256 TJie Proposed Pendulum Operations for India. [No. 4, 



longer pendulum will have lost two oscillations on the shorter one. 

 Hence all that is requisite in practice, is to observe as accurately as 

 possible the intervals between the successive coincidences ; the number 

 of vibrations made by the clock pendulum is determined by observ- 

 ations of the sun or stars, and then the number made by the detached 

 pendulum is computed by simple proportion.* 



The first pendulum observations of which any account is preserved 

 are those made by Picard at Paris and Uranienburg (Tycho Brahe's 

 observatory) and those by Richer at Cayenne in 1672. These last 

 observations are said to have attracted Newton's attention, as they 

 proved the variation in the length of the seconds pendulum in different 

 latitudes, and it is generally stated that Richer made the discovery by 

 accident. But it appears from Picard's address to the Prench academy 

 in 1671, that a variation had been already observed, and it is probable 

 that Richer's mission was undertaken partly with a view to thrOAV 

 light on the subject. Picard stated that " from observations made at 

 London, Paris, and Bologna, it would seem as if the seconds pendulum 

 required to be shortened in approaching the equatoi', but that on the 

 other hand, he is not sufficiently convinced of the accuracy of those 

 measurements, because, at the Hague, the length of the seconds 

 pendulum was found to be quite the same as at Paris, notwithstand- 

 ing the difference of latitude."f 



Near the end of the 18th century, Borcla made his celebrated expe- 

 riments for determining the length of the seconds pendulum at Paris. 

 His apparatus, which is named after him, consisted of a spherical ball of 

 platinum attached by grease to a brass cap which had been truly ground, 

 so as to fit it perfectly. The object of this attachment was to enable 

 the observer to turn the ball round in the cap at pleasure, so as to 

 destroy the effects of unequal density in different parts of it. A fine 

 wire carrying the cap was fastened to the lower end of a small 

 cylinder, passing through the knife edge, which carried on its upper 

 end a small moveable weight, by adjusting which the knife edge 

 and cylinder could be made to vibrate independently in the same 



* If r = daily rate of the clock and I the mean interval of the coincidences, 

 then the number of oscillations made by the pendulum in a day = n 



n = — — (86400 + r) the lower sign is to be used when the 



clock is losing. 



f Cosmos Vol. IV. page 25, Sabine's translation. 



