Barrels

Barrels are used by the Elves of Mirkwood to ship things like apples, wine, and many other foods that the elves eat. They are shipped back to their origin when they're empty via a brook that runs down the middle of the Mirkwood Elves' fortress, where they are pushed into the brook, go via a portcullis, and then fianally along the Forest River.

History
Barrels are only mentioned in The Hobbit, when Bilbo frees the Dwarves from the dungeons by using the ring to open up the locks of the cells that held the twelve dwarves while the elves are enjoying a feast. He goes, still wearing the one ring, and finds twelve barrels, puts a bit of straw in them, and seals them up. Then the elves go and push the barrels, which they believe to be empty. Bilbo, being unable to go into a barrel himself, grabs onto the last barrel and holds on the the bottom, using it like a lifering. Later, he finds another barrel and grabs onto it, and uses the two barrels like a raft. After a meal, the barrels get tied up into a raft, get pushed by elves and finally end up near Escargoth, where they are hauled back to shore by men. At this point, the barrels the dwarves were in were opened by Bilbo, Thorin, Fili and Kili, and many of the dwarves took a while to get back to normal, due to slightly leaky barrels and lack of air. A number seemed only half alive.

Quotes
"I do not wish to see apples again, after smelling them in my barrel the whole way." Fili

"Well, here we are!" Thorin after the raft-ride

"These barrels are unusually heavy. They don't seem to have been emptied. If it was daylight we would go and check what's inside." Elven raft-pushers