48 On the Motion of Bodies affected hj Frictiott. 



which the motions of bodies are retarded by friction h<1s 

 never, that I know of, been truly estabiishcd. Musschcn- 

 broek says that in small velocities the friction varies very 

 nearly as the velocity, but that in great velocities the friction 

 increases : he has also aUeinpted to prove that, by increasing 

 tlie weight of a body the friction does not always increase 

 exactly \\\ the same ratio ; and that the same body, if by 

 changing its position you change the magnitude of the sur- 

 face on which it moves, will have its quantity of friction 

 also changed. Helsliam and Ferguson, from the same kind 

 of experiments, have endeavoured to prove that the friction 

 does not vary by changing the quantity of surface on which 

 the body moves ; and the latter of these asserts that the 

 friction increases verv nearly as the velocity ; and that by 

 increasing the weight, the friction is increased in the suvie 

 ratio. These different conclusions induced me to repeat 

 their experiments, in order to see how far they were con- 

 clusive in respect to the principles deduced from ihcm : 

 when it appeared that there was another cause operating 

 besides friction, which they had not attended to, and which 

 rendered all their deductions totally inconclusive. Of those 

 who have written on the theory, no one has established it 

 altogether on true principles. Euler (whose theory is ex- 

 tremely elegant, and which, as he has so fully considered 

 the subject, would have precluded the necessity of ofi'ering 

 anv thmg further, had its principles been founded on ex- 

 periments) supposes the friction to vary in proportion to the 

 velocity of the body, and its pressure upon the plane, neither 

 of which are true : and others, who have imagined that 

 friction is a uniformly retarding force (and which conjecture 

 Vviil be confirmed by our experiments), have still retained 

 the other supposition, and therefore rendered their solutions 

 not at all applicable to the cases for which they were intended. 

 I therefore endeavoured by a set of experiments to deter- 

 mine, 



1st. Whether friction he a vtiiformly retarding force. 



2d!v, The qnantily of friction. 



Sdly, JVIiether the friction varies in proportlori to the pres- 

 sure or weisht. 



4thly, Whether the friction le the same, on uliicJiever of 

 its svrfacei a bodij moves. 



The experiments, in which T was assisted by mv ingenious 

 friend the Rev. Mr. Jones, Fellow of Trinity college, were 

 niade with the uniost care and attention ; and the several 

 results agreed so very exactly v/ith each other, that I do not 

 scruple lo pronounce them to be conclusive. 



2. A plane was adjusted parallel to the horizon, at the 



extremity 



