It was water to add to the radiator as water would boil off. My dad used them as we traveled across the open country.

red and blueno donkey, you sure this ain't it?
http://10engines.blogspot.com/2009/05/radiator-water-bag.html
"Vive la mort, vive la guerre, vive le sacre mercenaire."

I'm trying to remmeber the name of them. They were printed with a picture of a donkey in red and blue

I know this post is years late, but this bag is intended for drinking water. Any jug or bottle can be used to store water for the radiator, these were kept in the trunk. The canvas bag allowed water to leak and evaporate and thus cool the bag. It was huge in front to allow the windstream to better cool the water. A boiling radiator doesn't really "care" if the water is cool or not, as long as its under 212 degrees. I lived in LA and Bakersfield in the 1950 and 1960 and we had one of these bags.

Of course you have to add water to the radiator. It boils over and loses water because that bag is hanging there and blocking off the cooling air to half the radiator!
Another interesting anachronism from the 40-50s is the tubular shaped evaporative air conditioner that mounted at the top of the window, usually in the passenger's door.

Rsgre100% correct. They can still be seen in parts of the outback here in Australia. Back in the 60s and early 70s my dad always tied one on the front of the grill of his car during long trips into the country.

Before the days of antifreeze and recycling water resevoir tanks on the car radiators being standard equipment (maybe from the late 60's on), the water would literally evaporate enough that you had to add water every so often, especially in the desert country. The "desert bag" was one way to carry extra water with you in those situations. Cars have come a long ways since then!
Regards,
Steve

In the 1940s-1960s, you could drive to Las Vegas from Los Angeles and find gas stations selling the water bags. I remember seeing them in Bakersfield, Riverside, Palm Springs and Barstow for sale around 1960 when we drove to Las Vegas. We bought one and I remember my dad filling the radiator a few miles from town when the radiator was boiling over.

Used for drinking as well

It is a water bag that is cooled by moisture evaporation through the canvas cover.

Hey! thanks, I didnt think anyone was going to answer.

It's a water bag to supplement the water in the radiator and keep the engine from over-heating.