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two stars being determined in the usual manner, by observing them 

 on the same night, and in the same position of the instrument, gives 

 the sum of their zenith distances : and if on the next or some follow- 

 ing night y Draconis be observed, and after its passage the instru- 

 ment be turned half round, and the other star observed, then the dif- 

 ference of the measure, as read on the micrometer, will be the differ- 

 ence of the zenith distances of the two stars. These sums and differ- 

 ences, thus ascertained on different nights, will be independent of any 

 change that may happen to the instrument in the interval. This me- 

 thod affords the means of determining, with almost unlimited preci- 

 sion, the value of the small equations which become the subject of 

 investigation in the employment of the instrument. Thus all changes 

 of the position of the stars, occasioned by aberration, nutation, &c, 

 will produce double the effect on the small differential or subsidiary 

 angles, as measured by this method. For the investigations of these 

 small equations it will not be necessary to have determined either 

 the exact zenith distance of each star, or the exact difference of their 

 zenith distances, or the absolute magnitude of this subsidiary angle ; 

 its variation from time to time being the only important object of re- 

 search. The author is led to expect that this property may, at some 

 future period, be applied with advantage in investigations made with 

 moveable zenith instruments. 



March 20th, 1833. 

 MARK ISAMBARD BRUNEL, Esq., Vice-President, in the Chair. 



A paper was read, entitled, " Narrative of the Proceedings of Com- 

 mander Thomas Dickinson, of His Majesty's Sloop Lightning, while 

 employed in the Enterprise for the Recovery of the Public Stores and 

 other property sunk in His Majesty's late Frigate Thetis, on the south- 

 west side of the Island of Cape Frio." By Commander Thomas 

 Dickinson, R.N. Communicated by P. M. Roget, M.D., and J. G. 

 Children, Esq., Secretaries to the Royal Society. It was preceded 

 by the reading of a letter from the author to the Secretary, explain- 

 ing the reasons which induced him to lay this narrative before the 

 Royal Society, and place on the records of its proceedings the infor- 

 mation it contains relative to the commencement of an enterprise, 

 wholly planned and undertaken by himself, and which, under his 

 superintendence, was, by the great, persevering and meritorious ex- 

 ertions of his officers and crew, most successfully accomplished. 



The narrative commences with the statement of the consternation 

 produced at Rio de Janeiro on the receipt of the intelligence of the 

 loss of the Thetis, with a freight of about 810,000 dollars, on the 

 south-west side of the island of Cape Frio, and of the determination 

 of the author, on finding that no one seemed disposed to take any 

 step towards the recovery of the property thus lost, to make the at- 

 tempt himself, if he could obtain from the Commander-in-chief at that 



