'It obeys you as if it could hear you speak,' I responded in the same hushed tone; for as the moment of vision approached nearer, a nervousness which I could not wholly control pervaded my body, and made me fearful of betraying some symptom of unmanly agitation to my companion. Diana touched the spring which she had before pointed out to me; then laid her finger on her lip and drew me back a step. All the wheels were in motion; and grandly, slowly, almost imperceptibly, as the sweep of that far-distant planet which it was following in its course through space, the marvellous engine moved along its orbit. At the same moment a strain of subdued melody, resembling somewhat the music of Æolian harps heard far off, floated out and palpitated upon the still air of the vaulted room. The strain grew louder and clearer, then sank again to whisperings almost inaudible: and then once again increased in power and volume, seeming now like a chorus of angelic voices chanting a hymn of praise. I held my breath to listen, and, for a time, forgot surprise in the pure pleasure of the ear. 'What is it?' I whispered at length. 'I call it the song of the moon,' answered Diana. 'You will hear it whenever the moon's rays fall upon the glass. I love it the best of all.' 'There are others, then?' 'Each planet has its song, different from all the others: and the stars also: but those we cannot hear.' This was said so quietly, and with an air so grave, that I knew not whether my cousin expected me to take it seriously. 'Are you really an enchantress,' I asked, 'that you can bring down to earth the music of the spheres, as well as make their mysteries visible?' 'Why not? is one more wonderful than the other?' she returned, with a faint smile. 'But you must not expect me to tell you all my secrets at once, Cousin Will. Think of me as an enchantress for to-night. I am not the first who has practised magic in this tower. It was built they say, in the time of King Arthur, by the magician Merlin; and Friar Bacon once lived here, and worked upon the problem of the Speaking Head. But none of them could do what I can do, or ever saw what I have seen a thousand times!'