Page:Popular Astronomy - Airy - 1881.djvu/253

Rh the periodical comets, called Encke's comet, that many of the German astronomers expressed themselves doubtful of the truth of the law of gravitation. I made, and continued at proper intervals, for four years, a new set of observations of Jupiter's satellites, and I had the good fortune to find that the satellites were further from Jupiter than was supposed, that the space through which Jupiter drew them in an hour was greater than was supposed, and that the perturbations ought to be increased by about $1⁄50$ part. These measures of mine were verified by continental observers. On using the corrected perturbations, the computed and the observed places of the planets agreed perfectly.

For the perturbations produced by Mercury, Venus, and Mars, which have no satellites, we have no similar foundation for our computations; and here we can only go on a method of trial and error. For instance, assuming for calculation that one of these planets has as great a disturbing power as the earth, we can compute how much it will disturb another planet's motion in every position; and if the disturbing power be altered in any proportion, we know that the disturbance of the other