study, geometry. In 1727 he was invited by Catharine
I. to reside in St. Petersburg, and on accepting
this invitation he was made an associate of the Academy
of Sciences. A little later he was made professor
of physics, and in 1733 professor of mathematics. In
1735 he solved a problem in three days which some
of the eminent mathematicians would not undertake
under several months. In 1741 Frederick the Great
invited him to Berlin, where he soon became a member
of the Academy of Sciences and professor of mathematics; but in
1766 he returned to St. Petersburg.
Towards the close of his life be became virtually blind,
being obliged to dictate his thoughts, sometimes to
persons entirely ignorant of the subject in hand.
Nevertheless, his remarkable memory, still further
heightened by his blindness, enabled him to carry out
the elaborate computations frequently involved.

Euler's first memoir, transmitted to the Academy of
Sciences of Paris in 1747, was on the planetary perturbations.
This memoir carried off the prize that
had been offered for the analytical theory of the motions
of Jupiter and Saturn. Other memoirs followed,
one in 1749 and another in 1750, with further expansions
of the same subject. As some slight errors were
found in these, such as a mistake in some of the formulae
expressing the secular and periodic inequalities,
the academy proposed the same subject for the prize
of 1752. Euler again competed, and won this prize
also. The contents of this memoir laid the foundation