cbf94194_Euclid_5_Intro_html__Definitions
[0] 18. [Definitions] 
A perturbed proportion is when given three magnitudes and others equal to them in number it happens that (1) as, in the first group of magnitudes, a leading term is to a following term, so, in the second group of magnitudes, a leading term is to a following term, while (2) as, in the first group of magnitudes, a following term is to some other term, so, in the second group of magnitudes, some other term is to a leading term.
Euclid.5.Intro.html
Definitions