PHYSICAL AND MATHEMATICAL SCIENCES. 1 63

elected. His lectures were so thronged, that he wasoften obliged to adjourn to the open air, and his re-putation extended far and near. About this period heseems to have first invented the thermometer, or rathera thermoscope *; for the former name could not be pro-perly applied until the adaptation of a fixed scale at alater period. The instrument is mentioned in a letterfrom Sagredo in 1613, but how long before the inven-tion had taken place is uncertain. Even this date,however, is prior to that of Drebel (1620), who mayyet have devised it independently, and was certainly thefirst who introduced it in Holland.

Galileo ’s thermoscope was a tube ending in a bulb,inverted in a vessel of water, the bulb being occupiedby air. Leopold de’ Medici , some years afterwards,filled it with spirits of wine, and sealed the open endwhile the spirits were boiling. Mercury was finallysubstituted by Lana in 1670.

Galileo was soon after engaged in a dispute with Bal-thasar Capra, who appears to have pirated his inventionof a scale of proportional parts. He also turned histhoughts to the subject of magnetism, bestowing highcommendations on Gilbert, with whom the experi-mental investigation of this science had originated.

In 1609, proposals were made to him to return toPisa . Cosmo de’ Medici, who had now succeeded hisfather as grand duke of Tuscany , regretted that somasterly a genius had been allowed to leave the uni-versity, which he should naturally have adorned. Ga-lileo ’s answers to these overtures are extant, and give ahighly interesting and characteristic picture of hisviews, designs, and disposition: he, however, did notaccept the offer.

The year 1609, the same in which Kepler’s com-mentary on Mars appeared, is also for ever memorable,from Galileo ’s invention of the telescope. This, in-deed, is, in the minds of many, the sole important dis-covery associated with his name; whilst, again, some* hot; ex&riu t to observe; /t«trge«, to measure. /

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