nces of the planets are usually calculated.

These discoveries are known as Kepler's Laws, and are usually classified
as follows:--

1. 'The orbit described by every planet is an ellipse, of which the
centre of the Sun occupies one of the foci.

2. 'Every planet moves round the Sun in a plane orbit, and the
radius-vector, or imaginary line joining the centre of the planet and
the centre of the Sun, describes equal areas in equal times.

3. 'The squares of the periodic times of any two planets are
proportional to the cubes of their mean distances from the Sun.'[1]

These remarkable discoveries do not embrace all the achievements by
which Kepler has immortalised his name, and earned for himself the proud
title of 'Legislator of the Heavens;' he predicted transits of Mercury
and Venus, made important discoveries in optics, and was the inventor of
the astronomical telescope.

GALILEO GALILEI, the famous Italian astronomer and philosopher, and the
contemporary of Kepler and of Milton, was born at Pisa on February 15,
1564.

His father, who traced his descent from an ancient Florentine family,
was desirous that his son should adopt the profession of medicine, and
with this intention he entered him as a student at the University of
Pisa. Galileo, however, soon discovered that the study of mathematics
and mechanical science possessed a greater attraction for his mind,
and, following his inclinations, he resolved to devote his energies to
acquiring proficiency in those subjects.

In 1583 his attention was attracted by the oscillation of a brass lamp
suspended from the ceiling of the cathedral at Pisa. Galileo was
impressed with the regularity of its motion as it swung backwards and
forwards, and was led to imagine that the pendulum movement might prove
a valuable method for the correct measurement of time. The practical
application of this idea he afterwards adopted in the construction of an
astronomical clock.

Having become proficient in mathematics, Galileo, whilst engaged in
studying the writings of Archimedes, wrote an essay on 'The Hydrostatic
Balance,' and composed a treatise on 'The Centre of Gravity in Solid
Bodies.' The reputation which he earned by these contributions to
science procured for him the appointment of Lecturer on Mathematics at
the University of Pisa. Galileo next directed his attention to the works
of Aristotle, and made no attempt to conceal the disfavour with which he
regarded many of the do