C 201 3 
XVIII. Observations on the Planet Venus. By William 
Herschel, LL. D. F. R. S. 
Read June 13, 1793. 
The planet Venus is an object that has long engaged my 
particular attention. A series of observations upon it, which 
I began in April, 1777, has been continued down to the pre- 
sent time. 
My first view, when I engaged in the pursuit, was to as- 
certain the diurnal rotation of this planet ; which, from the 
contradictory accounts of Cassini and Bianchini, the former 
of which states it at 23 hours, while the latter makes it 
24 days, appeared to me to remain unknown, as to its real 
duration : for the observations of these gentlemen, how 
widely different soever with regard to time, can leave no 
doubt but that this planet actually has a motion on its axis. 
The next object was the atmosphere of Venus ; of the 
existence of which also, after a few months observations, I 
could not entertain the least doubt. 
The investigation of the real diameter, was the third ob- 
ject I had in view. 
To which may be added, in the last place, an attention to 
the construction of the planet, with regard to permanent ap- 
pearances ; such as might be occasioned by, or ascribed to, 
seas, continents, or mountains. 
