h render the exact computation of her elements so
difficult.

The Annual Equation, an irregularity discovered by Tycho Brahe, which is
produced by the increase and decrease of the Sun's disturbing force as
the Earth approaches or recedes from him in her orbit, had its value
first assigned by Horrox. This he calculated to be eleven minutes
sixteen seconds, which is within four seconds of what it has since been
proved to be by the most recent observations.

The Evection, an irregular motion of the Moon discovered by Ptolemy,
whereby her mean longitude is increased or diminished, was explained by
Horrox as depending upon the libratory motion of the apsides, and the
change which takes place in the eccentricity of the lunar orbit.

These discoveries were made by Horrox before he attained the age of
twenty years, and if his reputation had alone rested upon them his name
would have been honourably associated with those who have attained to
the highest eminence in astronomy.

Another achievement which adds lustre to Horrox's name consists in his
detection of the inequality in the mean motions of Jupiter and Saturn.

He also directed his attention to the study of cometary bodies, and
arrived at certain conclusions with regard to the nature of their
movements. At first, he believed like Kepler that comets were projected
in straight lines from the Sun; this supposition having been upheld on
account of the great elongation of their orbits. He next perceived that
their velocity increased as they approached the Sun, and decreased as
they receded from him. Afterwards he says, 'They move in an elliptic
figure or near it,' and finally he arrived at the conclusion that
'comets move in elliptical orbits, being carried round the Sun with a
velocity which is probably variable.' This theory has been verified by
numerous observations, and is now generally accepted by astronomers.

Horrox also made a series of observations on the tides. He notified the
extent of their rise and fall at different periods, and investigated
other phenomena associated with their ebb and flow. After having
continued his observations for some time, he wrote to his friend
Crabtree, and informed him that he had perceived many interesting
details which had not been previously described, and he hoped to be
able to arrive at some important conclusions with regard to their nature
and cause. Unfortunately, Horrox's writings on this subject, along with
many other impor