The letter presents a plan for a citadel designed by Détalonde Morival, whose rapid progress in mathematics and engineering is described as extraordinary. The author criticises complex mathematical pursuits as unnecessary, emphasising the practical value of a good engineer over abstract calculations. He advocates for Morival—a noble and capable soldier—to be employed in the service of the sovereign, highlighting his skills, dedication, and loyalty. Nearing the end of his life, the author seeks comfort in the sovereign’s continued support for Morival, asserting that such support would be a just response to the injustices Morival has faced. Additionally, according to T. D. N. Besterman, the letter is dated 2 January 1775.

The author presents a drawing of the Battle of Rosbac by D’Étalonde to the King, highlighting the profound admiration and dedication of an officer eager to serve in such battles. He expresses gratitude for the King’s support and the impact it has had on the officer’s loyalty, noting that the officer is more focused on serving under the King than seeking any favour in France. The author commends the King’s ability to balance significant matters with personal attentiveness and reflects on the recent, less impressive French coronation in contrast to the King’s multifaceted and impressive leadership. He concludes with a declaration of lifelong loyalty and devotion to the King.

The writer expresses deep personal loyalty to the monarch and comments on reports that Cosinski and other conspirators swore a religious oath to the Virgin Mary before attempting their crime. He notes with irony that many infamous assassins were also deeply religious. He suggests that the monarch’s presence, virtues, and humanity may have stirred natural feelings of compassion in the would-be assassin, countering the influence of religious fanaticism. The writer believes this horrific event will ultimately strengthen the monarch’s position and make rebellion shameful. He requests a brief reply, to be sent to Geneva, and closes with renewed expressions of respect and devotion.