Circle around the back of the privy, and there is a small door at the back. This door is below the level of the seats, and inside sit the boxes where all of the "stuff" drops. Presumably, some enslaved person's job was to cart the boxes down the hill and across the lane to the dung repository, or empty them somewhere else.
At any rate, for some reason the privy interests me. Using the restroom is a very private and intimate act, and in observing this particular one, we can be reminded that George and Martha Washington, and any number of their VIP guests, were really just human beings. George Washington put his pants on one leg at a time. He was bitten by mosquitoes just like we are. And, yes, from time to time he pooped. Even the greatest among us today - the richest, the strongest, the brightest, the most politically savvy, the most Scripturally knowledgable - is reduced to a state of humanity by this simple act. It is a great leveler in a world where a level playing field is seldom a reality.