Compendium of All Natural Science

Whence the velocity of motion may be discerned, and whence the velocity of rectilinear motion, and whence the velocity of orbicular motion, and whence the velocity of revolution. 20.

Moreover, it happens that everything which is moved locally is moved equally with its parts. Whence, if one part is moved most swiftly, the whole also will be perceived to be moved most swiftly.

Now, the velocity of rectilinear motion is recognized from that line which the center point of the moved body traverses; just as that of circular motion is perceived from that line which is described by the swiftest point of the mobile body. But the speed of revolution can be recognized from the angles that are generated around the center. Wherefore, those for which the same angles are produced at the center have an equal velocity of revolution.

Whence the velocity of other motions. 21.

But the velocity of the motion of increase and decrease, and likewise of compression and extension, is estimated from the quantity acquired or lost; and similarly for extension and compression, even if other factors are not equal. Likewise, the velocity of the motion which is from quality to quality is confirmed from the quality gained or lost. Under this condition, to be sure, that the velocity of all motions be weighed by the magnitude or quantity of time.