380 INVENTION OF THE BALANCE SPRING. 



Account of the " elastic balance, it is only with much trouble that I have 



bTwenTeRo ** ^'^^'^^ ^^^^^ ^P^^^^ Springs fit to succeed by their isochron- 



and Berthoud " ism. And again have I found them quite altered on al- 



respeciing u tempting to temper them when turned up. This operation 



" is, however, indispensable, if we want to give them the 



** quality of keeping a constant figure, a quality that is pre- 



•' ferable by much to that of the isochronism by the spi- 



" ral, in machines destined to undergo all the changes of 



" temperature, which never fail to alter the figure of springs 



" when they have not been tempered after being turned up. 



" I must add, that if my experiments do not agree with 

 *' those of M. L. R. they agree with those of Mr. Harri- 

 *' son : this celebrated artist always found, as well as I did, 

 *' that the great arcs of vibration were quicker than the small 

 ■' ones. 



*' M. L. R. after having set out with a principle contra- 

 *' dieted by experiment, makes the isochronism to consist 

 " only in the more or less of length of the spiral spring; 

 " whereas I have proved in my Traite des Horloges Marines^ 

 *' first, that we can arrive at isochronism without rendermg 

 *' the spring longer, but by making it broader and thinner, (No, 

 " 143). Secondly, that we can come at it by a great num- 

 " ber of close turns, (No. 154) ; or by renloring the spring 

 *' strojiger or weaker at the centre or outwards, (No. 157).' 

 *' I have shewn, that the lamma or wire must be made 

 " like a whip or lash, strongest at the centre, (No. 159). 

 " See also the Nos. 159, 222» 2-35, &c.; and, in general, 

 '* see in the table of matters of the Treatise of Marine C/ocks, 

 " at the word .s^zV^/, all the articles where it is treated of. 



" The quality of isochronism is precious, without doubt, 

 *' in a spiral spring, and I have insisttd on it in my Traite des 

 " Horloges Marines, not, as M. L. R. pretends, because the 

 '* justness of my marine clock is founded, like that of his 

 " watches, on the isochronism of the spiral ; not because I 

 " believe that, without this method, we shall ever have only 

 " feeble success in marine clocks. I have neither said nor thought 

 " so ; but because this method can render the making, regu- 

 " lating, or timeing of these machines, more expeditious and 

 " more easy, I have always looked on it merely as an useful 

 ^^ accessary, which, perhaps^ might render my clocks still 



" more 



