1 66 Mr, ViiircE on the Motion of 



the weight of a body the fridlon does not always Increafc exacftly 

 in the fame ratio; and that the fame body, if by changiiig 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 fri6lion does not vary by 

 changing the quantity of furface on which the body moves ; 

 and the latter of thefe aflerts, that the fridion increafes very 

 nearly as the velocity; and that by increafing the weight, 

 the friction is increafed in the fame ratio. Thefe different con- 

 clufions induced me to repeat their experiments, in order to fee 

 how far they were conclulive in refpe(5l to the principles de- 

 dticed from them : when it appeared, that there was another 

 caufe operating befides friction, which they had not attended 

 to, and which rendered all their dedu£l:ions totally inconcluiive. 

 Of thofe who have written on the theory, no one has efta- 

 blifhed it altogether on true principles : Euler (whofe theory 

 is extremely elegant, and which, as he has fo fully confidered 

 the fubje6t, would have precluded the neceffity of offering any 

 thing further, had its principles been founded on experiments) 

 fnppofes the fri6iion to vary in proportion to the velocity of the 

 body, and its preflure upon the plane, neither of which are 

 true: and others, who have imagined that friftion is a uni- 

 formly retarding force (and which conje61:ure will be confirmed 

 by our experiments), have ftill retained the other fuppofition, 

 and therefore rendered their folutions not at all applicable to 

 the cafes for which they were intended. I therefore endea- 

 voured by a fet of experiments to determine, 



I ft, Whether fri^ion be a uniformiy retarding force, 



2dly, The quantity of fiidlion. 



