Secondly, I define IMPETUS, _or quickness of motion, to be the swiftness or velocity of the body moved, but considered in the several points of that time in which it is moved. In which sense_ impetus _is nothing else but the quantity or velocity of endeavour. But considered with the whole time, it is the whole velocity of the body moved taken together throughout all the time, and equal to the product of a line representing the time, multiplied into a line representing the arithmetically mean_ impetus _or quickness_. Which arithmetical mean, what it is, is defined in the 29th article of chapter XIII. And because in equal times the ways that are passed are as the velocities, and the _impetus_ is the velocity they go withal, reckoned in all the several points of the times, it followeth that during any time whatsoever, howsoever the _impetus_ be increased or decreased, the length of the way passed over shall be increased or decreased in the same proportion; and the same line shall represent both the way of the body moved, and the several _impetus_ or degrees of swiftness wherewith the way is passed over. And if the body moved be not a point, but a strait line moved so as that every point thereof make a several strait line, the plane described by its motion, whether uniform, accelerated, or retarded, shall be greater or less, the time being the same, in the same proportion with that of the _impetus_ reckoned in one motion to the _impetus_ reckoned in the other. For the reason is the same in parallelograms and their sides. For the same cause also, if the body moved be a plane, the solid described shall be still greater or less in the proportions of the several _impetus_ or quicknesses reckoned through one line, to the several _impetus_ reckoned through another.