XXX Presidents Address. \_A.Vi^. 29, 



95 millions of miles was given for years and years as the Sun's distance ; 

 but in 1868 Stone shewed in the clearest manner possible that Encke 

 had made a complete mistake in interpreting the observations at 

 Otaheite, and that when this mistake was rectified the value given 

 was 8''*91. A somewhat similar mistake was made in the first result 

 published from the British observations of the 1874 transit, and the 

 error was again pointed out by Stone. I venture to say that no 

 astronomer, who has seen the phenomena himself, will question the 

 accuracy of Stone's corrections ; and yet these mistakes are constantly 

 trotted out to prove that the method is Avorthless. When the time 

 comes round again for the next transits in 2004 and 2012 they will 

 no doubt be observed, but the question of the Sun's distance will have 

 been settled long before that. In 1877 a notable attempt to solve 

 the problem was made by Mr. Gill, at the island of Ascension, by the 

 method known as that of " Diurnal parallaxes." The planet Mars at 

 some of its oppositions approaches the earth comparatively closely, 

 and is then considerably displaced with reference to neighbouring 

 stars, according as the observer on the surface of the earth sees it 

 in the morning or evening. ^ With a heliometer stationed on the island 

 of Ascension Mr. Gill determined the amount of this displacement 

 with a very high degree of accuracy and the value of the solar 

 parallax deduced was 8"'78. This value is considerably smaller than 

 what was judged to be the most probable value pointed out by other 

 good methods, but it is certainly entitled to great weight. 



The velocity of light in combination with the value of the constant 

 of aberration supplies us with another method of determining the 

 solar parallax. Recent determinations of the velocity of light have 

 been made by Cornu, Michelson and Newcomb. Newcomb's experi- 

 ments were by far the most complete and elaborate, and his result for 

 the velocity of light at the earth's surface is probably not in error as 

 much as twenty miles a second. A new value of the constant of 

 aberration was deduced about the same time, 1882, by Nyren of the 

 Poulkova Observatory ; and this value combined with Newcomb's 

 light velocity gives a parallax of 8"'79, which is strikingly confirma- 

 tory of Mr. Gill's result. There is one element of doubt, however, in 

 this method ; can we assume that light travels with exactly the same 

 speed at the earth's surface as it does in interstellar space ? We 

 cannot tell. In 1872 Galle, of Berlin, proposed that some of the 

 minor planets which come moderately near the earth should be pressed 

 into the service, and an attempt was made |in 1873 with the planet 



