] x iv INTRODUCTORY AND HISTORICAL. 



parallax of the sun may be assumed as 10" without danger of an error exceeding 0."25, thus 

 attaining* a most remarkable accordance with the results at which Cassini and Flamsteed had 

 arrived in the previous century. 



About 1760, Tobias Mayer struck out a new path, and deduced a value for the solar parallax 

 by means of the lunar theory. One of the equations for the perturbation in longitude depends 

 on the simple angle between sun and moon, and the coefficient had been, up to that time, 

 determined by the employment of 10". 8, as the amount of the solar parallax, which forms one 

 factor. Mayer, however, comparing! theory with observations, deduced empirically a new 

 value for the coefficient, whence he obtained 7". 8, which determination he esteemed subject to 

 an error of one twenty-fourth part, at the outside, since the co-efficient 1' 55", upon which it 

 depended, could not be in error by more than 5".J 



The transits of Venus, in 1761 and 1769, had long been awaited by astronomers with intense 

 interest, and were regarded throughout the world as the best means available for accurate 

 determination of the dimensions of the orbit of the earth, and of the value in terrestrial measures 

 of the unit in which celestial measures are necessarily expressed. A concise narration of the 

 efforts made to procure accurate data, and of the materials collected, may be found in the mas- 

 terly and standard discussion of the problem by Encke. 



The English sent|| Maskelyne to St. Helena ; and Mason and Dixon, the same who subse- 

 quently measured the arc of a parallel in America, were destined to Sumatra. Fortunately, 

 they were so much delayed that, in their apprehension lest they might not arrive in season at 

 their appointed station, they decided to observe at the Cape of Good Hope, a more desirable 

 situation, as the event proved. The French Academy sent the astronomer Pingre to the island 

 Rodrigues, a little more than 300 miles from Mauritius, in the Indian ocean. Le Gentil was 

 to have observed at Pondicherry, but the war deprived him of the means, inasmuch as the ship 

 containing his instruments was captured by the British. Another French astronomer, Chappe^ 

 was sent by the St. Petersburg Academy of Sciences to Tobolsk, in Siberia, and a Russian 

 astronomer, Rumowski, to Selinghinsk, near Lake Baikal, on the Mongolian frontier. Besides 

 these regularly organized expeditions, astronomers were everywhere on the alert. Not only 

 throughout Europe, but at the missionary stations of southern and eastern Asia, observations 

 of the ingress and egress of the planet were made with all the care and nicety which circum- 

 stances permitted. The various results deduced, there is no need of fully recapitulating here.^I 



They were far from harmonious, and led to much warm controversy, which, indeed, almost 

 assumed the form of a national dispute. Short obtained, by different methods, different results 

 between 8". 47 and 8". 67, and adopted the value 8". 65 as final. In a second memoir he arrived 

 at the values 8".49 and 8".63. Pingre found, on the other hand, 10".l; Ruinowski, 8". 33; 

 Hornsby, 9". 5 to 10"; Audifredi, ("Dadeius Ruffus,") 9J". To sum up, the transit of 1761 did 

 not contribute much to our knowledge of the astronomical unit. 



The transit of 1769 promised better results for many reasons. The uncertainty of the values 

 from the first transit stimulated to the greatest possible effort, which was also insured by the 

 consciousness that more than a century must elapse before another similar opportunity would 

 present itself; and beside all this, the transit itself was, upon the assumption that the most 

 advantageous points for observation would be occupied, (as in fact they were,) still further more 

 favorable than its immediate predecessor. Both ingress and egress were visible at numerous 



< Mfm. de I'Acad. da Sciences, 1760, 93. 

 f Theoria Luna;, 50, p. 50. 

 jlbid., 51, p. 52 



Die Entfernung der Sorme von der Erde, ma dem Venusdurchgang von 761, hergdeitd von J. F. ENOKE Gotha 1822 



|| ENKE, Enlf. d. Sonne, 5, pp. 11-32. 



U See, among others, SHORT, Phil. Tran 8 ., 1762, p. 611 ; 1763, 300. PINGRE, Mem, del' Acad., 1761, pp. 413, 483 ; 1764, 

 p. 339 ; 1765, p. 1. HOBNSBY, Phil. Trans., 1763, p. 467. RUMOWSKI, N. Comm. Acad. Petrop. XI, Out. p. 41, Mfm. pp 

 83 ' * 87 ' XI1 ' 875 ' PLANMANN ' ^^.Akad.Handl.,\US, 118; 1764, 144. Phil. Trans., 1768, 107. ATOIFKEI.I, Investi- 

 gaiio Parallax* Sulara, Rome, 1765. AUDIFBEDI, de Sola Parallaxi, Rome, 1766. 



