Casting the horse inverted: access door and pouring sequence
A hinged door cast into the body, venting through the legs, and filling the neck to line m n first
A heavily illustrated page on casting the bronze horse feet-up, with a perspective view of a vaulted furnace, plans of the casting pit, and the horse-form seen inverted among its gates. Leonardo specifies a hatch 7 ounces wide and one braccio long, cast into the body but fitted with male-and-female hinges so it can be fixed in place. He notes that the mouths of the body can dispense with vents except at the highest point, since the legs serve as the common escape for the enclosed air. The pouring sequence fills the neck through many mouths up to line m n before all the other mouths are unstopped at once.
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An access hatch cast into the horse with male-and-female hinges
The opening into the horse's body is to be 7 ounces wide and one braccio long. The little door is cast together with the horse but separated from its place and hung on male-and-female hinges so that it can be fixed shut.
Furnace void with a straight barrel-vault for stirring
A furnace is drawn whose interior void is a straight barrel-vault, shaped so that the melt inside it can be stirred.
Venting the enclosed air through the legs
All the mouths of the body may be made without vents, except at the highest place of the body. The legs act as the common exhalation for the enclosed air, over and above the bronze that fills them.
Pouring order: fill the neck to line m n, then open all mouths
This is how the horse should be cast: first fill the neck with bronze through many mouths, up to the line m n marked on the figure, and only then unstop all the other mouths at once.