Our knowledge of des Essar stems almost entirely from Boswell's French Theme of March 31, 1764, in which he tells us that "he was once a Capuchin [who] ran away twice from his monastery, and the second time he escaped to Brussels, where he lived for some time. He taught mathematics to people of the first rank. [...] Essar is assuredly of a distinguished family, though of a rather distant branch of it. [...] He established himself at Amsterdam [where he had] pupils of the right sort. He belonged to a literary society composed of men of wit and amiable women.... Finally monsieru des Essar married a young Hollandised Englishwoman. He came to Utrecht, where he has remained for some time. He still teaches mathematics, and he conducts the Gazette Francaise."
Life with Boswell:
Boswell probably knew des Essar though the Literary Society of Utrecht. Des Essar is mentioned four times throughout Boswell in Holland 1763-64.