some irregularities of the Moon's motion. Cassini
discovered the belts of Jupiter, and also the Zodiacal Light, and
established the coincidence of the nodes of the lunar equator and orbit.

JAQUES CASSINI, son of Giovanni, was born at Paris in 1677. He followed
in his father's footsteps, and wrote several treatises on astronomical
subjects. He investigated the period of the rotation of Venus on her
axis, and upheld the results arrived at by his father, which were
afterwards confirmed by observations made by Schroeter. Cassini made
some valuable researches with regard to the proper motion of the stars,
and demonstrated that their change of position on the celestial vault
was real, and not caused by a displacement of the ecliptic. He attempted
to ascertain the apparent diameter of Sirius, and made observations with
regard to the visibility of the stars. The Cassini family produced
several generations of eminent astronomers, whose discoveries and
investigations were of much value in advancing the science of astronomy.

OLAUS ROEMER, an eminent Danish astronomer, was born at Copenhagen
September 25, 1644. When Picard, a French astronomer, visited Denmark in
1671, for the purpose of ascertaining the exact position of
'Uranienburg,' the site of Tycho Brahe's observatory, he made the
acquaintance of Roemer, who was engaged in studying mathematics and
astronomy under Erasmus Bartolinus. Having perceived that the young man
was gifted with no ordinary degree of talent, he secured his services to
assist him in his observations, and, on the conclusion of his labours,
Picard was so much impressed with the ability displayed by Roemer, that
he invited him to accompany him to France. This invitation he accepted,
and took up his residence in the French capital, where he continued to
prosecute his astronomical studies.

In 1675 Roemer communicated to the Academy of Sciences a paper, in which
he announced his discovery of the progressive transmission of light. It
was believed that light travelled instantaneously, but Roemer was able
to demonstrate the inaccuracy of this conclusion, and determined that
light travels through space with a measurable velocity.

By diligently observing the eclipses of Jupiter's satellites, Roemer
perceived that sometimes they occurred before, and sometimes after their
predicted times. This irregularity, he discovered, depended upon the
position of the Earth with regard to Jupiter. When the Earth, in
traver