If in two ellipses having a common major axis we take two such arcs that their chords are equal, and that also the sums of the radii vectores, drawn respectively from the foci to the extremities of these arcs, are equal to each other, then the sectors formed in each ellipse by the arc and the two radii vectores are to each other as the square roots of the parameters of the ellipses.

Source, in full

Sect. 4, Lemma 26, Insigniores orbitae cometarum proprietates (1761) [Notable properties of comets' orbits] translated by Florian Cajori, A History of Mathematics (1906) p. 259, from the German of Michel Chasles, Geschichte der Geometrie, haupsächlich mit Bezug auf die neuern Methoden (1839) p. 183.