INVENTION OF THE BALANCE SPRING. 373 



JL-ondon was destroyed, 3d of Sept, 1666, Dr. Hooke was much 

 engaged in surveying the waste ground left by the fire, and ar- 

 ranging the different claims and properties of the public and 

 for individuals; by which it is probable he got much more 

 money than he would have got by prosecuting the business of 

 his longitude watches. I shall now give you some extracts 

 from M. Berthoud, by which you will see what a serious 

 quarrel took place between him and M, Le Roy, and, among 

 other things, about the theory of pendulum springs, &c. 



" I pass," says M. Berthoud,* " to a discovery of which M. Account of the 

 *' L. R. seems extremely jealous, that of the isochronism by a |^°^ '"^*^l^rov 

 *' certain length of the spiral spring, which he had proposed in and Berthoud 



*' enigma, in 1768, in his Expose Succinct, and which he 'fspecting 

 *= ' ' ' ^ ' timepieces. 



*' only divulged in 1770, in his Mesure du Temps en Mer : 



" he holds to it there, with so much the more reason, as he 



*' is persuaded, that on this property of the spring, the regu- 



*' larity of his marine watch particularly depended. I do nor, 



*' dispute with M. L. R. that he has not discovered this 



" property of the spiral spring, by which all the vibrations of 



" the balance become isochrone : but I complain, and witli 



" propriety and a just right, that he wants to dispute the dis- 



*' covery of its having been made on my part ; and dares to 



^' accuse me of being only his copyist : it is very easy for me to 



^- prove that I could not be so. 



?' The ill-timed jests that M. L. R. allows himself, and the 



'* tone of raillery which he affects on this occasion, shall not 



** prevent the truth of that which, in my Essai sur I'Horlogerie 



*' (torn i. page 168), I have said in speaking of my elastic 



*'• balance, and is this which follows:" " I had destined this 



'' machine to make experiments on the duration of great and 



*' small •vibrations of the same balance, which moves freely : for 



^ this purpose, I made the end of the pivot run on a very hard 



*' stone; and to lessen the friction of the pivots, they each run 



** between three rollers. I observed the number of vibrations 



*' which the balance made when it moved horizontally or ver- 



" tically, the velocity of the vibrations according to the dif~ 



■'^ ference of temperature ; and it must serve to measure the dif- 



* In his " Eclairssemens sur rinvcntion et la Construction dcs 

 ^' Horloges Marines." 



" fereni 



