$227] Transits of Venus 285 



different stations, or the difference in the durations of the 

 transit, can be without difficulty translated into difference 

 of direction, and the distances of Venus and the sun can 

 be deduced.* 



Immense trouble was taken by Governments, Academies, 

 and private persons in arranging for the observation of the 

 transits of 1761 and 1769. For the former observing 

 parties were sent as far as to Tobolsk, St. Helena, the 

 Cape of Good Hope, and India, while observations were 

 also made by astronomers at Greenwich, Paris, Vienna, 

 Upsala, and elsewhere in Europe. The next transit was 

 observed on an even larger scale, the stations selected 

 ranging from Siberia to California, from the Varanger Fjord 

 to Otaheiti (where no less famous a person than Captain 

 Cook was placed), and from Hudson's Bay to Madras. 



The expeditions organised on this occasion by the 

 American Philosophical Society may be regarded as the 

 first of the contributions made by America to the science 

 which has since owed so much to her ; while the Empress 

 Catherine bore witness to the newly acquired civilisation of 

 her country by arranging a number of observing stations 

 on Russian soil. 



The results were far more in accordance with Lacaille's 

 anticipations than with Halley's. A variety of causes pre- 

 vented the moments of contact between the discs of Venus 

 and the sun from being observed with the precision that 

 had been hoped. By selecting different sets of observations, 

 and by making different allowances for the various probable 

 sources of error, a number of discordant results were 

 obtained by various calculators. The values of the parallax 

 (chapter vin., 161) of the sun deduced from the earlier 

 of the two transits ranged between about 8" and 10"; while 

 those obtained in 1769, though much more consistent, still 

 varied between about 8" and 9", corresponding to a variation 

 of about 10,000,000 miles in the distance of the sun. 



The whole set of observations were subsequently very 

 elaborately discussed in 1822-4 and again in 1835 by 

 Johann Franz Encke (1791-1865), who deduced a parallax 

 of 8"*57i, corresponding to a distance of 95,370,000 miles, 



* For a more detailed discussion of the transit of Venus, see Airy's 

 Popular Astronomy and Newcomb's Potular Astronomy, 



