At the top of the drum, a basket holds chunks of dry ice. This basket can be lowered into the water with an external control. The top of the drum is sealed, save for an exit point for a large hose. When the basket is lowered into the water, the dry ice sublimes, and a thick, white carbon dioxide fog is formed. This fog is heavier than air, so it tends to hug the ground as it comes out of the hose.
Fog machines produce a very believable effect, and the fog produced is completely harmless.
Figure 8.4 shows a typical dry ice fog machine. A typical dry ice fog machine. Two similar foggers are owned by WPI, and often used in theatre productions. Often both smoke and fog are used together to produce a very interesting effect. As usual, it is important to not over-use either effect, as they tend to lose their impact the more that they are used.