INTRODUCTORY AND HISTORICAL. 



of this enterprise has been given by Lieutenant Gilliss in the introduction to the present volume.* 

 It now only remains to make use of the results of the expedition. 



2.-GENERAL STATEMENT OF THE PROBLEM. 



The proof-sheets of the present volume III of the observations of the planets Mars and Venus, 

 constituted the materials placed in my hands by Lieutenant Gilliss for the determination of 

 the best value of the solar parallax which they would afford. The end to which he had 

 devoted so much earnest and self-sacrificing effort, and for which he had labored so zealously 

 through obstacles and disadvantages of almost every description, and the scrupulous care 

 manifestly bestowed by him and the assistant astronomers upon the observations, demanded 

 commensurate exertions for the deduction of the final result an amount of labor uncontem 

 plated in the original plan of the observations, and necessarily entailing a delay of several 

 months. 



The observations of Mars and Venus, made at the Santiago Observatory, by Lieutenant 

 G-illiss, and under his direction, are in perpecfc conformity with the plan previously laid down, 

 and consist of four series of micrometric comparisons, comprising two oppositions of each 

 planet : 



The first series for Mars consists of observations on 46 different days, between 1849, Decem 

 ber 10, and 1850, January 31. 



The first for Venus contains observations on 51 different days, between 1850, October 19, and 

 1851, February 10. 



The second for Mars, observations on 93 days, from 1851, December 16, to 1852, March 15. 



The second for Venus, on 2*7 days, from 1852, May 29, to 1852, September 13. 



Making in all observations upon 217 days, extending over nearly three years. 



For combinining with these, according to the contemplated method, which requires a com 

 parison in declination with the same star, upon the same date at a northern observatory, 

 correspondent observations were found as follows : 



By the Naval Observatory at Washington. 



Mars, I. series ; nine. 

 Mars, II. series ; two. 

 Venus, I. series ; eight. 



By the Royal Observatory at Greenwich. 



Mars, I. series; four. 



By the Harvard Observatory at Cambridge. 



Mars, L series; Jive. 



In all, therefore, there were but twenty-eightf correspondent observations for both planets 

 during these three years, eighteen of these being during the first Mars series, and two during 

 the second ; while for Venus, the planet especially selected, there were eight during the first, 

 and none during the second opposition. The details of these correspondent observations are 

 as follows : 



See Astr. Nachr. XXXI, p. 247, XXXIV, p. 340, XXXVI, p. 77. 



1 The Washington observation of 1850, October 19, has been rejected by the advice of Mr. Ferguson. 



