obliquely, as the velocity of the perpendicular aſcent of the weight to the velocity of the hand that draws the rope, will ſuſtain the weight.
In clocks and ſuch like inſtruments, made up from a combination of wheels, the contrary forces that promote and impede the motion of the wheels, if they are reciprocally as the velocities of the parts of the wheel on which they are impreſſ'd, will mutually ſuſtain the one the other.
The force of the ſcrew to preſs a body is to the force of the hand that turns the handles by which it is moved as the circular velocity of the handle in that part where it is impelled by the hand is to the progreſſive velocity of the Screw towards the preſs'd body.
The forces by which the Wedge preſſes or drives the two parts of the wood it cleaves, are to the force of the mallet upon the wedge, as the progreſs of the wedge in the direction of the force impreſs'd upon it by the mallet, is to the velocity with which the parts of the wood yield to the wedge, in the direction of lines perpendicular to the ſides of the wedge. And the like account is to be given of all Machines.
The power and uſe of Machines conſiſt only in this, that by diminiſhing the velocity we may augment the force, and the contrary: Ffrom whence in all ſorts of proper Machines, we have the ſolution of this problem; To move a given weight with a given power, or with a given force to overcome any other given reſiſtance. For if Machines are ſo contriv'd that the velocities of the agent and reſiſtant are reciprocally as their forces, the agent will juſt ſuſtain the reſiſtant: but with a greater diſparity of velocity will overcome it. So that if the diſparity of velocities is ſo great as to overcome all that reſiſtance which com-
