33Q THE POPULAR SCIENCE MONTHLY 



swinging to and fro, and yet it had awaited the coming of Galileo to 

 read its inner meaning ! 



On the question of falling bodies Galileo ran counter to Aristotle. 

 That philosopher had taught that bodies in falling acquire velocity in 

 proportion to their weight. Galileo found by a simple but careful 

 study that all bodies acquire the same velocity in falling. With the 

 imprudence of youth he forthwith proclaimed the errors of Aristotle 

 from the house-tops, much to the scandal of his classical friends. In 

 answer to their protests he proposed an experiment, and this experi- 

 ment was made. The faculty of the University of Pisa together with 

 the interested or curious of the city gathered at the leaning tower — 

 " Pisa's leaning miracle/' as Whittier calls it. From the top and at the 

 overhanging side Galileo let fall a one-pound and a one-hundred-pound 

 shot. The two shot started, fell — and struck the ground together. As 

 Lodge exclaims, " The simultaneous clang of those two weights sounded 

 the death-knell of the old system of philosophy and heralded the birth 

 of the new." And yet, it is recorded that while some saw, and were con- 

 vinced, others — nor is their race extinct to-day — saw, but, consulting 

 their copies of Aristotle, disbelieved. 



Following in Galileo's foot-steps — perhaps more cautiously lest he 

 be made to suffer for it — Huygens (1629-1699) further developed and 

 extended the science of mechanics. In particular may be noted his 

 development of the theory of circular motion ; the invention, or at least 

 perfection, of the pendulum clock, and the determination of the accel- 

 eration of gravity from pendulum observations, a method which is 

 to-day the most accurate one in use. Huygens shares with Galileo all 

 the honors due a scholar and original worker of the first rank. They 

 resemble each other in character, in method and in the great value of 

 their labors. 



As bearing upon the law of gravitation the theory of circular motion 

 was of the very greatest value. Without it, Newton must either have 

 failed in his task or have discovered these principles for himself. As 

 a matter of fact he did the latter. Huygens was also the first to grasp 

 the significance of the following occurrence. In 1671 Jean Eichter, in 

 the course of astronomical work, carried a pendulum clock from Paris 

 to Cayenne in South America. The clock kept correct time in Paris, 

 but at the latter station it daily fell two and a half minutes behind 

 mean solar time. The pendulum was shortened to correct it, but on 

 returning to Paris it was found to gain time at the same rate that it 

 had before lost it. Huygens at once correctly explained this as due to 

 the rotation of the earth on its axis; thus furnishing the first experi- 

 mental evidence of the earth's rotation. 



The span of Galileo's life was from 1564 to 1642. He died dis- 

 credited by his church, deprived of his liberty and afflicted with blind- 



