368 THE PRESIDENT’S ADDRESS. 
as much as Luther in his, the leader of a revolt against authority. 
Though he seems to have satisfied himself of the truth of his views 
early in the century, he did not promulgate them till about 1540. 
They made their way slowly : it was not until after the middle of the 
seventeenth century that they were generally received in England. 
Bacon, the great apostle of induction, never assented to them, and 
Milton, writing about 1660, bases the machinery of Paradise Lost 
on suppositions inconsistent with the Copernican theory. 
The discoveries of Copernicus were followed vy those of Kepler, 
who established the following propositions regarding the solar system, 
namely :— 
(1) That the orbits of the planets are elliptical. 
(2) That the line connecting the sun and any planet sweeps over 
equal areas in equal times. 
(3) That the squares of the periodic times of the planets are in 
the same proportion as the cubes of their mean distances from the 
sun. Then follow the discoveries of Galileo, and in the latter half 
of the seventeenth century Newton appears on the scene to furnish 
a mathematical explanation of the motious of the heavenly bodies. 
About 158+ the laws of the equilibrium of fluids, which had been 
known to Archimedes, were rediscovered by Stevinus. In 1616 
Harvey discovered the circulation of the blood, About 1621 
Willebrod Snell discovered that the ratio of the sines of the angles 
of the incidence and refraction of a ray of light is constant for the 
same media. During the first half of the seventeenth century the 
three fundamental laws of motion were established, the most promi- 
nent name connected with them being that of a scientific man already 
mentioned, the astronomer and physicist, Galileo. During the same 
period Torricelli discovered the vacuum which goes by his name, 
and Pascal proved that the height of a column of liquid in a tube 
with a vacuum above it depends upon the weight of the column of 
air balanced by it. About 1650 Boyle established the law that the 
density of a gas varies as the pressure, and in 1651 Pecquet, a French 
physician, discovered the motion of the chyle. 
By the middle of the seventeenth century the violent perturbations 
caused by the great movements of the sixteenth century had for the 
most part ceased. Italy and Spain, having early rejected spiritual 
liberty, had fallen into decadence. The Thirty Years’ War, the last 
European religious war, had ended in 1648, leaving Germany 
