Simon if you happen to need to control a blast in a simple fashion without a normal diverter, you can just have short pipe on the blower separated from a longer pipe in the tuyere. You just alter the distance between the two, and the aim between them until your furnace is running at the rate you want. A few bricks can hold the pipes in position.
Andrew, I'm sure Mr. Weygers was careful not to burn down his tree. The devil is in the details, as in most things having to do with fire, Including bonfires, campfires, etc. His books are blacksmithing classics, and I admire them greatly.
I can hold my hand two feet above the 2' length of stovepipe I set on my current melting furnace comfortably, and that is while melting metal with a blower, not just forging small parts in a tin can forge with an atmospheric draft.
A longer pipe suspended well below a limb on a small atmospheric forge as shown will run quite cool. Most anything we do can be dangerous if approached without awareness. Running a blown exhaust direct into pine needles during a drought wouldn't be advisable, but I think most people wouldn't do that. I wouldn't even run my iron furnace in the clear under drought conditions. Even running a hand grinder can be danerous then. An illustration of what the author found useful does not absolve people from using good sense when doing the same.