1?JVENTI0N OF THE BALANCE SPRING. 37^ 



** ^only known by what he said of it before J770. But I Account of the 

 ^' have proved that I did know it; since, as we have'^g^^^^gp^^g^oy 

 " seen, that on the 10th of February, 1768, I had lodged and Berthoud 

 " with the Academy my Theory of the Isochronism of the [f^^pieccs. 

 *' Spiral. 



•' Is (it there) all that M. L. R. has told us, in 1770, of 

 ." this property of the spiral spring, in his Memoire siir la 

 ** Mesitre dii Temps en Mer, which contains the description 

 " of his present watches. I have sufficiently proved by the 

 " dates of our productions that I could not be his copier ; 

 " but 1 can yet prove in another manner that I could not 

 ** be so, since we agree not, either in the fundamental prin- 

 " ciple of our theor}^ on the isochronism by the spiral, or in 

 " all its consequences. 



*' First, M. L. R. says, that in all the experiments which 

 '•^ he has made on the time or duration of the vibrations of a 

 ■"• balance with the spiral spring, he has almost always found 

 " that the great vibrations are slower than the small ones. 



" All the experiments, on the contrary, which I have re- 

 *' lated in my Traite des Horloges on spiral springs, and 

 " a still greater number which I have made, and which 

 *' are not mentioned in that work, prove that, in general, 

 " the spiral renders the great vibrations of the balance 

 '* quicker than the stnall ones. See, in the Treatise on Ma- 

 " rine Clocks, the experiments of No. 137, 20(5, 212, 215, 

 " 216, 217, 218, 219, 220, 225, 227,228, 230, 232, the 

 " first of 233, and the No. 234, 928. The experiments of 

 " 207, 221, 226, aud the second of 233, are the only 

 •' ones which could give the great vibrations slower than the 

 ^' small ones: and still it is only by a long and difficult 

 " task that spirals can be brought to that point which alone 

 ** can assure us, that the spiral is susceptible of being made 

 " isochrone, a property which we obtain then by shortening 

 " it. Less fortunate than JVL L. R., who tells us (page 3 i 

 *' of his Memoire) that *' this operation (of seeking the point 

 *• where a spiral is isochrone) seemed at Jirst tedious, but 

 *' that practice renders it so easy, that at once he nowknoxvs 

 *' pretty nearly the length of the spring where all the vibra- 

 " tions are of equal duration." " I confess, on the contraiy, 

 " that, though aided with an excellent instrument in my 

 C c c 2 "■ elastic 



