I took one of the kids for a cycle down to where the old old gunpowder mills used to be. We stopped beside this

Am I right in thinking that this would have been used for making the charcoal for the gunpowder or was there some other purpose for it?

I think (I'll need to check) that was the base of a chimney stack.

I've not seen that before, where is it?

Go to Roslin and turn off down the road to Rosewell. (There are signs up saying the road is closed, but you are still allowed access to up to a certain point along it.) At the bottom of the hill, there's a hair-pin bend with gates on the right hand side. The old powder mill is through those gates.

If you can't park outside the gates, drive on just past the sewage works and you'll find plenty of parking in the Roslin Glen Park.

A dirt road leads right through the old mill and there are still a few remnants of the old buildings. About 25 years ago, however, there were many more and the place was like an old deserted village.