8 4

Sir Isaac Newton’s Book I.

y 5- T h e inclined plane affords also a means of railinga weight with less force, than what is equal to the weight itself. Suppose it were required to raise the globe A (in fig.4.7. ) from the ground BC up to the point, whose perpendi-cular height from the ground is E D. Is this globe be drawnalong the slant D F, less force will be required to raise it, thanif it were listed directly up. Here if the force applied to theglobe bear the same proportion only to its weight, as E D bearsto F D, it will be sufficient to hold up the globe; and there-fore any addition to that force will put it in motion, and drawit up; unless the globe, by pressing against the plane, where-on it lies, adhere in some degree to the plane. This indeedit must always do more or less, fince no plane can be made soabsolutely smooth as to have no inequalities at all; nor yet soinfinitely hard, as not to yield in the least to the pressure of theweight. Therefore the globe cannot be laid on such a plane,whereon it will stide with perfect freedom, but they must insome measure rub against each other ; and this friction willmake it necessary to imploy a certain degree of force more,than what is necessary to support the globe, in order to giveit any motion. But as all the mechanical powers are subjectin some degree or other to the like impediment from friction •I shall here only shew what force would be necessary to su-stain the globe, is it could lie upon the plane without caus-ing any friction at all. And I say, that is the globe weredrawn by the cord G H, lying parallel to the plane D F ; andthe force, wherewith the cord is pulled, bear the lameproportion to the weight of the globe, as E D bears to OF;

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