A development of the theory of special relativity is based on a principle of relativity for inertial frames as applied to mechanical phenomena only. The Lorentz-transformation equations and the familiar mass-variation formula are deduced from the observation that mass does vary with speed; but in these the speed of lightcis everywhere replaced by a universal speedk—namely, the speed at which the mass of any particle must become infinite—which arises as a consequence of the assumptions. The experimental result that the free-space speed of light with respect to the earth is equal tokthen implies the same free-space speed of light with respect to every inertial frame—the usual starting point of the theory.