162 STATESMEN AND SAGES presence, but they set aside the evidence of their senses and quoted Aristotle as much' as before. The enmity arising from these disputes rendered his situa- tion so unpleasant, that in 1 592, at the invitation of the Venetian commonwealth, he gladly accepted the professorship of mathematics at Padua. The period of his appointment being only six years, he was re-elected in 1598, and again in 1606, each time with an increase of salary; a strong proof of the esteem in which he was held, even before those astronomical discoveries which have immortalized his name. His lectures at this period were so fully attended that he was some- times obliged to adjourn them to the open air. In 1609 he received an invita- tion to return to his original situation at Pisa. This produced a letter, still extant, from which we quote a catalogue of the undertakings on which he was already employed. " The works which I have to finish are principally two books on the ' System or Structure of the Universe,' an immense work, full of philos- ophy, astronomy, and geometry ; three books on ' Local Motion,' a science en- tirely new, no one, either ancient or modern, having discovered any of the very many admirable accidents which I demonstrate in natural and violent motions, so that I may, with very great reason, call it a new science, and invented by me from its very first principles ; three books of mechanics, two on the demon- stration of principles and one of problems ; and although others have treated this same matter, yet all that has been hitherto written, neither in quantity nor other- wise, is the quarter of what I am writing on it. I have also different treatises on natural subjects on Sound and Speech, on Light and Colors, on the Tides, on the Composition of Continuous Quantity, on the Motions of Animals, and others besides. I have also an idea of writing some books relating to the military art, giving not only a model of a soldier, but teaching with very exact rules every- thing which it is his duty to know, that depends upon mathematics, as the knowledge of castrametation, drawing up of battalions, fortification, assaults, planning, surveying, the knowledge of artillery, the use of instruments, etc." Out of this comprehensive list, the treatises on the universe, on motion and me- chanics, on tides, on fortification, or other works upon the same subjects, have been made known to the world. Many, however, of Galileo's manuscripts, through fear of the Inquisition, were destroyed, or concealed and lost, after the author's death. In the same year, 1609, Galileo heard the report that a spectacle-maker of Middleburg, in Holland, had made an instrument by which distant objects ap- peared nearer. He tasked his ingenuity to discover the construction, and soon succeeded in manufacturing a telescope. His telescope, however, seems to have been made on a different construction from that of the Dutch optician. It con- sisted of a convex and concave glass, distant from each other by the difference of their focal lengths, like a modern opera-glass ; while there is reason to believe that the other was made up of two convex lenses, distant by the sum of their focal lengths, the common construction of the astronomical telescope. Galileo's attention naturally was first turned to the moon. He discovered that her surface, instead of being smooth and perfectly spherical, was rough with mountains and
