C 455 3 
On the Rotatory Motion of the Stars on their Axes . 
I shall now add a few remarks on the subject of the rota- 
tion of the fixed stars on their axes. This motion has been 
lately mentioned in a paper, where I could not have an oppor- 
tunity to enter into the reasons why it ought to be admitted.* 
The discovery of the period of a Herculis furnishes me with 
an opportunity to say a few words upon the subject, as every 
addition to the list of periodical stars increases our knowledge 
of the construction of the celestial bodies. Not so much be- 
cause now one star more is known to be subject to periodical 
changes in its lustre ; for this would indeed be of no great con- 
sequence. But we ought not to be satisfied with merely in- 
rolling this circumstance among the list of facts we are ac- 
quainted with. The rotatory motion of stars upon their axes 
is a capital feature in their resemblance to the sun. It appears 
to me now, that we cannot refuse to admit such a motion, and 
that indeed it may be as evidently proved as the diurnal mo- 
tion of the earth. 
Dark spots, or large portions of the surface, less luminous 
than the rest, turned alternately in certain directions, either 
towards or from us, will account for all the phasnomena of 
periodical changes in the lustre of the stars, so satisfactorily, 
that we certainly need not look out for any other cause. Let 
us, however, take a review of any objections that might be 
made. 
The periods in the change of the lustre of Algol, j Q Lyrac, 
S Cephei, and y Antinoi, are short ; being only 3, 5, 6, and 7 
days respectively : those of 0 Ceti, the changeable star in 
* Phil. Trans, for the year 1795, Parti, page 68. 
MDCCXCVI. 3N 
