Newton saw that a mountain mass might be used, and weighed against the earth by finding how much it deflected the plumb-line at its base. The density of the mountain could be found from specimens of the rocks composing it, and the distance of its parts from the plumb line by a survey. The deflection of the vertical would then give the mass of the earth. Not long after Newton's death the mountain experi ment was actually tried. The honor of making the first experiments on gravitation belongs to Pierre Bouguer (q.v.), whose splendid work does not appear to have received the credit due it.
Having established the law of gravitation throughout the solar system, it was natural to infer the universality of its action. We know on the one hand, by observing the motion of the planets and satellites, the asteroids, and the comets, that the law holds with great exactness for all these bodies; on the other hand, experi ments of Cavendish with balls of lead, and of others, verify its exactness down to very short measurable distances: and though we are un able with our present appliances to determine the orbits of double stars and of other stellar systems, still we seem to be fully justified in as suming that in these cases also the law stated above holds, at least, very approximately.
The track was first laid down by Newton, based on astronomical observations, and only made firmer and broader by every later observa tion. Important work in Europe has recently been done in gravitational experiments by the late Prof. U. Jolly, and by Profs. Braun, Boys and Poynting, who, with others, have advanced beyond the results of Henry Cavendish (q.v.), whose device, known as the Cavendish experi ment, for determining the density of the earth, has so long interested scientists. The latest re search has verified Newton's celebrated guess that "the quantity of the whole matter of the earth may be five or six times greater than if it consisted all of No inquiry on gravitation has showed that it is related to anything but the masses of the attracting and the attracted bodies. It appears to have no relation to physical or chemical con dition of the acting masses or to the interven ing medium. This independence of gravitation of any quality but mass, bars the way to any explanation of its nature or source.
There is a point respecting the law which is almost universally passed over without notice, although it is one of the most important ques tions with respect to the construction of any theory to account for gravitation; namely, the exact proportionality of the gravitating forces of any two bodies to their masses. The most delicate experiments show no deviation from the exactness of this law; nor has the most accurate observation of planetary bodies sufficed to detect any such deviation. This is the fact proved by the well-known guinea-and-feather experiment, in which it is shown that though a mass of gold and a feather do not fall equally fast under ordinary circumstances, because of the unequal resistance of the air in the two cases, yet that, the air being removed by means of the air-pump, they fall with equal velocity. The experiment proves that the force of gravity in the two cases is exactly proportional to the mass of the guinea and of the feather. Newton showed the same thing himself with far greater minuteness by vibrating balls of various mate rials similarly suspended. In this, which is known as Newton's pendulum experiment, it is shown that pendulums of equal length vibrate in equal times whatever be the material and the masses of which the bobs of the pendulum are made. By this experiment, when performed with all the nicety at command, it is probable that any deviation amounting to a ten-thou sandth or a hundred-thousandth part of the whole amount considered could be detected. Planetary motions prove the law to even a greater degree of accuracy. It is curious that this portion of the law, though it is only proved by experiment and observation, is hardly ever, if ever, referred to by popular writers. It is either assumed without pretense of proof, or is passed over without remark.
Notwithstanding the vast interest and im portance of the study which this subject pre sents, and all the labors of eminent scientists in endeavors to solve its complex problems, it still remains to be said that the world is yet"without any theory which can really be considered as explaining gravitation. Consult Mackenzie, 'The Laws of Gravitation> (1900).