Swamp coolers for lowriders

Not to sound like an idiot, but what exactly is a swamp cooler? Maybe if you can tell me what it looks like or what it does, I can help. Is there anything else this might be called instead of a swamp cooler?

I have also heard them called cool cans. It is the large cylendrical "can" that can be seen mounted on the passengers door on many early lowriders

Swamp coolers  they where used out in the south west they use them houses or trailer in arizona they made them for cars in the 50's the only thing i can say is you might have to scatch build one if you cant find it online shouldnt be to hard to make one just researtch some pic's should be able to find them useing google. If you have to make one hers a link to evilbay that has some pics http://search.ebay.com/swamp-cooler_W0QQfnuZ1