by the arch BC, the line CD bciog fuppofed horizontal : And 
if ic falls back, whether it be by the perpendicular DB , or 
by the arch CB, ic will re-take inthe point B a velocity equal 
to that, which had raifed it to the point in C or in D. Which 
two Suppofitions are well eftabliflied by Galilee and divers 
other Geometricians , abftrafledly from the Refiftance of the 
Air and of other Impediments; and they are alfo, (faith our 
Author) very near conform to Experiment, the refiftance of the 
Air notwithftanding. But he takes them in this Treatife in an 
exafl precifenefs , to make the Demonftrations the more iotel* 
ligible. 4. That the fmall vibrations of ^ Pendulum divcmdidt 
in times fenjibly equal, although they defcribe unequal arches : 
But for the facility ©f the DeroonftrationS;, 'tis here fuppofed, 
that thefe times are precifely equal* 
From the Experiments which he bath made with the motion 
of Balls that have no Spring,this general Confequence is drawn, 
That if a body not-fpringy thould impel another body not- 
fpringy and unlhakeable, it would remain without motion, and 
not turn back , there being no new caufe at all for a motion 
that way. And thus he remarketh, that 'tis much eafier to flop 
aballthat is rolling , and to make ic lofe its motion , than to 
drive it back with the fame celerity 5 becaufe^ that befides the 
force which is requifite to ftop it, there needs another to give 
to ic again its former velocity. 
So that 'cis certain to ourAuchor,(as it is to divers othcrs)thac 
all motions of Reflexion are made by a Spring. And, though at 
firft it feems difficult to believe, that bodies of the hardnefs of 
Ivory and Steel(for example) which do refleft bodies as hard as 
themfelve&^are fiexible,and capable of having fuch an impreflion 
made on them as is required for a Springs yet our Author 
eafily refolves this fcruple, by referring his Reader to thofe 
fmall imprcffions and dints, remaining in Iron after ic bath been 
ftruck by a bard body, though Iron be harder than Ivory, and 
a'moft as hard as Steel. To which he adds, that 'twere impof- 
fible, thataGlafs-ballor aBaflof baked Earth flhould break, 
if it did not change its figure when it is with great force thrown 
againft: another hard body. And in regard we fee , that thefe 
Balls keep their roundnefs when having been ftruck they break 
not, the;^mtift needs (faith he) exa&ly retake their former fi- 
