PHYSICAL ann LITERARY. 136 
_yed more eafily amongft one another, than 
ea? a” ee 
thefe of a firm body, whofe parts are clofely 
united by attraction: yet {till there. muft be 
a confiderable adhefion of the particles of 
fand,-on account of their irregular figures, 
and the inequalities on their furfaces. When 
aman applies his hand to a hole in the fide 
of a veffel of water, he fuftains a preffure 
from the tendency of the particles to flide 
down and make their efcape: but, when his 
hand is laid on the fide of a fand bank, there 
is no fuch preflure to be felt; becaufe the 
particles are intangled together, and fup- 
port one another. Thus a body laid upon a 
rough inclined plain, may remain fixt, by. 
the refiftance from attrition alone; whereas, 
if the plain be fmooth, fome other power 
_ becomes neceflary to hinder the defcent of 
the body. 
THE mechan action and reaction of 
i ee: or the changes with regard to moti- 
| 
2 
on and reft, which take place in the. collifi- 
ons of non-elaftic bodies, have been already 
confidered; and were found to be entirely 
confiftent with the moft abfolute zvertie. But 
that. real actions and equal oppofite reactions, 
“obtain in the active powers of attraction and 
repulfion, 
