 Heytesbury deals with augmentation as with pure increase in size, which is the same process as rarefaction; more precisely, he deals with the speed of rarefaction as measured by the effect produced. There are three ways of “measuring” the speed of augmentation acquired in time: (1) by the maximum quantity acquired; (2) by the latitude of rarity or rareness; and (3) by the ratio of new size to previous size ([RSS] 1494: fol. 60rb). He holds the third position, according to which the ratios correspond to different degrees of augmentation. Augmentation, like the two other motions, can be uniform or difform. An augmentation is uniform if in equal intervals of time, the body undergoing augmentation increases in size by the same ratio. While discussing the above mentioned positions, Heytesbury makes broad use of the new calculus of ratios invented by Kilvington and Bradwardine.