vitiated almost all the observations made on the planets up to within a few years, the correction for irradiation. This was the case here. The received measures, beginning with Bradley and ending with Todd, had almost without exception been made in transit, and, as no regard had been paid to the contracting effect of irradiation, had been invalidated in consequence. The new method supplied almost exactly the amount needed to explain the right ascensions, a second of arc, and in precise accordance with the place which the discrepancy demanded.
About the mass there has been, and still is, great uncertainty. This is because it can only be found from the perturbing effect it has on Venus, the Earth, or Encke's comet. Modern determinations, however, are smaller than the older ones; thus Backlund in 1894 got from the effect on Encke's comet only one-half the mass that Encke had, fifty-three years before. Probably the most reliable information comes from Venus, which Tisserand found to give for Mercury 17100000 of the mass of the Sun, or 121 of the mass of the Earth. If we take 17000000 as the nearest round number, we find the planet's density to be 0.66 that of the Earth.
The same observations that disclosed at Flagstaff the planet's size revealed a set of markings on his face so definite as to make the rotation period unmistakable.
