Chap. II. A N T I E N T M E r A P H Y S I G S. 343 



It appears to me, therefore, evident, that this Law of Motion 

 •will apply only to Motion produced by the a£tion of Body upon 

 Body. But the queftion is, Whether it will apply to all Motion, 

 even of that kind ? 



In order to determine this qucflion, we muft diftinguilli two ways 

 in which Body adling upon Body produces Motion. The firft is by 

 Pulfion, that is, when the Body that ads upon the other propclls it, 

 fo as to make it go on of itfelf, without the Body propelling it ; 

 and this way of moTing Body is commonly called Impulfe. For 

 producing this kind of Motion, it is neceflary that the Body impelled 

 fhould have a certain degree of elafticity ; for it is by that quality 

 that it acquires the force which carries it on by itfelf. A Body, 

 therefore, fuch as nvool^ ^'>'^Jkf^ ^^^y^ "ot being compa<Sted or prefled 

 together, will not go on in that way, whatever the force or quality 

 of the Body impelling may be; but it can only be carried on in- 

 the other way, which I am now to explain, that is, by Trufion. 



When a Body is mOT^ed by another in this way, it does not leave 

 the Body moving, but is carried on along with it ; and in this way, 

 not only foft unelaftic Bodies, luch as thofe juft now mentioned, are 

 moved, but alfo elaftic Bodies, if they are not propelled with vio- 

 lence by the Moving Body, fo as to produce what Sir Ifaac Newton 

 calls an idus ovjiroke, but are moved only by the preflure of the o- 

 ther Body. 



Again, this Trufion operates upon Body in two feveral ways ; 

 for, either the Moving Force a£ts upon that part of the Body which' 

 is oppofite to the direction in which the Body is moved, or it ads 

 upon the fame fide. In the firfl: cafe, I call it protrujion ; in the o- 

 ther cafe, it is well known by the common name of draiving or 

 pulling, 



Thefcr 



