Mechanics.

iSx

la their Structure; concerning all whichwe may observe in general, that they con-

sist

is not upright, so it will not require a Power equal toits whole Weight; but being in the Nature of the Mo-tion on an Inclined Plane, (since the Body bears on theprominent Parts all the while) the Power which movesit will be proportional to but a Part of its Weight only ;and this will vary with .the various Degrees of Smooth-ness or Asperity between the subbing Surfaces, and the •other concurring Circumstances.

6. I find by Experiment, that a Body ABCD (of PI. X.Wood, Brass, &c.) laid on the Surface EFGH, will Fig. i.be drawn along by a Weight P, nearly equal to one

third of its own Weight; if the Surfaces be hard andwell polifh’d, it will be less than a third Part ; but ifthe Parts be soft or rugged, it will be much greater.

Thus also the Cylinder of Wood AB, if very smooth,and laid on two well-polisti’d Supporters C, D, (having Ps X-been first oil’d or greas’d) and then charged with the 2.

Weight of two Pounds in the two equal Balls G, H, itwill require an additional Weight x (equal to about athird Part of the two Pounds) to give Motion to, orovercome the Friction of the said Cylinder.

7. Now this additional Weight, as it causes a greaterPressure of the Cylinder, will likewise encrease theFriction, and therefore require the Addition of anotherWeight y, equal to the third Part of its own ; for thesame Reason the Weight y will require another z, athird Part less ; and so on ad infinitum. Hence uponSupposition that the Friction is precisely equal to a thirdof the Weight , the first Weight with all the additionalones, viz. 2, f, *7, &c. will be a Series of Num-,bers in Geometrical Progression decreasing. Now theSum of all those Terms, except the first (i. e. the Sumof all the infinite Number of additional Weights x -|- y-f- z, &c.) is found (by a well known Theorem inArithmetic) to be equal to one Pound. So that if theWeight of the Cylinder be inconsiderable, the Way to

over*