Cloud Rings (2006) by Ned Kahn.
"A series of devices set into a sunken courtyard that continuously shoot rings of fog up into the space between two buildings. The billowing rings are seen from the street as well as from the windows of the surrounding buildings. This exhibit produces rings of mist (similar to smoke rings) a few times every minute. The torus shaped smoke ring travels straight up. The exhibit uses an ultrasonic humidifier to generate the mist, which collects under the domed top and is ejected out when the exhibit pulses. A gear-motor rotates a cam, which pushes a diaphragm against spings to store energy for the pulse."