ON ASTRONOMY. 123 



From the transit of lYOl he finds P =: 8". 53 

 From the transit of 1769 he finds P = 8". GO 



Mean P = 8". 5 7 



The sun's distance is therefore very nearly 95,300,000 miles. From 

 this all the other planetary distances are easily obtained by Kepler's 

 Third Law. Airy, in^his Lectures on Astronomy, puts the distance 

 at 95,500,000, with a probable error of not more than 500,000 miles. 



It is worthy of remark, as showing that all true methods are in 

 harmony with each other, that Professor Burg, in 1824, following the 

 footsteps of Mayer, deduced the solar parallax from analytical in- 

 vestigations of the Lunar Theory, and found for a result 8". 62, very 

 nearly coincident with that of Encke. Laplace had obtained, in the 

 same way, 8". 61, a result almost identical with that of Burg. 



These remarkable coincidences of analysis with observation in the 

 remote and untrodden fields of science furnish, as Laplace suggests, 

 one of the most striking proofs of universal gravitation. • 



Before bringing these remarks to a close, I must call your attention 

 to one other effort to approximate still nearer to this i'undamental 

 element in the solar system, an effort recommended by the ablest 

 astronomers, at home and abroad, and deemed worthy of the combined 

 labor of different countries. I refer to the United States naval astro- 

 nomical expedition to the southern hemisphere during the years 1849, 

 1850, 1851, and 1852, under Lieutenant J. M. Gilliss. 



In 1847 Dr. Gerling, a distinguished astronomer and mathema- 

 tician of Marburg, in Germany, called the attention of astronomers 

 to the importance of repeating the observation on Mars in opposition, 

 and especially of making observations on Venus during the inferior 

 conjunction, for the more exact determination of the sun's parallax. 

 Pie suggested reasons for believing that, with the increased power 

 and accurac}^ of instruments, together with the delicate and refined 

 methods of observation now in use, most valuable results, additional 

 to any hitherto realized, might be obtained. He particularly urged 

 the importance of observations on Venus, which had not heretofore 

 been rendered available for this purpose. The principle of this will 

 be understood by reference to figure 14. 



Fi(j. M 



Two observers at and 0', by observation obtain the angle at 

 Venus, viz : V 0', in the same manner as before given in the case 

 of MarSj from which the distance E V can be computed. The planet 



