CLAIRAUT (Alexis-Claude). Autograph letter [to Émilie Du Châ - Lot 59

Lot n° 59
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CLAIRAUT (Alexis-Claude). Autograph letter [to Émilie Du Châ - Lot 59
CLAIRAUT (Alexis-Claude). Autograph letter [to Émilie Du Châtelet]. S.l., [May 1741]. 4 pp. in-4 , letter incomplete from the end. AN IMPORTANT SCIENTIFIC POLEMIC WHICH OPPOSED THE LEARNED EUROPE ON THE MEASUREMENT OF FORCE. Some of the scholars followed Descartes and Newton in defending the notion of "quantity of motion". The perpetual secretary of the Academy of Sciences, Dortous de Mairan, was in favour of this theory, as was Voltaire, who in 1741 presented a memorandum to the Academy of Sciences entitled Doutes sur la mesure des forces motrices et sur leur nature. Another group of scholars supported the notion of "living force" introduced by Leibniz, for example Maupertuis, Clairaut (who was one of the rapporteurs at the Académie des Sciences for Voltaire's memoir), and the Marquise Du Châtelet. The latter published in 1740 Institutions de physique in which, among other things, she dared to criticize Dortous de Mairan on the question of the measurement of forces. Alexis-Claude Clairaut addresses the question in a general way before proposing particular remarks on chapter xvii of the Institutions de physique de la marquise Du Châtelet, entitled "Du repos, et de la chute des corps sur un plan incliné". The future would prove Leibniz right against Descartes and Newton on this question. "I was beginning to be extremely bored with your silence, Madame, and I was about to reproach you for it when your obliging letter relieved me of my worries. I was afraid that you would be displeased that I had not written you directly the same things as I had written to M. de Voltairre, and having received no reply from either you or him, I did not know what to think. I SAW WITH GREAT PLEASURE THAT YOU WERE HAPPY WITH WHAT I WAS TELLING MR. DE VOLTAIRRE. TO TELL YOU THE TRUTH, I EXPECTED THAT MY LETTER WOULD NOT DISPLEASE YOU & I DID NOT COUNT ON MR. DE VOLTAIRRE'S APPROVAL. I would, however, have been delighted if he had told me what he thought of it and if he had discussed this matter with me, but HE TREATED ME AS AN INDIGENOUS CALCULATOR OF MATTERS THAT REQUIRE A PHILOSOPHICAL MIND AND HONOURED WITH LETTERS TO ENJOY PEOPLE WHO, IN ORDER TO KNOW CALCULATION POORLY, DO NOT KNOW PHYSICS BETTER. I don't want to dwell on the reproaches I could make to him because that would be to make you a little too... Besides, you have caught me in a crisis of occupation[s] which hardly allowed me to do what you asked me about your book. I have, however, read with great care and severity two chapters of those in which my ministry can be the least useless. I have made a few remarks which I enclose here and which you may use as you please. Perhaps I am wrong, you will judge... You seem to believe that politics holds me back on the question of the living forces. I protest the contrary. If I said it was a matter of words, it is because I think it is a matter of words for all people who are really aware. THE DIFFERENCE I MAKE IN THE TWO PARTIES IS THAT MOST OF THOSE WHO ARE FOR THE LIVING FORCES HAVE SUFFICIENT PRINCIPLES NOT TO BE MISTAKEN IN QUESTIONS OF MECHANICS, WHEREAS THE GREATER NUMBER OF THOSE OF THE OTHER PARTY COMMIT A THOUSAND PARALOGISMS. Those of the English party who, in order to compare the living force of bodies in motion with the dead force on which everyone agrees, will confine themselves to saying that they regard the living force as the sum of the blows of gravity or of such other pressure as one may wish during the time it has been exerted, will not allow themselves to be dismayed. But they must stick to that. They will be told that they do not want this definition, & that they take strength for the quantity of obstacles overcome, and they will have nothing to say either. I prefer the second definition, but the other does not seem absurd to me. I do not know if either party would be wrong in wanting to make a single principle that measures living force, because it seems to me that for this principle to be good, it would be necessary to be able to derive from it what must happen in all particular cases, just as in the case of dead forces we can calculate all their effects when we know the space they cause to travel in a given instant. If we say that the force is like the velocity, we shall be led to believe that the indentations are like the velocities, and we shall be mistaken on a thousand occasions, but if we say simply that the force is like the fourth of the velocity, if we did not know more, we would believe that two bodies which act against each other with velocities reciprocal to their masses would not be in equilibrium. You will ask me what living force is if it cannot be reduced to a clear principle. Perhaps this is as vague a question as if one were asking to estimate and
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