PORTION OF THE COSMOS. THE SOLAll DOMAIN. 349 



in parts of the semi-major axis. The diameter of this planet is 

 1694 German, or 6776 English, geographical miles; its 

 ma ss 40! 8'a - 9 > ^ s v l um e 0'957, and its density 0'94, as 

 compared to the Earth. 



Of the transits of the two inferior planets, first announced 

 by Kepler in his Rudolphine Tables, it is that of Venus 

 which, by the aid it affords towards the determination of 

 the Sun's parallax and the distance of the Earth from the 

 Sun thence derived, is most important in its bearings on 

 the theory of the entire planetary system. According to 

 Encke's complete investigation of the transit of Yenus which 

 happened in 1769, the Sun's parallax is 8"-57116 (Ber- 

 liner Jahrbuch fur 1852, S. 323). On the proposal of 

 a distinguished mathematician Professor Gerling, of Mar- 

 burg since 1849, a new investigation respecting the Sun's 

 parallax has been undertaken by the orders of the Go- 

 vernment of the United States of ]\orth America, It is 

 designed to obtain the parallax by means of observations of 

 the planet Yenus near its eastern and western elongation, 

 as well as by micrometric measurements of the differences 

 in Eight Ascension and Declination of well-determined 

 fixed stars, made at places on the Earth's surface differing 

 considerably in latitude and longitude (Schum. Astr. 

 Nachr. No. 599, S. 363 ; and No. 613, S. 193). The 

 astronomical expedition charged with the prosecution of this 

 undertaking, and which is commanded by a highly-informed 

 officer Lieutenant Gilliss, of the United States Navy 

 has proceeded to Santiago de Chile. 



The rotation of Yenus upon its axis was long the subject 

 of many doubts. Dominique Cassini, in 1669, and Jacques 

 Cassini, in 1732, found 23 hours 20 minutes as its period; 



