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1 66 ilfr. Vince on the Motion of 
the weight of a body the fridtion does not always increafe exactly 
in the fame ratio} and that the fame body, if by changing its 
pofition you change the magnitude of the furface on which it 
moves, will have its quantity of friction alfo changed. Hel- 
sham and Ferguson, from the fame kind of experiments, 
have endeavoured to prove, that the fridtion does not vary by 
changing the quantity of furface on which the body moves ; 
and the latter of thefe afierts, that the fridtion increafes very 
nearly as the velocity; and that by ihcreafing the weight, 
the fridtion is increafed in the famer atio. Thefe different con- 
clufions induced me to repeat their experiments, in order to fee 
how far they Were conclufive in refpedt to the principles de- 
duced from them : when it appeared, that there Was another 
caufe operating befides fridtion, which they had not attended 
to, and which rendered all their dedudtions totally inoonclufive. 
Of thofe who have written on the theory, no one has efta- 
bli filed it altogether on true principles : Euler (whofe theory 
is extremely elegant, and which, as he has fo fully confidered 
the fubjedt, would have precluded the neceffity of offering any 
thing further, had its principles been founded on experiments) 
fuppofes the friction to vary in proportion to the velocity of the 
body, and its p refill re upon the plane, neither of which are 
true: and others, who have imagined that fridtion is a uni- 
formly retarding force (and which conjedture will be confirmed 
by our experiments), have ft ill retained the other fuppofition, 
and therefore rendered their dilutions not at all applicable to 
the cafes for which they were intended. I therefore endea- 
voured by a fit of experiments to determine, 
ifi, Whether fridtion be a uniformly retarding force. 
.idly, The quantity of fridtion. 
jdly. 
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