His situation was now worse than before, for about this time, 2nd July, 1591, his father died after a short illness, leaving his family in very narrow circumstances. In this distress the Marquis del Monte again appeared as a friend in need. Thanks to his warm recommendation to the Senate of the Republic of Venice, in the autumn of 1592 the professorship of mathematics at the University of Padua, which had become vacant, was bestowed on Galileo for six years.[24] On 7th December, 1592, he entered on his office with a brilliant opening address, which won the greatest admiration, not only for its profound scientific knowledge, but for its entrancing eloquence.[25] His lectures soon acquired further fame, and the number of his admirers and the audience who eagerly listened to his, in many respects, novel demonstrations, daily increased. During his residence at Padua, Galileo displayed an extraordinary and versatile activity. He constructed various machines for the service of the republic, and wrote a number of excellent treatises, intended chiefly for his pupils.[26] Among the larger works may be mentioned his writings on the laws of motion, on fortification, gnomonics (the making of sun-dials), mechanics, and on the celestial globe, which attained a wide circulation even in copies, and were some of them printed long afterwards—the one on fortification not until the present century;[27] others, including the one on gnomonics, are unfortunately lost. On the wide field of inventions two may be specially mentioned, one of which was not fully developed until much later. The first was his proportional circle, which, though it had no special importance as illustrative of any principle, had a wide circulation from its various practical uses. Ten years later, in 1606, Galileo published an excellent didactic work on this subject, dedicated to Cosmo de’ Medici, and in 1607 a polemical one against Balthasar Capra, of Milan, who, in a treatise published in 1607, which was nothing but a plagiarism of Galileo’s work disfigured by blunders, gave himself out as the inventor of the instrument. Galileo’s reply, in which he first exhibited the polemical dexterity afterwards so much dreaded, excited great attention even in lay circles from its masterly satire.[28] The other invention was a contrivance by which heat could be more exactly indicated. Over zealous biographers have therefore hastened to claim for their hero the invention of the thermometer, which, however, is not correct, as the instrument, which was not intended to measure the temperature, could not be logically called a thermometer, but a thermoscope, heat indicator. Undoubtedly it prepared the way by which improvers of the thermoscope arrived at the thermometer.[29]