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12 long afterwards—the one on fortification not until the present century; 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. 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.

Before proceeding further with Galileo's researches and discoveries, so far as they fall within our province, it seems important to acquaint ourselves with his views about the Copernican system. From a letter of his to Mazzoni, of 30th May, 1597, it is clear that he considered the opinions of Pythagoras and Copernicus on the position and motion of the earth to be far more correct than those of Aristotle and Ptolemy. In another letter of 4th August of the same year