Chap. 13. /* rre.iftfe of BODIES. 131 



brance of the many admirable myfterks of nature which that 

 great man hat! tai-ght the world, we have taken liberty to di 

 pute againrt him : bccaufe this difficulty feemeth to hove driven 

 himagainll his Genius, to believe that in fuch motions there 

 muft be allowed a quality imprinted into the moved body to 

 cawfe themrw'k our wholcjcope both in this,and in all other oc- 

 cafions where like qualities are urgedjs to prove them fijperflu- 

 ous and ill grounded in naturejand to be but mere terms to con- 

 found & leave in the dark whofoever is forced to fly unto them. 



CHAP. XIII. 



Of three forts of violent wot ion, Reflettion>Vndu- 

 lation, and Refraftion. 



THe motion we have hft fpoken o becaufe it is ordinarily I( 

 either in part or wholy contrary to gravity (which is ac- That reflexion 

 counted the naturall motion ofmoft bodicsjuieth to be called vi- 

 olcnt or forred. And thus.you have delivered unto you the na- 

 tures and cauies both of naturall and of forced motioiijyet it re- 

 maineth that we advcrtifeyou of fbmc particular kinds of this 

 forced motion,whieh fcem to be different from it, but indeed art 

 not. As firft, the motion cf reflection: which if we do but confi- 

 tier how farced motion is made; we (hall find that it is nothing 

 elfe but a forced motion,whofe line whereupon it is madejs asic 

 were fhapped in two by the encounter of a hard body. For even 

 as we fee in a fpout of water that is ftrongly (hot againft a wall, 

 the water following driveth the precedent parts firft to the wal, 

 and afterwards coming themfelves to the wall, fbrccth them a- 

 gain another way from the wall : right fd, the latter parts of the 

 torrent ofaire.whieh is caufed by the force that occafioned the 

 forced motion , driveth the former parts, firft upon the refiftant 

 body, and after w.ird> t^ain from it. But this is more eminent in 

 light then in any oH rr S)dy,becaufe light doth lefle riflent gra- 

 vity ; and fo cbferveth the pure courfc of the ftroke, better then 

 any other b< x A 4 } ; frr-m which others do for the moft part decline 

 ibme w ?. y i rsafo n of the: r we |o 1 -. -. . 

 Now tl lar bw of reflexion is, that the line incidcrk^Si: 



the line f ixrl-^^^n rntsd malr equal angles with that line of the Re ^ exion 1S 



r /i i l r r C -ii r t madc at C( l ua11 



rchltcnt li rr i .-cics w ch is m the 'amc fuperncies with themfelrcs. angi. 

 The cciiuonftration vvhercor^that gvcat wit Renatus Des Cartes 



I 2 hath 



