This paper was first published in 1898, but posthumously republished in 1917, as a challenge to Einstein's then new theory of general relativity, J. for Math. & Phys., Zeitschrift f. Mathematik und Physik, V43, pp 93-104.
If it is assumed that the hitherto unexplained advance by 41`` per century of Mercury's perihelion is caused by the delay of time spent for the spatial propagation of gravity, it follows that this value equals the velocity of light, of thermal radiation and of electric waves. Attention has to be paid to what can, on the one hand, really be proven by computation and observation and what, on the other hand, is presumed in the first place without any proof. If the gravity between two masses is transmitted from the first body to the second and back again with some lag of time, one finds that this necessarily gives rise to an advance of the planets perihelion. It is, however, impossible to prove that the actual value of the perihelions advance, although it could not be deduced from disturbances of any type, could not have another origin than the presumed time lag. If this presumed origin gave a value for the propagation velocity of gravity different from the velocity of light, this would have no further meaning. Just the coincidence of both velocities vindicates this presumption and, hence, the notion of a finite propagation velocity of gravity.
