296 HISTORY OP ASTRONOMY. 



the times of opposition of that planet in 1849 and 1852. 

 To facilitate the observations, and to secure concert of 

 action, so that observers in every part of the world 

 might use the same stars of comparison, Lieutenant 

 Gilliss prepared an ephemeris of these planets and suit- 

 able stars of comparison during the critical periods. 



The expedition set sail in 1849, and arrived safely at 

 Valparaiso. The observatory was located at Santiago, 

 the capital of Chili, where observations were commenced 

 in December, 1849. During that season the weather 

 was exceedingly favorable for observations. Of the fifty- 

 two pre-appointed nights remaining of the first series 

 of observations on Mars, there were only four when no 

 observations could be made ; and within these forty-eight 

 nights, 1400 observations of the planet were accum- 

 ulated. During the second series on Mars, comprising 93 

 days, between December 16, 1851, and March 15, 1852, 

 about 2000 differential measures were made on seventy- 

 eight nights, and meridian observations on eighty nights. 

 Between October 19, 1850, and February 10, 1851, dif- 

 ferential measures of the planet Yenus and comparing 

 stars, were made on fifty-one nights ; and there were 

 seventy-three meridian observations, at which time its 

 diameter also was measured. Owing to its very fre- 

 quent tremulous motion in the evening twilight, the 

 differential measures, when approaching its eastern 

 stationary terms, were often found dimcuTt, and rarely 

 afforded much satisfaction. But in the morning twi- 

 light the atmosphere was tranquil, and generally so clear 



