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HISTORY OF SCIENCE. 



ance of air. But the Aristoteleans quoted chapter and verse of their 

 master's works, and ridiculed the idea that the fact could be otherwise 

 than he had stated. As Galileo says, " they fancied that science was 

 to be studied like the yEneid or the Odyssey, and that the true read- 

 ing of nature was to be discovered by the collation of texts." There 

 is at Pisa a well-known structure, the famous Leaning Tower, of which 

 the peculiarity seems as if providentially designed to offer remarkable 

 facilities for putting the question to the test of experiment. Galileo 

 invited the defenders of Aristotle's doctrine to witness for themselves 

 the result of allowing two unequal weights to fall to the ground from 

 the lofty impending gallery. Yet though they saw with their own eyes 

 the unequal weights strike the ground at the same instant, they asserted 

 as strenuously as before that a weight of ten pounds must naturally fall 

 ten times as fast as a weight of one pound ! And they declined to 

 accept Galileo's experiment as settling the case, considering that what 

 they termed the natural velocities were here from some unknown cause 

 interfered with. It seems at first sight marvellous that men should 

 have for centuries accepted upon authority a falsity which could have 

 been so easily exposed ; but it should be remembered 

 that Aristotle's axiom was one of the foundations of the 

 accepted system of philosophy, and men are always very 

 reluctant to have their settled convictions disturbed. 

 Galileo's exposure of the folly of some of these opinions 

 produced in the minds of the Aristoteleans not convic- 

 tion of their errors, but ill-will towards himself, and, to 

 escape from their malicious machinations, he left Pisa 

 to accept the chair of mathematics at Padua. The in- 

 come attached to this was extremely small, and he was 

 obliged to add to his resources by occupying much of 

 his time by private teaching. Notwithstanding these 

 engagements, he found opportunities for pursuing his 

 investigations, and, while at Padua, he composed several 

 works. While here it is said he invented the air-ther- 

 mometer, which contrivance has been usually attributed 

 to Santorio. Galileo's was merely a glass tube termi- 

 nating in a bulb, the tube dipping beneath the surface 

 of a liquid which also partly occupied the tube, \rhile 

 the bulb, which was at the top, remained filled with air. 

 (The arrangement of the apparatus is shown in Fig. -47.) 

 It was the expansion or contraction of the air which 

 indicated the temperature, and though the instrument 

 was sensitive to small changes of temperature, the 

 height of the liquid in the tube was affected also by FI G. 47- 

 changes of barometric pressure. The thermometer was 

 improved by one of Galileo's pupils, the son of the Duke of Florence, 

 who brought it into the form we now use it as a spirit-thermometer, in 



