AS A MENTAL OPERATION n 



the phases of Venus, solar spots, and finally the moon's 

 librations. But he was no mere observer, and made 

 experiments as well as observations on gravity. Before 

 the close of the sixteenth century he dropped bodies 

 of different weights from the Tower of Pisa, and found 

 that they did not fall, as had been supposed by the 

 Aristotelians, with velocities in proportion to ' their 

 weights, but, as Bacon learnt from him, with velocities 

 nearly uniform, though not quite so on account of the 

 resistance of the air which opposes greater delay to 

 lighter bodies. Many years afterwards, after the death 

 of Bacon, he followed this discovery of the uniformity of 

 gravity by that of its uniform acceleration. The latter 

 discovery was, like the former, experimental. Galileo 

 had smooth inclined planes made ; and then, by rolling 

 balls down them and measuring the times and spaces 

 of descent, he discovered inductively that the space 

 fallen is always as the square of the time of falling ; so 

 that, if a body in one second of time falls about 16 feet, 

 in two seconds it will have fallen 64 feet, four times as 

 far (time 2-squared), in three seconds 144 feet, nine 

 times as far (time 3-squared). Nothing could better 

 illustrate the power of the empirical method than these 

 quantitative experiments of Galileo. They may enable 

 us to dispense with later examples, such as Newton's 

 experimental discovery of the dispersion of light and 

 the numerous experimental analyses of modern chemis- 

 try. Finally, Galileo's employment of the empirical 

 method, even before the appearance of Bacon's Novum 

 Organum, does not detract from the originality of the 

 latter. Galileo used method by genius; but Bacon 

 made the first logical theory of the empirical method, 

 and has enabled many ordinary men almost to rival 

 Galileo by showing them how to use its rules. 



