Voltaire writes that after leaving Paris in July he asked a young man named Delamare to give de Mairan the text on the driving forces that he had kindly lent Voltaire, adding that he does not know if the young man returned it to him. He adds that it would be happy for the young man if he had stolen it in order to learn but notes that it is a treasure that is not for his use. Voltaire writes that the day before his departure, he asked Mr Pitot if he had read the text and that he replied that he had not. From this, Voltaire concludes that in de Mairan’s academy the same thing happens as at assemblies of actors: everyone thinks only of their own role and they play is worse for it. He writes that he asked Pitot if he thought that the quantity of motion was the product of the mass by the square of the velocities and that Pitot had agreed, saying that Leibnitz and Bernoulli seemed convincing to him. However, Voltaire notes that shortly after his arrival at Cirey, Pitot sent him a letter claiming to have read de Mairan’s text, which he describes as a masterpiece, and noting that it had changed his mind on the matter. Voltaire then thanks de Mairan for the enlightenment and pleasure he gained from studying under his instruction, flattering de Mairan, and remarking that he greatly admires how de Mairan distinguishes accelerated movements which are like the square of speeds and times, from forces that are only due to speeds and times. He makes reference to three articles (Articles 22, 23, and 24) that he particularly admired.
Voltaire than asks de Mairan for his help on a problematic matter that has been occupying him for a few days. He states that it is an experiment contrary to the first foundations of catoptrics and notes that the foundation of the experiment is that one must see the object at the point of intersection of the cathetus and of the reflected ray but that his fundamental rule is, on many occassions, false. He provides a diagram to explain his findings, noting that he should see the object A at the point of intersection D but instead sees it at L, K, I, H, and G successively. He asks if this does not prove that we do not perceive distances by means of angles that are formed in our eyes, noting that he often sees the object as very close and large although the angle is small.
He ends the letter by saying that Taquet and Barrou have been unable to solve the problem he poses and asks if de Mairan would mind giving him his opinion. He adds that Gabrielle Émilie Le Tonnelier de Breteuil, marquise Du Châtelet is worthy of reading de Mairan’s work and finds that there is no one who is more made to give a taste of the truth than him. She has ordered Voltaire to assure de Mairan of his esteem and pays him he compliments.