A few words may here be added on the scientific character of Galileo; in this respect he was, with the exception of Kepler, the first man of his age.
He has the credit of being the discoverer of the first law of motion; but whether he fully realised this all-important law, or whether it was one of those happy guesses which we sometimes find to have been made by men who are the precursors of great discoverers, but who do not perceive the full scope and the ultimate bearing of the truths on which they have lighted, I need not here discuss. He did, however, state the law in a Dialogue on mechanics, published in 1638, in these words:
“I imagine a movable body projected in a horizontal plane, all impediments [to motion] being removed; it is then manifest from what has been said more fully elsewhere, that its (the body’s) motion will be uniform and perpetual upon the plane, if the plane be extended to infinity.”
This of course involves the principle of the first of the three laws of motion, the Newtonian laws, as they are frequently called, because the man whose name they bear was the one who used them clearly and consistently as the basis of a great astronomical theory. The law, as now usually stated, is fuller and more explicit than that given by Galileo, and may be enunciated thus: “Every body perseveres in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed on it.”
It is, however, greatly to the scientific credit of Galileo that before the close of his life he should have emancipated himself from the erroneous idea that circular motion alone is naturally uniform, and should have stated in the language just quoted the true mechanical doctrine, unknown to his predecessors, unknown even to Kepler, a doctrine which involved nothing less than a revolution in the conception of the laws of motion. Nor was this his only contribution to the science of mechanics; he it was who first understood the law that regulates the velocity of falling bodies; he perceived that they were acted upon by an uniformly accelerating force, that of terrestrial gravity, and that the velocity at any given point is proportional to the time of descent.