78 



APPENDIX, No. III. 



able, especially in the steadiness of the tempera- 

 ture and the uniformity of the clock's rate ; hut 

 as they were decidedly most favourable in the 

 case of Mr. Foster's experiments, I have no hesi- 

 tation in considering his as the most entitled to 

 credit. 



Mr. Foster is the gentleman to whose co-opera- 

 tion I owed so much when observing the comet at 

 Valparaiso ; an account of which, in a letter to 

 Dr. Wollaston, appeared in the Transactions of the 

 Royal Society for 182:2. His present work speaks 

 sufficiently for itself ; but I should be doing him 

 scanty justice by confining myself to such a refer- 

 ence, without also stating that, occupied as I was 

 with professional duties, it would have been hope- 

 less to have undertaken these experiments with- 

 out the valuable assistance of a person who, besides 

 being free to attend exclusively to the subject, was 

 thoroughly skilled in all its details. 



This zealous officer has since been promoted to 

 the rank of Lieutenant, and after being elected a 

 Fellow of the Royal Society, accompanied Captain 

 Parry on his voyage to the N.W. in 1824 and 25. 

 The very important experiments which he has 

 made in those regions, and which afterwards ap- 

 peared before the public, justified the high promise 

 which I had some years ago the honour to make 

 in his favour. 



Being desirous of presenting an account of these 

 operations to the Royal Society before the vacation 

 of 1823, 1 had not time to repeat the experiments 

 in London before the above letter was read. Since 

 that period, however, I ascertained by careful ob- 

 servation, that the number of vibrations made by 

 the pendulum now, did not accord with that which 

 resulted from the experiments made in London 

 before the voyage. The number of vibrations of 

 this pendulum, in London, in May 1820, before 

 the voyage, was .... 86235.98 



The number in August 1823, after the 



voyage, was 86236.95 



The difference being . 



.97 



As it was not possible that so great a difference 

 could arise from errors of observation, it became 

 an object of anxious inquiry to discover the cause. 

 Captain Kater was disposed to assign it to an ac- 

 cident which had happened to the pendulum at 

 San Bias, but which I, at first, imagined inadequate 

 to such an effect. The accident was this : the 

 pendulum, when not in use, was, as usual, raised 

 by means of a screw, so that the knife edge was 

 lifted clear of the agate planes on which it vibrated 

 during the experiments. This screw being too 

 small, or having some flaw in it, unexpectedly 

 broke at San Bias before the experiments there 

 were begun ; and although the knife edge was not 

 raised more than the twentieth of an inch, yet, 

 as the pendulum weighed more than 1 51bs., the 

 fall might, he thought, have altered the form of 

 so delicate an e<lge in a slight degree, and thus 

 have virtually lessened the distance between the 

 point of suspension and the centre of oscillation ; 

 for if the knife edge be supposed to have become 

 cylindrical, the virtual point of suspension, as has 

 been demonstrated, would be at the distance of 



the radius of curvature of this cylindrical portion 

 below its surface, and the number of vibrations of 

 course be greater than before. 



As the whole pendulum had acquired a coating 

 of oxide, with the exception of the tail-piece, 

 which was lackered, I was desirous of ascertain- 

 ing in what manner, and to what degree, its 

 vibrations would be affected by this partial addition 

 of weight ; and for this purpose the following ex- 

 periments were made : — The vibrations of the 

 pendulum in its oxydized state having been deter- 

 mined, 10 grains of weight were affixed at § 

 of the length of the bar, measured through the 

 ball, from the point of support, that being sup- 

 posed to be near the centre of oscillation of the 

 oxide. This had for its object to discover, before 

 cleaning the pendulum, what would be the effect 

 of an addition of weight at that place. On swing- 

 ing it accordingly, the number of vibrations was 

 increased 0.83 in 24 hours. It was then taken" to 

 the Mint, and the weight, carefully determined by 

 Mr. Barton in one of his delicate balances, was 

 found to be 15 lb. lOoz. 14 dwts. 12^ grs. It was 

 next cleaned by Captain Kater, by means of 

 diluted sulphuric acid, and afterwards washed with 

 a solution of soda in water, and being effectually 

 dried, was again weighed, when it was found to 

 have lost exactly 24f grains. Coincidences were 

 now taken on three succeeding days, and the 

 number of vibrations of the pendulum in its clean 

 state proved to be fewer than when it was coated 

 with oxide by only 0.73 of a vibration. Since no 

 more than £ part of the oxide removed could be 

 oxygen, only \ of the above difference between its 

 vibrations when clean and when coated, or 0.14, 

 can be ascribed to additional weight since it was 

 formerly swung in 1 820 ; the real difference, how- 

 ever, to be accounted for, being 0.97, this cause is 

 manifestly inadequate to the effect. I therefore 

 thought it right, after attentively considering every 

 other possible manner in which the pendulum 

 could have been altered, to adopt the idea which 

 had been suggested, and which was eventually 

 proved to be correct, since the knife edge, upon 

 removal after the experiments were over, was 

 found to be distinctly rounded. To obtain the 

 most correct results, I accordingly used the vibra- 

 tions made in London in 1820, to compare with the 

 experiments made before the accident, and the 

 vibrations recently determined in London for com- 

 paring with those made after it ; an arrangement 

 rendering the resulting ellipticities entirely in- 

 dependent of that circumstance. 





ABSTRACT OF THE 



MOST EXACT RESULTS AT EACH 

 STATION. 



Stations. 



Diminution 

 of Gravity 

 from Pole 

 to Equator. 



Ellipticity. 



Length of 

 Equat. Pend. 



Galapagos 0° 32' 0" N. 

 SanBlas, 210 30'25"N. 

 Rio, . 22° 55' 22" S. 



.0051412 

 .0054611 

 .00554.31 



1 

 284.98 



1 

 .313.55 



302^37 



39.017196 



39.00904 



39.01206 



