2-56-3. William Thomson to H. Poincaré

Nov. 7/91

The University, Glasgow

Dear Mr. Poincaré,

I am sorry you had the trouble of returning the little proof. I ought to have written at the top of it ‘‘need not be returned’’, as I have now done! But I only send it to you again in writing to thank you for kindly returning it to me when you naturally thought it had been enclosed by mistake with the other papers. It appeared in the Philosophical Magazine at the beginning of this month.11According to Poincaré’s letter (§ 56.2), the proof in question is a printer’s proof, corresponding to Thomson’s demonstration of a theorem in point dynamics, or what Thomson calls ‘‘kinetic trigonometry’’ (W. Thomson 1891a).

The December number will contain a continuation of my paper on Periodic Motion, to which will be appended a short (too short) reference to your great work on the subject, which only came to my knowledge last week in Cambridge where Mr. Cayley lent me the volume of the Acta formed by your paper. I have been much interested in it so far as I have yet been able to look through it.22Thomson published a postscript (W. Thomson 1891b) to his earlier paper on periodic motion of a finite conservative system (W. Thomson 1891c). As Barrow-Green (1997, 148) observes, Thomson finds Poincaré’s conjecture on the denseness of periodic solutions to be less general than a similar statement made by Maxwell, but notes that his own investigation of the instability of periodic motion concurs with that of Poincaré (1890), and comments: ‘‘M. Poincaré’s investigation and mine are as different as two investigations of the same subject could well be, and it is very satisfactory to find perfect agreement in conclusions.’’ On Poincaré’s paper and its several consequences see the correspondence with Mittag-Leffler (§§ 1.89 et 1.90), the overview by Nabonnand (1999), and detailed studies by Barrow-Green (1994) and Andersson (1994).

Believe me, yours very truly,

William Thomson

ALS 1p. Private collection, Paris 75017.

Time-stamp: "19.03.2015 01:56"