INVENTION OF THE BALANCE SPRING. 3Bl 



*" more perfect. But two indispensable qualities in the spi- 

 " i-al are, first, the constancy of forccy (abstraction being 

 *' naade of the accidental changes produced by the action 

 " of heat and cold) ; secondly, the constancy of figure; and 

 *' it is on these two qualities, of the first necessity, that I have 

 " always insisted, and that I still insist on. 



M. Le Roy gave the isochronism to the balance by a certain 

 length in the flat spiral spring: and M. Berthoud, to obtain the 

 same, condemns the cylindrical helix as being unfit for this 

 purpose, and us<"s the flat helix tapered thinnest outward. 

 Mr. Arnold used the cylindrical helix ; and Mr. Earnshaw 

 recommends tapering the spring thinnest inward. These di- 

 versities of opinion still serve to confirm what Dr. Hooke 

 observed in the numerous experiments which he made with 

 springs ; namely, that he could obtain the isochronism by 

 twenty different ways. 



As you had requested me,. I shall now endeavour, Mr. 

 Nicholson, to furnish you with some accounts of the detached 

 'scapement, and shall give you, as far as I have been able to 

 discover it, the history of its invention and subsequent pro- 

 gi-ess. 



. " I pass " (says M. Berthoud *) " to the detent 'scapement Observations of 

 *' having free vibrations (the detached), over which M. L. R. gpectin" the" 

 •^' pretends to have such an inconteslible and exclusive right." first free es- 

 " M. B." (says he) " relates, that the late M. de Camus had ^^^P^*"*^"'- 

 ♦' told him that the deceased M. du Tertre was the first who 

 ^' had this idea. He assures us besides, that in 1754, he him- 

 *' self had contrived one of this sort ; and that when in Lon- 

 f* don, in If 66, Mr. Mudge had shewn him one similar, or 

 " nearly so. We perceive clearly for what purpose he makes 

 .'♦' all these quotations, but that they may only prevent the 

 *' truth being known, of what, in 1748, the Academy had de- 

 " dared, in speaking of the first 'scapement which had 

 •** appeared with free vibrations, and which I had presented to 

 ,V it, that the idea of it seemed new to the Academy, and 

 f* susceptible of many advantages." 



* In his " Eclairssemens siir I'Invention et la Construction des 

 fj Horloges Marines." 



It 



