y Amontons, and the principle of virtual velocities; he
described the camera obscura before Baptista Porta, understood aerial
perspective, the nature of coloured shadows, the use of the iris, and
the effects of the duration of visible impressions on the eye. He wrote
well on fortification, anticipated Castelli on hydraulics, occupied
himself with the fall of bodies on the hypothesis of the earth's
rotation, treated of the times of descent along inclined planes and
circular arcs, and of the nature of machines. He considered, with
singular clearness, respiration and combustion, and foreshadowed one of
the great hypotheses of geology, the elevation of continents.

[Sidenote: Stevinus continues the movement in Natural Philosophy.] This
was the commencement of the movement in Natural Philosophy; it was
followed up by the publication of a work on the principles of
equilibrium by Stevinus, 1586. In this the author established the
fundamental property of the inclined plane, and solved, in a general
manner, the cases of forces acting obliquely. Six years later Galileo's
treatise on mechanics appeared, a fitting commencement of that career
which, even had it not been adorned with such brilliant astronomical
discoveries, would alone have conferred the most illustrious distinction
upon him.

[Sidenote: Discovery of the laws of motion.] The dynamical branch of
Mechanics is that which is under most obligation to Galileo. To him is
due the establishment of the three laws of motion. They are to the
following effect, as given by Newton:

(1.) Every body perseveres in its state of rest or of uniform motion in
a right line unless it is compelled to change that state by forces
impressed thereon.

(2.) The alteration of motion is ever proportional to the motive force
impressed, and is made in the direction of the right line in which that
force is impressed.

(3.) To every action there is always opposed an equal reaction, or the
mutual actions of two bodies upon each other are always equal, and
directed to contrary parts.

Up to this time it was the general idea that motion can only be
maintained by a perpetual application, impression, or expenditure of
force. Galileo himself for many years entertained that error, but in
1638 he plainly states in his "Dialogues on Mechanics" the true law of
the uniformity and perpetuity of motion. Such a view necessarily implies
a correct and clear appreciation of the nature of resistances. No
experi