A fresh privilege was procured (Jan. 21, 1746), and as Le Breton’s capital was insufficient for a project of this magnitude, he invited three other booksellers to join him, retaining a half share for himself, and allotting the other moiety to them.  As Le Breton was not strong enough to bear the material burdens of producing a work on so gigantic a scale as was now proposed, so Diderot felt himself unequal to the task of arranging and supervising every department of a book that was to include the whole circle of the sciences.  He was not skilled enough in mathematics, nor in physics, which were then for the most part mathematically conceived.  For that province, he associated with himself as an editorial colleague one of the most conspicuous and active members of the philosophical party.  Of this eminent man, whose relations with Diderot were for some years so intimate, it is proper that we should say something.

D’Alembert was the natural son of Madame de Tencin, by whom he had been barbarously exposed immediately after his birth.  “The true ancestors of a man of genius,” says Condorcet finely upon this circumstance, “are the masters who have gone before him, and his true descendants are disciples that are worthy of him.”  He was discovered on a November night in the year 1717, by the beadle, in a nearly dying condition on the steps of the church of St. John the Round, from which he afterwards took his Christian name.  An honest woman of the common people, with that personal devotion which is less rare among the poor than among the rich, took charge of the foundling.  The father, who was an officer of artillery and brother of Destouches, the author of some poor comedies, by and by advanced the small sums required to pay for the boy’s schooling.  D’Alembert proved a brilliant student.  Unlike nearly every other member of the encyclopaedic party, he was a pupil not of the Jesuits but of their rivals.  The Jansenists recognised the keenness and force of their pupil, and hoped that they had discovered a new Pascal.  But he was less docile than his great predecessor in their ranks.  When his studies were completed, he devoted himself to geometry, for which he had a passion that nothing could extinguish.  For the old monastic vow of poverty, chastity, and obedience, he adopted the manlier substitute of poverty, truth, and liberty—­the worthy device of every man of letters.  When he awoke in the morning, he thought with delight of the work that had been begun the previous day and would occupy the day before him.  In the necessary intervals of his meditations, he recalled the lively pleasure that he felt at the play:  at the play between the acts, he thought of the still greater pleasure that was promised to him by the work of the morrow.  His mathematical labours led to valuable results in the principles of equilibrium and the movement of fluids, in a new calculus, and in a new solution of the problem of the precession of the equinoxes.[99]