378 W. Harkness—The Solar Parallax. 
TRANSIT OF 1761. 
1763. Hornsby (PTr, 1763, p. 494),-.--------- ed Te ge 9”°73 
1763. Short (PTr, 1763, p. 340), ee a sc ee oe 8°56 
Sree, Fiery tiene, £100, p. Sere ee eet 10°10 
' 1767. Planman (PTr, 1768, p. 127), 
TRANSIT OF 1769. 
1770. Euler (Novi Commentarii Ac. Sc. Petropol., t. xiv), 8”°8 
1771. enaty etc t at Oe eo ce eS a 8°78 
1771. Lalande ( Ac, 1771, SN, 2 | Rg SE Spal Rapiell -  agpeaalap tg 8°62 
Fett, MMMROUN er Fotos ek. CS See ee ae eS 8°723 
BRR le oe ge i cs ge teen ee: 8°63 
1772, Pingré EO 21 e, PALO ec on wet eae Se 8°80 
te ees es eee ow ees 8°43 
1814 Delambre (Astron. Théorique et Pratique, t. i, p 
Sentec ve sone mo aba ek lh ak pee co 8°552 
cea Dw Saou (Traité Analytique des Mouvements 
Apparent des reel Celestes, t. i, pp. 451-491),-- 8° ‘85 
1832. fee: (MAS, v, 58 
1865. Powalky (Conn. de tanith 1867 Additions, p. 22),-- 8°832 
1868. E. J. Stone (MNt, vol. xxviii, p. 264), 8°91 
TRANSITS OF 1761 anD 1769. 
1835. Encke (Abhand. der Akad. zu nies 1835, Math. 
K1., s. 309), 
en ae ee 8°571 
TRANSIT OF 1874. 
1877. Airy (The Observatory, 18977, vou ij p. 149); 26522 8°760 
1878. Tupman (MNt, 1878, vol. xxxviii, p. 455), ..------ 8°846 
The large differences in the parallaxes obtained by different 
astronomers from the same observations are due to the circum- 
stance that, as the instants of contact are rendered uncertain by 
the intervention of various disturbing phenomena, many of the 
observers record two or three different times, corresponding to 
as many different phases which they endeavor to describe, and 
thus the resulting eon are influenced to a certain extent 
by the interpretation put upon these descriptions. The interior 
contacts give better results than the exterior ones, but in any 
case the probable error is large. From sixty-one selected ob- 
servations of interior contacts of the transit of December, 1874, 
discussed by Col. Tupman (MNt, 1878, vol. xxxviii, 20 on page 
450, and 41 on p. 458), I find the probable error of an observed 
time of contact to be +4°59, which corresponds to a probable 
error of +0’15 in the distance between the centers of the sun 
and Venus. Actual errors of from twenty to Suey. seconds in 
the observed times of contacts are by no means uncommon. 
Observations with heliometers.—A few emer: were used 
in observing the transit of December, 1874, but I am not aware 
