Now, since these two forces must be equal in order equally to resist the compression of the vortex, we shall have the equation vvccld /D, or vvc because ld equals C/D, or finally vvd=. By replacing andx in this last equation by their values, viz., the circumferences divided by the times of revolutions, or the circumferences by the diameters proportional to them, we shall have =d tt, or tt: :