This is Leonardo's graphic vision of the casting pit for the Sforza horse. In the center, is the assembled outer mold of the horse. It is flanked on four sides by multiple furnaces - two rectangular and two circular- of the type he had designed for casting cannon to melt the bronze before it flowed into the mold. He could calculate how much bronze would be needed because he knew bow much wax or potters' clay -which was used for the "thickness" - already had occupied the space between the outer and the inner molds.
Tubes connecting one of the furnaces to holes in the mold carry the molten bronze. In a note transcribed here he refers to the fact that some of the tubes - "spouts" - would have to be vented. The sketch at the bottom shows "how the horse shall be cast" -upside down. The diagram and note at top right describe a hole about 12 by 23 inches for getting inside the finished horse. Through this hole would be removed the earthen materials that had formed the inner mold during casting.