02 PHILOSOPHICAL TRANSACTIONS. [aNNO 1770. 



eclipse, on the 4th of June, for determining the longitude. I observed with a 

 12 feet achromatic telescope of Dollond, the end at O"^ 7™ 55' apparent time. 

 The celebrated M. L. Euler, who has computed several observations of this 

 eclipse made in different places, finds, by my observation, the difference of 

 meridians between Paris and Ponoi 2^ "ZS"" 33', that is, 38° 5 l' East of Paris. 



2°. On the observation of the transit of Venus. — I observed, with the same 

 telescope, the interior contact at the entry at \0^ 15™ 4' apparent time. I have 

 computed very scrupulously the effect of parallax on the moment of this 

 contact; I made use of the same elements that M. de la Lande gives in his 

 memoir, printed in 1764, excepting that I assume the nearest distance of the 

 centres of Venus and the sun, seen from the centre of the earth, 10' 27"; which 

 quantity I deduced from the whole duration, observed at Hudson's Bay, by 

 Messrs. Dymond and Wales, as given in the newspapers. I find the effect of 

 parallax 7™ 3' of time, by which I must have seen the contact sooner than at 

 the earth's centre. The computation of my observation gives the moment of 

 the conjunction at 12'' 46"" 21^' apparent time at Ponoi, and the geocentric 

 latitude of Venus for that moment 1 0' SS^.Q. 



If the nearest distance of the centres be taken 5" less, I find the effect of 

 parallax 7" 1 P of time, that is, 8* greater, the latitude becomes 5" less, and the 

 moment of the conjunction 1*" 28' later. 



3°. I have made a great number of observations for determining the force of 

 gravity and the length of the simple pendulum swinging seconds. I used an 

 invariable pendulum which M. de la Condamine got constructed at Quito, when 

 the French academicians went thither to measure a degree of the meridian, which 

 he was pleased to send me to Petersburg; this pendulum, which is no other than 

 a simple steel rod fixed to a lentille, made at Para 98740 oscillations in 24 hours 

 of mean time, and at Paris 98891 in the same time. I made experiments with 

 this same pendulum at Petersburg, before my departure for Lapland, and have 

 repeated them since my return fhither. They give the number of oscillations in 

 24 hours of mean time 9894 1, having been careful to preserve constantly the 

 same temperature, and to cause the pendulum to swing very small arcs. At 

 Ponoi, I found the number of oscillations 98946. Hence it follows, that tbe 

 simple pendulum, which beats seconds at Petersburg, vi'ill be 441.02 lines, Paris 

 measure, that is -jV_ Un. longer than the pendulum which beats seconds at Paris ; 

 and the pendulum at Ponoi will be 441 .22 lin. that is t^v lines longer than that 

 of Paris. 



The excess of the Paris pendulum, above that at the equator, has been 

 determined by the academicians 1.50 lin. and admitting Sir Isaac Newton's 

 principle, and Huygens's, that the increase of gravity, in approaching the pole, 

 follows the ratio of the square of the sine of latitude, we should find I.98 lin. 

 for the excess of the Petersburg pendulum above that at the equator, instead 



