the surface behind. That is the siphon action that is created by the air pressure. Turn a gravity airbrush upside down and drop the cup lip into water and crank up the air pressure and you'll see that siphoning does in fact happen.
Bottom feed airbrushes are completely siphon action. They pull the paint up from the bottle or color cup, and require the most air pressure. Without sufficient air pressure, the paints stays put.
Side feed are somewhere between those. The "artesian" action delivers the paint and the siphon takes over. Air pressure is even weaker than gravity, and so you require slightly more air pressure than a completely gravity feed system.
Hence the reason I said it's half and half. Most people are only familiar with gravity or siphon/bottom. Side feed is halfway between the two. Throwing in artesian would only confound and confuse people.