Recovery of Treasure from the wreck of the Thetis. 867 



prosecutes the subject by a series of analytical investigations, which 

 are not susceptible of abridgement, but from which he obtains formula? 

 which do not quite agree with those of M. Plana. 



A paper was also read, entitled, " Some Suggestions relative to the 

 best method of employing the new Zenith Telescope lately erected at 

 the Royal Observatory." By John Pond, Esq., F.R.S., Astronomer 

 Royal. 



During the observations made by the author, in the course of last 

 summer, with the new zenith telescope lately erected at the Royal 

 Observatory, for the purpose of measuring the zenith distance of 

 7 Draconis, it occurred to the author to avail himself of subsidiary 

 observations on another star, of about the fifth magnitude, which has 

 nearly the same zenith distance towards the south that 7 Draconis 

 has towards the north, and which passes the meridian between 20 

 and 30 minutes, in time, after it. The angular distance between the 

 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 eithei 

 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 20. — A paper was read, entitled, " Narrative of the Proceed- 

 ings of Commander Thomas Dickinson, of His Majesty's Sloop Light- 

 ning, 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 nouth-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 pre- 

 ceded by the. reading of a letter from the author to the Secretary, ex- 

 plaining 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 



