Chimney endings/cowls?

[chimneyguy]

Greetings!
I can't find information about those chimney endings ( two symmetrical cones)

http://upload.wikimedia.org/wikipedia/commons/1/1d/Cowls.jpg

(second from right, but without the cap). I see lots of them in industrial are. Can somebody explain the role and how it works?
I'm not an engineer.

Regards chimneyguy

 

[KiwiMace]

I think on this type there is an airgap between the cones for drainage of rainwater and perhaps some induced airflow to extend the plume. There might even be a mesh screen in the wide part.