an the Planet Venus. 
*53 
Dec. 1791, and the 11th Jan. 1792,* the differences are more 
considerable. Thus, for example, from the most distant ob- 
servation, on the 28th Dec. 1789, at 5 h p. m. to the 27th Feb. 
1793, at 6 h 41' p. m. are 1157 days 1 hour and 41 minutes, 
which dividing into 1189,28 revolutions, might occasion some 
doubt. But, 
(a) In each separate period, several observations correspond 
as well as can be desired : 
(i G) The period is only assigned nearly, but the interval of 
more than four years is very long, so that an error of seconds 
may occasion such an excess ; and accordingly the abovemen- 
tioned time would divide even with a period of 2$ h 2 1' 19": and 
(7) In such computations, no regard is paid to the inequa- 
lities of the planet, nor to the middle of the duration of the 
phenomenon : wherefore so considerable a length of time can 
never be divided exactly by the period ; as my observations 
of the rotation of Jupiter likewise could not, under similar 
circumstances, though the period of that rotation is sufficiently 
well known.-f 
( b ) A like doubt might arise from the phenomenon being 
sometimes not at all or very doubtfully perceived, about the times 
of the greatest elongations, even at the hours when it was to be 
expected, according to the period. Hitherto, however, during 
more than four years, only three instances of this have occur- 
red to me; which were in the years 1790 and 1791, and about 
the time of the late western elongation, in August, 1793; in 
which last I only twice perceived barely a trace of a somewhat 
rounded form on the southern horn. Moreover, as often 
* See Beob. iiber die sebr betriichtlichen Gebirge und Rotation der Venus, § 26 to 30. 
t Beytriige zu den neuesten Astronom. Entd. p. 1 to 138. 
MDCCXCV. X 
