45^ Dr. Herschei/s Remarks on the 
Hydra, and that in the neck of the Swan are long, amounting 
to 33 1 ’ 394 > ar, d 497 days. Will not a doubt arise whether 
the same cause can be admitted to explain indiscriminately 
phenomena that are so different in their duration ? 
To this it may be answered, that the whole force of the ob- 
jection is founded upon our very limited acquaintance with the 
state of the heavens. Hitherto we have only had seven stars whose 
periodical changes have been determined. No wonder then that 
proper connections between their different periods were want- 
ing. But let us now place a Herculis among the list, which is 
not less than 6 o days in performing one return of its changes. 
Here we find immediately, that the step from the rotation of 
a Herculis to that of o Ceti, is far less considerable than that 
from the period of Algol to the rotation of a, Herculis ; and 
thus a link in the chain is now supplied, which removes the 
objection that arose from the vacancy. 
There is, however, another instance of a slow rotatory mo- 
tion ; and it is doubly instructive upon this occasion. In one 
of my former papers it has been shewn, that the 5 th satellite 
of Saturn revolves on its axis in 79 days ; this not only shews 
that very slow rotatory motions take place among the celestial 
bodies ; but from the arguments that were brought to prove 
its rotation, which I believe no astronomer will oppose, we are 
led to apply the same reasoning to similar appearances among 
the fixed stars. A variation of light, owing to the alternate 
exposition of a more or less bright hemisphere of this periodi- 
cal satellite, plainly indicates that the similar phenomenon of 
a changeable star, arises from the various lustre of the different 
parts of its surface, successively turned to us by its rotatory 
motion. 
