124 PROCEEDINGS OF THE AMERICAN ACADEMY 



astronomers or governments with negligence, or want of appreciation 

 towards this golden opportunity. The solar parallax which Encke de- 

 duced from an elaborate discussion of all the observations, fifty years 

 after they were made, is 8".57116. This coiTesponds to a solar dis- 

 tance of 95,360,000 statute miles. 



Although transits of Venus will take place in 1874 and 1882, and 

 astronomers already begin to talk of preparing for them, I have the 

 authority of Encke for declaring that, in comparison with that of 1769, 

 the next two transits will be so unfavorable, that nothing short of 

 jierfection in the construction of instruments, and in the art of ob- 

 serving, can compensate for the natural disadvantage ; — so that the 

 reduction of the possible error in the sun's parallax within the limit 

 of the hundredth of a second is hopeless for at least two centuries 

 more. 



2. The solar parallax may also be derived from the parallax of 

 Mars, when this planet is in opposition. In 1740, the French as- 

 tronomer, Lacaille, was sent to the Cape of Good Hope, and from 

 the parallactic angle observed between the direction of Mars, as seen 

 from that station and from the Observatory of Paris, deduced from 

 observations of declination, the horizontal parallax of Mars was 

 computed, and consequently that of the sun. The solar parallax 

 thus found was 10".20, with a possible error not exceeding 0".25. 

 Henderson, by comparing his own observations of the declination of 

 Mars at its opposition in 1832, with corresponding observations 

 at Greenwich, Cambridge, and Altona, computed the solar parallax 

 at 9".028. 



The United States Naval Astronomical Expedition to Chili, under 

 the charge of Lieutenant J. M. Gilliss, during the years 1849-52, had 

 for its object the advancement 'of our knowledge of the solar parallax, 

 partly by observations of Mars at opposition, and partly by observa- 

 tions of Venus during the retrograde portion of her orbit, and especi- 

 ally at the stationary points, in conformity with a method suggested by 

 Dr. Gerling ; the whole to be compared with simultaneous observa- 

 tions at the northern observatories. Although the observations at 

 Chili were made on 217 nights, covering a period of nearly three 

 years, the co-operation of northern astronomers was so insufficient that 

 only 28 corresponding observations were made. On this account tlie 

 second conjunction of Venus was useless; the other conjunction of 

 Venus and the second opposition of Mars were of little value, and 



