ACDB  =lp, where l denotes the length c D of the

rod and p the distance AB through which it has been moved . This distance, as measured by the rolling of the wheel, which acts as a curvometer, will be called the " roll " of the wheel and be denoted by w . In this case p =w, and the area P is given by P =wt . Let the circumfer- A ence of the wheel be divided into say a hundred equal parts u; then w registers the number of u's rolled over, and w therefore gives the number of areas lu contained in the rectangle . By suitably selecting the radius of the wheel and the length 1, this area lu may be any convenient unit, say a square inch or square centimetre . By changing 1 the unit will be changed . Again, suppose the rod to turn (fig . 7) about the end .