Why does a bottle of milk keep better when it stands in water in a porous pot in a draught?

Water evaporates from the pot cooling it.

The water evaporating requires heat to change its state, and this heat comes from the rest of the water still inside the jar first, and that in turn takes heat from the milk bottle. It would be little use putting the milk into the pot and covering the pot with something, as that would prevent the evaporated water from departing. If the pot was covered, the local humidity would go up, reducing the chances of evaporation. The need for a porous pot is that a large surface area is provided by such, as the water can pass very slowly through the pot. Evaporation from the surface pf the water inside the pot will be far less than that on the outside of the pot. Basically, it's an ancestor of the fridge...