NATURAL PHILOSOPHY. 



129 



long and laborious voyages into both hemispheres." The accordance 

 of the results obtained by the two methods is gne of the most striking 

 proofs of universal gravitation. Pontecoulant makes the solar parallax, 

 by this method, 8^.63. Lubboch, by combining Airy's empirical deter- 

 mination of the coefficient with the mass of the moon, as he finds it from 

 the tides (viz., Jy) makes the solar parallax 8". 84. If the mass of -fa 

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

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

 parallax. Moreover, Leverrier, in his Theory of the apparent motion 

 of the Sun, deduces a solar parallax of 8". 95 from the phenomena of 

 precession and nutation. 



The conclusions of this 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. 



The three astronomical methods present solar distances, which, even 

 if we select the most trustworthy decision of each, differ by three or 

 four millions of miles ; that is, by three or four per cent, of the whole 

 quantity. Though the best products of the first and third methods 

 were at one time within a million of miles of each other, an increase of 

 lunar observations, and especially improvements in the lunar tables, 

 have now carried that difference up to four millions of miles. If Fou- 

 cault's experiment were allowed to give the casting vote, it would de- 

 cide in favor of the third method ; thus making the reflection of La- 

 place, already quoted, still more memorable. 



In regard to the commonly receive'd distance of the sun, which is 

 based upon Encke's profound discussion of all the observations made at 

 the last two transits of Venus, the case stands thus : Encke decides, 

 from the weights of the observations, discussed in the light of the math- 

 ematical principle of least squares, that the probable error of the sun's 

 distance, as given by the transits, does not exceed of the whole 



