298 INVENTION OF CHRONOMETERS, kc. 



" instant it begins to move, it assumes a figure that is not a 

 *' perfect spiral, both when the spring opens and closes, by the 

 '* contrary vibrations." But this explanation is not conclusive, 

 and requires some modifications. A spring, of whatever form 

 it may be, only acts when its figure suffers an alteration, and 

 for that reason a relative change must take place, as well in 

 What way be the cylindrical as the spiral, or any other spring. What is the 

 of^a balance" shape most favourable to isochronism, is another question to 

 spring ? be decided by very delicate experiments, which we do not know 



to have been ever made. At present, some watchmakers think 

 that the helical spring does not possess any advantage with re- 

 gard to that property ; but as the opinion of other persons is 

 in the affirmative, while all the manufacturers, as far as our 

 knowledge goes, agree in considering the cylindrical form as 

 more easily managed than the other, its application seems en- 

 titled to the merit of a practical improvement. 

 Arnold may be That application* is ascribed by common report to Mr. 



*^^'^!."^t'^^'^ t^ Arnold, and we see no reasonable ground to dispute it to him, 

 the first applier ' t> r » 



of the helical except the instance before noted, and the evidence of the late 

 spring C^®^" Josiah Emery, who declared before the'Committee of the House 

 of Commons, in the case of Mr. Mudge,* that he had read an 

 account, in an English paper, of that sort of spring, under the 

 name of cylindrical spring, about a year or two before Mr. Ar- 

 nold took out his patent. This account was contained in an 

 advertisement from Bow-street, relative to a number of watches 

 that had been stolen in France and brought into England ; but 

 Mr. Arnold perhaps never saw it, and may have thought of 

 the application of the helical spring to watches without pre- 

 vious hint or assistance. 

 Earnshaw de- Mr. Earnshaw, in the explanation of his timekeepers, pre- 



uies the iso- gented to the Board of Longitude, after noticing the insufficien- 



chroiiism ot ... 



the helical cy of the cylindrical spring, states that he had, by long perse- 



spring J ^" verance, found, how to make springs increasing in thickness to 

 asserts, that he ' _ i to » 



had discovered the outer end, in order to effect the isochronism of the vibra- 



t&lxhlffit-^ tions. This method of obtaining isochronal vibrations had 



— b'it it was been long before explained by Berthoud, with regard to the 



fore\y'Ber-' ^P^^^^ spring, in that part of his Treatise on Marine Time- 



thoiid. 



* See note 2, p. 7. 



f Report from the said Committee, pp. 104 and 105. 



pieces 



