Dimensions: 77x47x47 cm Materials: durmast, metal Constructor: Muséo Techni, Montréal Date: 1987
Working model after Leonardo da Vinci, Madrid Ms. I (BNM), fol. 101v
A cone-tipped axle rotates between three spheres in a nearly hemispheric support. Leonardo notes that "3 balls under the spindle are better than 4, because 3 balls are of necessity always touched and equally moved by the spindle. Using 4 there would be the danger that one of them is left untouched, awaiting the opportunity to produce friction."