372 IHfENTTON OF THE BALANCE SPRlNe. 



Le Royobsenr- M. Le Roy the eldest, in his Memorial on the best Manner 

 l'^^^^^^^°^l°^'(}f Measurhig Time at Sea, which was published in 1770, 

 the isochroniam at the end of the Voyage by M. Cassini, gives us, among other 

 pends'lOOvea i^pi'O'^'^^^ents which he had made use of in his timekeeper, 

 after Hooke, one, which regarded the isochronism of the pendulum spring, 

 and which would have remained, as he says, hitherto unknown, 

 had he not discovered this theory. His own words are. '* It is 

 " only some time ago, that I have at last discovered, as I shall 

 " more particularly explain, this important fact, which hence- 

 ^' forth must serve as a basis to the theory of watches, and as 

 " a guide to the workmen to construct them ; namely, thai 

 " there is in every spring of a sufficient extent^ a certain length 

 ^^ where all the mhrations, great and small ^ are isochrone : that the 

 ^' length being found, if you shall shorten this f^pring, the great 

 *' 'vibrations ivill be quicker than the small ones : if on the con- 

 *' trary_you lengthen it, the small arcs luill be finished in less time 

 '* than the large ofies. It is from this important property of 

 *' the spring, hitherto unknown, that particularly depends, as 

 *' we have by it seen, the regularity of Aiy marine watch. 

 *' After what has gone before, we perceive that the justness of 

 *' watches depends in a great measure on the length given to 

 " the spiral or regulating spring ; if with the same 'scapement 

 *' certain watches, or such as have, for example, the cylinder or 

 *' horizontal 'scapement, go ill, whilst that others of the same 

 *' sort are very regular, we here see the cause of it. This new 

 " observation may be of great use in the construction of clocks, 

 " whether small, or with second pendulums, where the pendu- 

 ** lum is suspended by a spring : indeed, we see from what has 

 " been said, that there ought to be there, such a length in the 

 " suspension spring, where all the vibrations of these pendulums 

 '' may be made isochrone." 

 ■FcsumptioB of Jtis evident, from what has been stated, that Dr. Hooke has 

 ° *' the fairest claim to the honour of these important inventions 



and discoveries. I mean that of the watch pendulum spring, 

 which he seems to have made in every possible form. And 

 that he was master of the theory of springs, al)out which so 

 much work of late years has been made, is equally evident. 

 Among other reasons why he did not sooner publish an ac- 

 count of these inventions and discoveries, there js one. which 

 though not generally known, it may be proper to mention here. 

 After the dreadful conflagration, by which the greater part of 



