Smoke Rings

Smoke rings are much more than an amusement of nicotine addicts. The October issue of New Scientist had a nice article on what are properly called vortex rings. There are people trying to make thrusters with them. They could be efficient and simple. At last there was a bit of math on the subject: for the most powerful ring, the length of the cylinder should be 4 times the diameter.
An old issue of Invention and Technology magazine had an article on them In World War II, there was a plan to make a giant machine to toss around planes in formation. It never was built, but the guy later sold a small version to a toy company. They did build a small version with a ring about three feet across that traveled at several hundred miles an hour and made an eerie screaming noise.
Smoke rings have fascinated me since I was a kid at the New York Worlds Fair in 1964. We never knew why, but giant smoke rings would occasionally shoot up into the sky. I think they wee being used to send cooking smoke out of the area.
There is a new toy called the Zero Blaster, and it uses fog machine fluid to make nice rings.
There is also a larger device, the Airzooka, sometimes sold at Spencer's Gifts.
I built a large box once. It was a four-foot cube, with a two-foot hole. It could make leaves and hair jump at 30 yards. The slightest obstacle would break the ring. They unravel. I want to rebuild it and use a fog machine to fill it.
Mount Etna, the volcano in Italy, blew smoke rings in July 2000:
http://www.decadevolcano.net/photos/smokerings.htm