OF ARTS AND SCIENCES : APRIL U, 1S63. 125 



even the first opposition of Mars led to no significant result. Dr. 

 B. A. Gould has computed the solar parallax from the first opposition 

 of Mars, observed at Chili, at 8".50. 



3. The solar pai'allax can also be computed from the law of uni- 

 versal gravitation. The principle may be thus stated : The motion of 

 the moon round the earth is disturbed by the unequal attraction of the 

 sun on the two bodies. The magnitude of the disturbance will be in 

 some proportion to the distance of the disturber, when compared with 

 the relative distance of the two disturbed bodies ; and this ratio of 

 distances is the inverse ratio of the parallaxes of the sun and moon. 

 By selecting one of the perturbations in the moon's longitude particu- 

 larly adapted to this purpose, Mayer, as early as 1760, computed the 

 solar parallax at 7".8. In 1824, Burg calculated this parallax, from 

 better observations, at 8". 62. Laplace gives it at 8'''.61. Fontenelle 

 had said that Newton, without getting out of his arm-chair, found 

 the figure of the earth more accurately than others had done by going 

 to the ends of the earth. Laplace makes a similar reflection on this 

 new triumph of theory. " It is wonderful that an astronomer, without 

 going out of his observatory, should be able to determine exactly 

 the size and figure of the earth, and its distance from the sun and 

 moon, simply by comparing his observations with analysis, the 

 knowledge of which formerly demanded long and laborious voyages 

 in both hemispheres. The accordance of the results obtained by 

 the two methods is one of the most striking proofs of universal 

 gravitation." Pontecoulant makes the solar parallax by this method 

 8".63. Lubbock, by combining Airy's empirical determination of the 

 coetficient with the mass of the moon, as he finds it from the tides, 

 (namely, -g\,) makes the solar parallax 8".84. If the mass of J^ is 

 substituted, the parallax is changed to 8".81. Finally, Hansen, in his 

 new Tables of the Moon, adopts 8".8762 as the value of the solar 

 parallax. Moreover, Leverrier, in his " Theory of the Apparent Mo- 

 tion of the Sun," deduces a solar parallax of 8".95 from the phenom- 

 ena of precession and nutation. 



The conclusions of this whole review are summed up in the following 

 table, in which the values of the solar parallax, and of the sun's distance 

 by the three methods of Astronomy, and by the experiment of Foucault, 

 are placed in juxtaposition. Also the different velocities of light found 

 by astronomical observations and by experiment. 



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