The great principle of the _Principia_ is universal gravitation, "That every particle of matter in the universe is attracted by or gravitates to every other particle of matter, with a force inversely proportional to the squares of their distances." By the laws of gravity, Newton was enabled to calculate the quantity of matter in the sun, and in all the planets, and even to determine their density, results which Adam Smith said "were above the reach of human reason and experience." He ascertained that the weight of the same body would be twenty-three times greater at the surface of the sun than at the surface of the earth, and that the density of the earth was four times greater than that of the sun. He found the true figure of the earth; he explained the phenomena of the tides. Of the "Principia," Sir David Brewster says, in his able life of Sir Isaac Newton, it is "a work which will be memorable not only in the annals of one science or of one country, but which will form an epoch in the history of the world, and will ever be regarded as the brightest page in the records of human reason,--a work, may we not add, which would be read with delight in every planet of our system,--in every system of the universe. What a glorious privilege was it to have been the author of the 'Principia'! "There was but one earth upon whose form, and tides, and movements, the philosopher could exercise his genius,--one moon whose perturbations and inequalities and actions he could study,--one sun whose controlling force and apparent motions he could calculate and determine,--one system of planets whose mutual disturbances could tax his highest reason,--one system of comets whose eccentric paths he could explore and rectify,--and one universe of stars to whose binary and multiple combinations he could extend the law of terrestrial gravity. "To have been the chosen sage summoned to the study of that earth, these systems, and that universe, the favored lawgiver to worlds unnumbered, the high priest in the temple of boundless space,--was a privilege that could be granted but one member of the human family;--and to have executed the last was an achievement which, in its magnitude, can be measured only by the infinite in space, and in the duration of its triumphs by the infinite in time. That sage,--that lawgiver,--that high priest was Newton." The "Principia" created the greatest interest throughout Europe, but met with violent opposition. While Laplace said it would take "pre-eminence above all the other productions of human genius," the majority could not believe that great planets were suspended in empty space, and retained in their orbits by an invisible power in the sun. When Newton presented copies to the heads of colleges, some of them, Dr. Babington of Trinity among the number, said, "they might study seven years before they understood anything of it."