28 GREAT MEN OF SCIENCE 



than was generally assumed for a long time to be the case. 

 The Pisa professorship, which was not a very good one, 

 was exchanged by Galileo after three years for a better 

 one at Padua, where he remained for eighteen years, which 

 were certainly the most fruitful and happy of his life. Here 

 he further developed his investigations on the phenomena 

 of motion. But a second science was also founded by him, 

 that of the strength of materials. These investigations also 

 were published very late, in Dialogues concerning Two New 

 Sciences} Galileo developed the fundamental idea of tensile 

 strength, combined this with the theory of the lever, and so 

 deduced laws concerning the breaking strength of rods, 

 prisms, and cylinders of various dimensions loaded in various 

 ways. He recognised and explained the advantages of 

 special forms of support differing from the prism in respect 

 of greater strength combined with less weight, and the ad- 

 vantages of the tubular form as compared with the solid 

 cylinder. These form the first beginning of a practical theory 

 of the strength of materials, and also of the theory of elasti- 

 city. A noteworthy part of this work is the way in which 

 Galileo specially treats of the nature of breaking strength 

 itself; that is, the cause of the solidity of solid bodies. One 

 cause he sees in the Tower of the Vacuum,' which presses 

 bodies together from outside, whereby he forms a perfectly 

 correct conception of the pressure of the air at a time when 

 anything of the kind was completely unknown. At that time 

 all visible effects of this pressure, as for example the rise of 

 water by the suction of a pump, were ascribed to an indefinite 

 hatred, on the part of nature, for empty space, the so-called 

 horror vacui. Galileo measures the Tower of the Vacuum' 

 by a special experiment with a cylinder and piston, and 

 further, by the limiting height of eighteen ells (thirty feet) 

 beyond which a perfect pump cannot suck the water from 

 a well; this was a matter of observation. From the force 

 1 Eng. trans, by H. Crew and A. de Salvis, 19 14. 



