The subject of floating bridges having been discussed at one of the scientific parties which had assembled at the house of Salviati, a difference of opinion arose respecting the influence of the shape of bodies on their disposition to float or to sink in a fluid. Contrary to the general opinion, Galileo undertook to prove that it depended on other causes; and he was thus led to compose his discourse on floating bodies,[25] which was published in 1612, and dedicated to Cosmo de Medici. This work contains many ingenious experiments, and much acute reasoning in support of the true principles of hydrostatics; and it is now chiefly remarkable as a specimen of the sagacity and intellectual power of its author. Like all his other works, it encountered the most violent opposition; and Galileo was more than once summoned into the field to repel the aggressions of his ignorant and presumptuous opponents. The first attack upon it was made by Ptolemy Nozzolini, in a letter to Marzemedici, Archbishop of Florence;[26] and to this Galileo replied in a letter addressed to his antagonist.[27] A more elaborate examination of it was published by Lodovico delle Colombe, and another by M. Vincenzo di Grazia. To these attacks, a minute and overwhelming answer was printed in the name of Benedetti Castelli, the friend and pupil of Galileo; but it was discovered, some years after Galileo's death, that he was himself the author of this work.[28] [25] Discorso intorno alle cose che stanno in su l'acqua, o che in quella si muovono. Opere di Galileo, vol. ii. pp. 165-311. [26] Opere di Galileo, vol. ii. pp. 355-367. [27] Ibid. 367-390. [28] These three treatises occupy the whole of the third volume of the Opere di Galileo. CHAPTER IV. _Galileo treats his opponents with severity and sarcasm--He is aided by the sceptics of the day--The Church party the most powerful--Galileo commences the attack, and is answered by Caccini, a Dominican--Galileo's Letter to the Grand Duchess of Tuscany, in support of the motion of the Earth and the stability of the Sun--- Galileo visits Rome--Is summoned before the Inquisition, and renounces his opinions as heretical--The Inquisition denounces the Copernican System--Galileo has an audience of the Pope, but still maintains his opinions in private society--Proposes to find out the Longitude at Sea by means of Jupiter's Satellites--His negociation on this subject with the Court of Spain--Its failure--He is unable to observe the three Comets of 1618, but is involved in the controversy to which they gave rise._