SEQUEL TO COPERNICUS. 427 



a circular motion; and ascend or descend when 

 they have, in addition to this, a rectilinear motion 

 by which they endeavour to get into their own 

 place. But his disciples soon began to question 

 the Aristotelian dogmas, and to seek for sounder 

 views by the use of their own reason. " The great 

 argument against this system," says Msestlin, "is 

 that heavy bodies are said to move to the center 

 of the universe, and light bodies from the center. 

 But I would ask, where do we get this experience 

 of heavy and light bodies ? and how is our know- 

 ledge on these subjects extended so far that we 

 can reason with certainty concerning the center 

 of the whole universe ? Is not the only residence 

 and home of all the things which are heavy and 

 light to us, the earth and the air which surrounds 

 it? and what is the earth and the ambient air 

 with respect to the immensity of the universe ? It 

 is a point, a punctule, or something, if there be 

 anything, still less. As our light and heavy bodies 

 tend to the center of our earth, it is credible that 

 the sun, the moon, and the other lights, have a 

 similar affection, by which they remain round as 

 we see them, but none of these centers is necessarily 

 the center of the universe." 



The most obvious and important physical diffi- 

 culty attendant upon the supposition of the motion 

 of the earth was thus stated. If the earth move, 

 how is it that a stone, dropped from the top of 

 a high tower, falls exactly at the foot of the 



