294: HISTORY OF ASTRONOMY. 



found in simultaneous observations of either Yenus or 

 Mars from two remote points of the globe. If an as- 

 tronomer in a high northern latitude observes the posi- 

 tion of one of these bodies when upon his meridian, and 

 another astronomer in a high southern latitude does 

 the same, a comparison of these two observations will 

 give the parallax of the planet, from which we can 

 compute its distance from the earth. The most favor- 

 able time for observing Mars is when it is nearest the 

 earth; that is, at its opposition; and the most favorable 

 time for observing Yenus is when it is stationary, or 

 near its inferior conjunction. The two places of ob- 

 servation must be in opposite hemispheres, as remote as 

 possible from each other, and it is desirable that they 

 should both be under the same meridian.. 



Lieutenant Gilliss, of the United States Navy, pro- 

 posed an expedition to Chili, for the purpose of making 

 observations upon Dr. Gerling's plan in connection with 

 the National observatory at "Washington. The Ameri- 

 can Philosophical Society, and the American Academy 

 of Arts and Sciences commended this plan to the favor- 

 able action of Congress ; and in August, 1848, Congress 

 authorized the fitting out of the expedition, under the 

 direction of the Secretary of the Navy. Lieutenant 

 Gilliss, to whom the plan of the expedition was mainly 

 due, was appointed to take charge of the expedition; 

 and passed midshipman (now lieutenant) Archibald 

 MacEea, and Henry C. Hunter, were appointed as assist- 

 ants. Suitable wooden buildings, to serve as an ob- 



