Leonhard Euler, a Swiss mathematician, born in Basel, April 15, 1707, died in St. Petersburg, Sept. 7, 1783. He was intended for the church, and entered the university of Basel; but the influence of the Bernoullis diverted his attention to philosophical pursuits. At the age of 19 he graduated, after having already attracted the notice of the French academy of sciences by a memoir upon some points of naval architecture. In the following year he went to St. Petersburg, where after several disappointments Daniel Bernoulli obtained for him the professorship of natural philosophy; and in 1733 he became professor of mathematics. His publications on the nature and propagation of sound, curves, the integral calculus, the movement of celestial bodies, etc, had gained him wide reputation; and in 1741, at the invitation of Frederick the Great, he left St. Petersburg for Berlin, where he remained 25 years. During this time he continued to hold his Russian appointments, and even drew a portion of their salary, receiving at the same time from all parts of Europe the most flattering marks of respect. When the dominions of Frederick were invaded by a Russian army in 1760, and a farm belonging to Euler was laid waste, the empress Elizabeth immediately reimbursed his losses.

These generous acts, among other motives, induced him to accept an invitation from the empress Catharine II. to return to St. Petersburg in 1766. He had during some years previously suffered from weakness of the eyes; and soon after returning to Russia he became so nearly blind as to be able only to distinguish very large chalk marks on a blackboard. The affection was the consequence of fever brought on by a calculation, for which his fellow academicians demanded four months, but which Euler completed in three days. He continued almost blind during the remainder of his life; but by constant exercise he acquired a power of recollection of mathematical formulas and figures almost incredible. Some of his most valuable works were composed after his loss of sight; among them, his "Elements of Algebra" and " New Theory of the Motions of the Moon." In more than 50 years of incessant labor he composed 30 separate works, chiefly in Latin, and more than 700 memoirs or treatises. The whole could not be contained in less than 40 large 4to volumes, and embrace every existing branch of mathematics, and almost every conceivable application of them.

To Euler belongs the credit of improving the analytic method, according to the system of Leibnitz and the Bernoullis, and of uniformly applying it to scientific investigations. Nor was he less remarkable for his popular expositions of the principles of his favorite science. His "Letters to a German Princess," which have been translated into English, and several times reprinted, throw a clear light on the most important facts in mechanics, optics, acoustics, and physical astronomy, and, though to some degree superseded by the progress of modern discovery, will always remain a model of perspicuous statement and felicitous illustration. His "Introduction to Algebra," translated by Prof. Farrar of Harvard college as preliminary to the Cambridge course of mathematics, has never been surpassed for its lucid and attractive mode of presenting the elements of that science. Euler was a man of simple, reserved, and benevolent mind, with a strong devotional sense and religious habit. He undertook to prove the immateriality of the soul, and had the courage to defend revelation at the court of Frederick II. He was twice married, his second wife being the aunt of his first, and had 13 children, only four of whom survived him.

The eldest son was his assistant and successor at St. Petersburg, and the second was physician to Catharine II.