190 PHYSICAL SCIENCE IN THE MIDDLE AGES. 



and indisputable; such, for instance, as that "in fluids the upper parts 

 do not gravitate on the lower ;" that " a lighter fluid will not gravitate 

 on a heavier ;' that " levity is a positive quality of bodies as well as 

 gravity." So long as these assertions were left uncontested and un- 

 tried, men heard and repeated them, without perceiving the incon- 

 gruities which they involved : and thus they long evaded refutation, 

 amid the vague notions and undoubting habits of the stationary period. 

 But when the controversies of Galileo's time had made men think with 

 more acuteness and steadiness, it was discovered that many of these 

 doctrines were inconsistent with themselves, as well as with experi- 

 ment. We have an example of the confusion of thought to which 

 the Aristotelians were liable, in their doctrine concerning; fallino- bodies. 



' O O 



" Heavy bodies," said they, " must fall quicker than light ones ; for 

 weight is the cause of their fall, and the weight of the greater bodies 

 is greater." They did not perceive that, if they considered the weight 

 of the body as a power acting to produce motion, they must consider 

 the body itself as offering a resistance to motion ; and that the effect 

 must depend on the proportion of the power to the resistance ; in 

 short, they had no clear idea of accelerating force. This defect runs 

 through all their mechanical speculations, and renders them entirely 

 valueless. 



We may exemplify the same confusion of thought on mechanical 

 subjects in writers of a less technical character. Thus, if men had any 

 distinct idea of mechanical action, they could not have accepted for a 

 moment the fable of the Echineis or Rernora, a little fish which was 

 said to be able to stop a large ship merely by sticking to it. 1 Lucan 

 refers to this legend in a poetical manner, and notices this creature 

 only in bringing together a collection of monstrosities ; but Pliny re- 

 lates the tale gravely, and moralizes upon it after his manner. " What," 

 he cries, 2 " is more violent than the sea and the winds ? what a greater 

 work of art than a ship ? Yet one little fish (the Echineis) can hold 

 back all these when they all strain the same way. The winds may 



tj J V 



1 Lucan is describing one of the poetical compounds produced in incantations. 



Hue quicquid fcetu genuit Natura siuistro 



Miseetur: non spuma canum quibus unda timori est 



Viscera non lyncis, noil durse nodus hysenoa 



Defuit, et cervi pasti serpente medullas ; 



Non puppes retinens, Euro tendente rudentes 



In mediis Echineis aquis, oculique draconum. 



Etc. Phcirsalia, iv. 670. 



Pliu. Hist. -V. xxxii. 5. 



