chapter  II
Pages 13

F we draw a straight line and fix on any point 0 I in it, we may suppose a force acting on the point 0, along the line AB, either towards A or B ; then

I. If it acts towards A, its direction is OA, if towards B, its direction is

OB. It is not necessary, therefore, to distinguish between the line of action and direction of force. If a force acts from 0 to A and another from 0 to B, the directions may be indicated by positive and negative signs, as for instance + P or - P. Now if the force OA contains the same units as OB but of opposite direction, the whole system will be in equilibrium, because the action of the force OA balances the action of the force OB.