the case.
But, a more capital feature still has been more recently brought to
light by observation, although previously familiar to the author, who,
in endeavoring to verify the theory, seriously injured his sight, by
observing with inadequate instrumental means. This is the periodicity of
the spots.

We have already observed, that there is reason to suppose that the
action of the inner vortex of the earth is probably greater than that of
the outer vortex, on account of the conflicting currents by which it is
caused. And the full development of this vortex requires, that the
central vortex or mechanical axis of the system shall be nearly
tangential to the surface. In this position, the action of the central
vortex is itself at a maximum; and, when the planets of the system are
so arranged as to produce this result, we may expect the greatest number
of spots. If the axis or central vortex approaches to coincidence with
the axis of the sun, the lateral vortices disappear, and the central
vortex being then perpendicular to the surface, is rendered ineffective.
Under these circumstances, there will be no spots on the sun's disc.
When, on the other hand, all the planets conspire at the same side to
force the sun out from the mechanical centre of the system, the surface
is too distant to be acted on by the central vortex, and the lateral
vortices are also thrown clear of the sun's surface, on account of the
greater velocity of the parts of the vortex, in sweeping past the body
of the sun. In this case, there will be but few spots. The case in which
the axis of the vortex coincides with the axis of the sun, is much more
transient than the first position, and hence, although the interval
between the maxima will be tolerably uniform, there will be an
irregularity between a particular maximum, and the preceding and
subsequent minimum.

The following table exhibits the solar spots, as determined by Schwabe,
of Dessau:

Year of observation. Groups of spots observed. Number of days.
1826 118 277
1827 161 273
1828 225 282
1829 199 244
1830 190 217
1831 149 239
1832 84 270