Royal Astronomical Society. 531 



The three values of /x, are then 



p, = +o"-n 



jx, = + -25 



JM , 1 = +0 '31 

 or, taking the mean /&, = 0"'22. 



This, of course, is only given as an approximate estimation ; but 

 it seems difficult to resist the conclusion that the mass of Venus 

 should be augmented by a quantity which cannot be put lower than 

 one-tenth, and is probably considerably larger. An augmentation 



of one-tenth would make this mass ogt - ;;o , of two-tenths, - .. . ., ^ . 



365308 334806 



III. On a Method of Determining the Latitude at Sea. By M . 

 C. L. von Littrow, Adjoint- Astronomer at the Imperial Observatory 

 at Vienna. Communicated by the Rev. W. Whewell, Master of 

 Trinity College, Cambridge. 



IV. On the Rectification of Equatoreals by Observations of Stars 

 on the Meridian and at an Hour-Angle of Six Hours. By M. C. L. 

 von Littrow. Communicated by the Rev. W. Whewell. 



V. The Parallax of a Centauri deduced from Mr. Maclear's Ob- 

 servations at the Cape of Good Hope in the years 1839 and 1840. 

 By Professor Henderson. 



An abstract of the principal contents of this paper will be found 

 in Professor Henderson's letter, contained in the last Monthly No- 

 tice, viz. that for March 1842*. In addition, the author gives the 

 following facts relating to the history of the observations of the 

 star a Centauri. The earliest recorded observations which he has 

 found are those of Richer, at Cayenne, in 1673, and ofHalley, at St. 

 Helena, in 1677 ; but neither of these astronomers mentions it as 

 being double. Feuillee appears to have been the first person who 

 observed it to be double, his observations being made at Conception, 

 in Chili, in July 1709, with a telescope of 18 feet focal length. He 

 estimates their magnitudes as being of the third and fourth, the 

 smaller star being the more westerly, and their distance as equal to 

 the apparent diameter of the smaller star {Journal des Observations 

 Physiques, &c, par Louis Feuillee, tome i. p. 425 ; Paris, 1714). 



La Condamine observed the star during the expedition to Peru 

 for measuring an arc of the meridian (see Philosophical Transactions 

 for 1 749, p. 142). He estimated it as being of the first magnitude, 

 and recognised its duplicity ; and he remarked that the larger star 

 was northward of the other, and to the east of it. From La Caille's 

 observations in 1751-2, the distance of the two stars appears to 

 have been 22"*5. Maskelyne observed them at St. Helena in 1761 

 (see Philosophical Transactions for 1764, p. 383), and estimated 

 them as being of the second and fourth magnitudes. Their distance, 

 as observed with a divided object-glass micrometer, he found to be 

 from 15" to 16". From this time to the time of the institution of 

 the Paramatta Observatory, the author has met with no observations 

 of the distance of the stars. Mr. Dunlop, in the years 1 825-6, found 



* See present volume, p. 482. 



2N2 



