PHYSICAL anp LITERARY. ° 29 
and, by its impenetrability, muftibe carried” 
along with the other. ‘Gs 
But, in order to form an accurate notion 
of this operation of percuflion, two prelimi- 
nary points mutft be fettled. The firft is, to 
determine the precife nature of the action ex- 
erted by a body at reft'when impelled by one 
in motion, Does the body at reft return 
ftroke for ftroke, or impulfe for impulfe? 
Does it truly react, or does it barely refift ? 
That it refifts only, and does not.react, » will 
_be evident from a fingle confideration, which 
is, that motion is effential to impulfe, not at 
all to refiftance. Now, if the body at reft 
reacts, by returning ftroke for f{troke; it mutt 
rufh upon the impingeing body, and carry it 
backwards at leaft for fome {mall fpace: 
which cannot be; becaufe, once admitting a 
retrograde motion, the wires infite of the two 
bodies muft carry them on in the fame direc- 
tion, which is contrary to fact. Jt is obvi- 
ous, then, that there can be no repercuffion 
or reaction of the body at reft. All the ob- 
4 -ftruction it gives to the action of the impinge- 
sing body, muft be merely by refiftance. The 
other point to be fettled is, whether the ope- 
___ fation be inftantaneous, or whether it be per- 
‘ formed 
