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the end of the four years, the dialogues being now completed except for introduction and index and a final revision, Galileo determined to go once more to Rome to arrange for the printing. Castelli, now the Pope's mathematician, highly approved this decision, as also did Riccardi, the chief censor, and others, who retailed remarks made by the Pope, disclaiming any responsibility for the action taken in 1616 against the Copernican doctrines. But when it came to the point the Pope insisted that the title originally chosen, "Dialogues on the Flux and Reflux of the Tides," did not indicate the real purpose of the work, which was a discussion of the relative merits of the systems of Copernicus and Ptolemy. Besides requiring that this be made more clear, and that the subject be treated as merely a hypothesis. Urban laid it down that his own favourite argument, which has already been mentioned, must be inserted at the end. Galileo accepted the conditions, and the manuscript was passed by Riccardi as censor after a few alteration's had been made by his assistant.

Thus in the summer of 1630 we find Galileo back in Florence with the coveted Imprimatur granted on the understanding that a preface and conclusion as demanded by the Pope should be added. The plague was then raging in Florence, and it spread to the suburb of Bellosguardo where Galileo lived. One of his employees, a glass-blower, was attacked by it and died, and Vincenzio