Bodies affecied hy Fri^lion. ij^; 



than the weight on an equal part of A, as A is greater, than a f 

 if therefore the friction was in proportion to the weiglit, c^^en's 

 paribus J it is manifefl, that the fridtlon on a would be equal to 

 the fri£lion on A, the whole friclion being, upon fuch a fuppo- 

 fitlon, as the weight on any given part of each furface multi- 

 plied into the number of fuch parts or into the whole area, 

 which products, from the proportion above, are equal. But 

 from, the laft experiments.it has been proved, that the fri6lioii 

 on any given furface increafcs in a lefs ratio than the weight; 

 confequently the. fridlion on any given, part of a has a lefs ratio 

 to the fridion on an equal part of A than A has to ^, and 

 hence the fridlion on^ is lefs than the friftion on A, tliat is, 

 the fmalleft furface has always the lead friction. But as this 

 conclufion. is contrary to the generally received opinion, I have 

 thought it proper to confirm the fame by a fet of experiments. 

 But before I proceed to relate them, I will beg leave to recom- 

 mend to thoie, who may afterwards be induced to repeat them9 

 the following cautions, which are extremely necefl'ary to be 

 attended to. Great care muft be taken that the two furfaces 

 have exactly the fame degree of roughnefs ; in order to be 

 certain of which, fuch bodies mirfl: be chofcn as have no knots 

 in them, and whofe grain is lb very regular that when the 

 two furfaces are planed with a fine rough plane, their rough- 

 nefs may be the fiime, which will not be the cafe if the body 

 be knotty, or. the grain irregular, or if it happens not to run 

 in the l^\me direction on both furfaces. When vou cannot de- 

 pend on the furfaces having the fame degree of roughnefs, the 

 beft way will be to pafl:e fome fine rough paper on each fur- 

 face, which perhaps will give a more equal degree of rough- 

 nefs than can be obtained by any other method. Now as the 

 proof which I have already given depends only on the motiori' 



of. 



