From the peculiarity of the laws which this power called gravity obeys, it has been inferred that it acts from centres of force; it is proved that its power diminishes in the inverse ratio of the square of the distance, and that the gravitating power of every material body is in the direct proportion of its mass. In astronomical calculations we have first to learn the mass of our earth. Experiment informs us that the density of our hardest rock is not above 2·8; but from the enormous pressure to which matter must be subjected, at great depths from the surface, the weight of the superincumbent mass constantly increasing, it is quite certain that the earth’s density must be far more than this. Maskelyne determined the attraction of large masses by a plummet and line on the mountain Schehallion.[20] Cavendish, with exceedingly delicate apparatus, observed the attraction of masses of known weight and size upon each other. Applying the powers of arithmetical calculation, and the data obtained from the small experiments to the larger phenomena, Maskelyne determined the earth’s mean density to be 4·71, whilst Cavendish made it 5·48, but the more recent refined investigations of Baily have determined it to be 5·67.[21]
From data thus obtained by severe inductive experiments and mathematical analyses, the astronomer, by observing the deviations of a distant star, is enabled to determine the influence of those stellar bodies near which it passes, and, hence, to calculate the relative magnitudes of each. The accuracy of the law is in this way put to the severest test, and the precision of astronomical prediction is the strongest proof of its universality and truth.
Rolling onward its lonely way, in the far immensity of our system, the planet Uranus was discovered by the elder Herschel,--so great its distance that its diminished light could scarcely be detected by the most powerful telescopes; but since its discovery its path has been carefully watched, and some irregularities noticed. Most of these disturbances were referable to known causes; but a little alteration in its rate of motion observed when the planet was in one portion of its vast orbit was unexplained. Convinced of the certainty of Newton’s law, and having determined that the attraction of known masses was insufficient to produce the disturbance observed, these deviations were referred to the gravitating influence of a mass beyond the known limits of our Solar System. By the investigations of Adams in England,[22] and Le Verrier in France,[23] the place of the hypothetical mass was determined, and its size computed. As a grand confirmation of the great law, and to the glory of those two far-searching minds, who do honour to their respective countries and their age, the hypothesis became a fact, in the discovery of the planet Neptune in the place determined by rigorous calculation. Astronomy affords other examples of the sublime truth of the law of gravitation, than which science can afford no more elevated poetry.