It is quite certain that this tale is untrustworthy as it stands. We have seen above that Trivet's date for the arrival of the Empress at London is similarly, beyond doubt, erroneous.[369] That the citizens, when they suddenly rose against the Empress, may also have blockaded Geoffrey in his tower, not only as her ally, but as their own natural enemy, is possible, nay, even probable. But that he ventured forth, through their ranks, to Fulham, when thus blockaded, is improbable, and that he captured the bishop as an enemy of the Empress is impossible, for the Empress herself had just installed him,[370] and we find him at her court a month later.[371] At the same time Trivet, we must assume, cannot have invented all this. His story must preserve a confused version of the facts as told in some chronicle now lost, or, at least, unknown.[372] On this assumption it may, perhaps, be suggested that Geoffrey was indeed blockaded in the Tower, but that when he accepted the Queen's offers, and thus made, as we shall see, common cause with the citizens, he signalized his defection from the cause of the Empress by seizing her adherent the bishop,[373] and holding him a prisoner till, as Holinshed implies, he purchased his freedom, and so became free to join the Empress at Oxford.[374] And now let us come to the subject of this chapter, the lost charter of the Queen. That this charter was granted is an historical fact hitherto absolutely unknown. No chronicler mentions the fact, nor is there a trace of any such document, or even of a transcript of its contents. And yet the existence of this charter, like that of the planet Neptune, can be established, in the words of Sir John Herschel, "with a certainty hardly inferior to ocular demonstration." The discovery, indeed, of that planet was effected (_magnis componere parva_) by strangely similar means. For as the perturbations of Uranus pointed to the existence of Neptune, so the "perturbations" of Geoffrey de Mandeville point to the existence of this charter.