At last his enemies made it too hot for him at Pisa.  He resigned his chair (1591), but only to accept a higher position at Padua, on a salary of one hundred and eighty florins,—­not, however, adequate to his support, so that he was obliged to take pupils in mathematics.  To show the comparative estimate of that age of science, the fact may be mentioned that the professor of scholastic philosophy in the same university was paid fourteen hundred florins.  This was in 1592; and the next year Galileo invented the thermometer, still an imperfect instrument, since air was not perfectly excluded.  At this period his reputation seems to have been established as a brilliant lecturer rather than as a great discoverer, or even as a great mathematician; for he was immeasurably behind Kepler, his contemporary, in the power of making abstruse calculations and numerical combinations.  In this respect Kepler was inferior only to Copernicus, Newton, and Laplace in our times, or Hipparchus and Ptolemy among the ancients; and it is to him that we owe the discovery of those great laws of planetary motion from which there is no appeal, and which have never been rivalled in importance except those made by Newton himself,—­laws which connect the mean distance of the planets from the sun with the times of their revolutions; laws which show that the orbits of planets are elliptical, not circular; and that the areas described by lines drawn from the moving planet to the sun are proportionable to the times employed in the motion.  What an infinity of calculation, in the infancy of science,—­before the invention of logarithms,—­was necessary to arrive at these truths!  What fertility of invention was displayed in all his hypotheses; what patience in working them out; what magnanimity in discarding those which were not true!  What power of guessing, even to hit upon theories which could be established by elaborate calculations,—­all from the primary thought, the grand axiom, which Kepler was the first to propose, that there must be some numerical or geometrical relations among the times, distances, and velocities of the revolving bodies of the solar system!  It would seem that although his science was deductive, he invoked the aid of induction also:  a great original genius, yet modest like Newton; a man who avoided hostilities, yet given to the most boundless enthusiasm on the subjects to which he devoted his life.  How intense his raptures!  “Nothing holds me,” he writes, on discovering his great laws; “I will indulge in my sacred fury.  I will boast of the golden vessels I have stolen from the Egyptians.  If you forgive me, I rejoice.  If you are angry, it is all the same to me.  The die is cast; the book is written,—­to be read either now, or by posterity, I care not which.  It may well wait a century for a reader, as God has waited six thousand years for an observer.”