To Da Vinci I shall have occasion to allude more particularly on a subsequent page.  Of his works still remaining in manuscript, two volumes are at Milan, and one in Paris, carried there by Napoleon.  After an interval of about seventy years, Da Vinci was followed by the Dutch engineer, Stevinus, whose work on the principles of equilibrium was published in 1586.  Six years afterward appeared Galileo’s treatise on mechanics.

To this great Italian is due the establishment of the three fundamental laws of dynamics, known as the Laws of Motion.

The consequences of the establishment of these laws were very important.

It had been supposed that continuous movements, such, for instance, as those of the celestial bodies, could only be maintained by a perpetual consumption and perpetual application of force, but the first of Galileo’s laws declared that every body will persevere in its state of rest, or of uniform motion in a right line, until it is compelled to change that state by disturbing forces.  A clear perception of this fundamental principle is essential to a comprehension of the elementary facts of physical astronomy.  Since all the motions that we witness taking place on the surface of the earth soon come to an end, we are led to infer that rest is the natural condition of things.  We have made, then, a very great advance when we have become satisfied that a body is equally indifferent to rest as to motion, and that it equally perseveres in either state until disturbing forces are applied.  Such disturbing forces in the case of common movements are friction and the resistance of the air.  When no such resistances exist, movement must be perpetual, as is the case with the heavenly bodies, which are moving in a void.

Forces, no matter what their difference of magnitude may be, will exert their full influence conjointly, each as though the other did not exist.  Thus, when a ball is suffered to drop from the mouth of a cannon, it falls to the ground in a certain interval of time through the influence of gravity upon it.  If, then, it be fired from the cannon, though now it may be projected some thousands of feet in a second, the effect of gravity upon it will be precisely the same as before.  In the intermingling of forces there is no deterioration; each produces its own specific effect.

In the latter half of the seventeenth century, through the works of Borelli, Hooke, and Huyghens, it had become plain that circular motions could be accounted for by the laws of Galileo.  Borelli, treating of the motions of Jupiter’s satellites, shows how a circular movement may arise under the influence of a central force.  Hooke exhibited the inflection of a direct motion into a circular by a supervening central attraction.

The year 1687 presents, not only an epoch in European science, but also in the intellectual development of man.  It is marked by the publication of the “Principia” of Newton, an incomparable, an immortal work.