96 THE MATHEMATICAL PRINCIPLES [BOOK 1 its breadth AB is supposed diminished • in infinitum, becomes less than any given space. And therefore (by Lem. I) the figures inscribed and circumscribed become ultimately equal one to the other; and much more will the intermediate curvilinear figure be ultimately equal to either* Q.E.D. LEMMA III. The same ultimate ratios are also ratios of equality, when the breadth^ AB, BC, DC, fyc., of the parallelograms are unequal, and are all di minished in infinitum. For suppose AF equal to the greatest breadth, and complete the parallelogram FAaf. This parallelo gram will be greater than the difference of the in scribed and circumscribed figures ; but, because its breadth AF is diminished in infinitum, it will be come less than any given rectangle. Q.E.D. COR. 1. Hence the ultimate sum of those evanes cent parallelograms will in all parts coincide with the curvilinear figure. A BF C D E COR. 2. Much more will the rectilinear figure^comprehendcd under tne chords of the evanescent arcs ab, be, cd, (fee., ultimately coincide with tl.c curvilinear figure. COR. 3. And also the circumscribed rectilinear figure comprehended under the tangents of the same arcs. COR. 4 And therefore these ultimate figures (as to their perimeters acE) are not rectilinear, but curvilinear limi's of rectilinear figures. LEMMA IV. If in two figures AacE, PprT, you inscribe (as before) two ranks of parallelograms, an equal number in each rank, and, when their breadths are diminished in infinitum. the ultimate ratios of the parallelograms in one figure to those in the other, each to each respec tively, are the same; I say, that those two figures AacE, PprT, are to one another in that same ratio. For as the parallelograms in the one are severally to p the parallelograms in the other, so (by composition) is the < sum of all in the one to the sum of all in the other : and so is the one figure to the other; because (by Lem. Ill) the former figure to the former sum, and the latter figure to the latter sum, are both in the ratio of equality. Q.E.D. COR. Hence if two quantities of any kind are any how divided into an equal number of parts, and those A £