The severity of this order was keenly felt by Galileo. While he remained at Arcetri, his seclusion from the world would have been an object of choice, if it had not been the decree of a tribunal; but to be debarred from the conversation of his friends in Florence,--in that city where his genius had been idolised, and where his fame had become immortal, was an aggravation of punishment which he was unable to bear. With his accustomed kindness, the grand duke made a strong representation on the subject to his ambassador at the court of Rome. He stated that, from his great age and infirmities, Galileo's career was near its close; that he possessed many valuable ideas, which the world might lose if they were not matured and conveyed to his friends; and that Galileo was anxious to make these communications to father Castelli, who was then a stipendiary of the court of Rome. The grand duke commanded his ambassador to see Castelli on the subject; to urge him to obtain leave from the pope to spend a few months in Florence, and to supply him with money, and every thing that was necessary for his journey. Influenced by this kind and liberal message. Castelli obtained an audience of the pope, and requested leave to pay a visit to Florence. Urban instantly suspected the object of his journey; and, upon Castelli's acknowledging that he could not possibly refrain from seeing Galileo, he received permission to visit him in the company of an officer of the inquisition. Castelli accordingly went to Florence; and, a few months afterwards, Galileo was ordered to return to Arcetri. During Galileo's confinement at Sienna and Arcetri, between 1633 and 1636, his time was principally occupied in the composition of his "Dialogues on Local Motion." This remarkable work, which was considered by its author as the best of his productions, was printed by Louis Elzevir, at Amsterdam, and dedicated to the count de Noailles, the French ambassador at Rome. Various attempts to have it printed in Germany had failed; and, in order to save himself from the malignity of his enemies, he was obliged to pretend that the edition published in Holland had been printed from a MS. entrusted to the French ambassador. Although Galileo had for a long time abandoned his astronomical studies, yet his attention was directed, about the year 1636, to a curious appearance in the lunar disc, which is known by the name of the moons libration. When we examine with a telescope the outline of the moon, we observe that certain parts of her disc, which are seen at one time, are invisible at another. This change or libration is of four different kinds; viz. the diurnal libration, the libration in longitude, the libration in latitude, and the spheroidal libration. Galileo discovered the first of these kinds of libration, and appears to have had some knowledge of the second; but the third was discovered by Hevelius, and the fourth by Lagrange.[30]