What am I? Where am I? we may imagine to have been the first exclamations of the first man awakening to conscious existence. Here, in our Age of Reason, we have been dealing with the same thoughts. They are the same which, as we have seen, occupied Greek intellectual life. [Sidenote: Roman astronomical ideas.] When Halley's comet appeared in 1456, it was described by those who saw it as an object of "unheard-of magnitude;" its tail, which shook down "diseases, pestilence, and war" upon earth, reached over a third part of the heavens. It was considered as connected with the progress of Mohammed II., who had just then taken Constantinople. It struck terror into all people. From his seat, invisible to it, in Italy, the sovereign pontiff, Calixtus III., issued his ecclesiastical fulminations; but the comet in the heavens, like the sultan on the earth, pursued its course undeterred. In vain were all the bells in Europe ordered to be rung to scare it away; in vain was it anathematized; in vain were prayers put up in all directions to stop it. True to its time, it punctually returns from the abysses of space, uninfluenced by anything save agencies of a material kind. A signal lesson for the meditations of every religious man. [Sidenote: More correct ideas among some of the clergy.] Among the clergy there were, however, some who had more correct cosmic ideas than those of Calixtus. A century before Copernicus, Cardinal de Cusa had partially adopted the heliocentric theory, as taught in the old times by Philolaus, Pythagoras, and Archimedes. He ascribed to the earth a globular form, rotation on its axis, and a movement in space; he believed that it moves round the sun, and both together round the pole of the universe. [Sidenote: The geocentric and heliocentric theories.] By geocentric theory is meant that doctrine which asserts the earth to be the immovable centre of the universe; by heliocentric theory that which demonstrates the sun to be the centre of our planetary system, implying, as a necessary influence, that the earth is a very small and subordinate body revolving round the sun. [Sidenote: The geocentric doctrine adopted by the Church.] I have already, in sufficient detail, described how the Roman Church had been constrained by her position to uphold the geocentric doctrine. She had come to regard it as absolutely essential to her system, the intellectual basis of which she held would be sapped if this doctrine should be undermined. Hence it was that such an alarm was shown at the assertion of the globular form of the earth, and hence the surpassing importance of the successful voyage of Magellan's ship. That indisputable demonstration of the globular figure was ever a solid support to the scientific party in the portentous approaching conflict.