INTRODUCTORY AND HISTORICAL Ixvii 



publish better observations than he knew how to make, he had changed his record of the 

 inomeut of commencement, after computing it by means of elements so erroneous that his published 

 value was out by 40'. Kejectingall observations which had passed under the revision of Father 

 Hell, except in those isolated instances in which the original notes were decipherable and intel- 

 ligible, Encke redetermiued the longitude of Wardoehuus and thus obtained the means of 

 correcting the original equations of condition for the transit of 1769 and solved them anew. 



This revised computation gave as the value resulting from the transit of 1769, 



8".5931 0".0460, 



being less by 0".01 than the previous determination from the same transit. Combining this, 

 as before, with the value given by the transit of 1761, he found the mean value 



8".57116 0".0370, 

 thus decreasing the former value by the comparatively unimportant amount 0". 00644. 



This last result has, since 1835, furnished the standard value for the parallax of the sun and 

 corresponding to a mean distance from the earth of 2,406J terrestrial semidiameters or 82 719 316 

 equatorial miles.* Assuming with Encke, f the Toise de Perou as 6 394 564 feet, or what is 

 equivalent, 1 statute mile = 825 701 toises, Bessel's determination:]: of the earth's dimensions, 

 according to which the length of a quadrant is 5 131 179.81 toises, and the major semi-axis 

 3 272 077.14 toises, gives for the earth's mean distance from the sun in statute miles 



95 360 000 412 000. 



In 1824, the same year in which Encke's discussion of the transit of 1769 appeared, Professor 

 Burg, like Mayer before him, deduced a value of the parallax from the lunar theory, and 

 urged the employment of this method as capable of affording better results than those which 

 could be anticipated from any other method earlier than the Percws-transit of 1874. Burg's 

 value, resulting, of course, from a lunar theory more refined than was attainable in the time of 

 Mayer, was 8". 62= 0".035. Laplace too, in the same manner, obtained|| the solar parallax 

 from this equation as 26.58 centesimal seconds or 8". 61, a result thoroughly accordant with 

 Encke's determination, and adds, "II est tres remarquable q'un astronome, sans sortir de son 

 observatoire, en comparant seulement ses observations a 1'analyse, cut pu determiner exactement 

 la grandeur et I'aplatissement de la terre, et sa distance au soleil et a la lune; elements dont 

 la connaissance a etc le fruit de longs et penibles voyages dans les deux hemispheres. L'accord 

 des resultats obtenus par ces deux methodes est une des preuves les plus frappantes de la 

 gravitation universelle. ' ' 



A couple of years previous to the discovery of Father Hell's forgeries, Henderson, at the Cape 

 of Good Hope, had made a series of observations^ of declinations of Mars, during the opposition 

 of 1832, which he compared with simultaneous observations at Greenwich, (two instruments, 

 the mural circle by Troughton, and that by Jones,) Cambridge, and Altona. The several 

 resultant values** were 9".076, 9". 343, 8". 588, and 9". 028, and the final mean by weightsff gave 

 him 9". 028. All these values are larger than either of Encke's adopted ones, and their great 

 discordance precludes reliance upon any of them. 



The method proposed by Professor Gerling for obtaining a still closer approximation to the 

 actual value of the solar equatorial parallax has been already developed, with full statements of 

 its relative advantages, both by the astronomer who suggested itJJ and by the one under whose 

 direction, the Astronomical Expedition to Chile was organized and carried out. The history 



Berlin Aslr. Jahrb. 1852, p. 323. f DOVE, Moans und Metaen. p. 39. EMCKE, Berl. Ast. Ja/trb. lS52,j>. 321. 



t Astr. Nadir. XIX, 97. 



Attr. Nachr. Ill, pp. 346, 348. 



|| Systems du Monde, Liv. IV, (Euvret VI, p. 264. 



Tf Aslr. Nachr. X, p. 162. 



o Astr. Nachr. XI, pp. 296, 404. 



ft Ibid. p. 404. Monthly Notices R. Astr. Soc. Ill, 39. 



J| Astr. Nachr. XXV, p. 363, XXVI, p. 195. 



Sjjj Intrcxl. to this volume, anil A. N. 



