id
on an atomic solid. Analogy will only justify comparisons of like with
like. The tangent of a comet's orbit, also, can only be tangential to
the circular motion of the ether at and near perihelion, which is a very
small portion of its period of revolution. As far as the tangential
resistance is concerned, therefore, it matters little whether its motion
be direct or retrograde. If a retrograde comet, of short period and
small eccentricity, were discovered moving also near the central plane
of the vortex, it would present a very serious objection, as being
indicative of contrary motions in the nascent state of the system. There
is no such case known. So, also, with the inclinations of the orbits; if
these be great, it matters little whether the comet moves in one way or
the other, as far as the tangential current of the vortex is concerned.
Yet, when we consider the average inclination of the orbit, and not of
its plane, we find that the major axes of nearly all known cometary
orbits are very little inclined to the plane of the ecliptic.

In the following table of all the periodical comets known, the
inclination of the major axis of the orbit is calculated to the nearest
degree; but all cometary orbits with very few exceptions, will be found
to respect the ecliptic, and never to deviate far from that plane:

+--------------------------------------------------------------------+
| Designations | Periodic | Inclination | Motion | Planetary |
| of the Comets. | times. | of | in Orbit. | Intervals. |
| | | Major Axes | | |
|--------------------------------------------------------------------|
|Encke | 1818 | 3 years. | 1d | Direct |Mars & Ceres.|
|--------------------------------------------------------------------|
|De Vico | 1814 | | 2 | Direct | |
|Fayo | 1843 | | 4 | Direct | Ceres |
|De Avrest| 1851 | From | 1 | Direct | |
|Brorsen | 1846 | five | 7 | Direct | and |
|Messier | 1766 | to | 0 | Direct | |
|Clausen | 1743 | six | 0 | Direct | Jupiter. |
|Pigott | 1783 | or | 4 | Direct | |
|Pous | 1819 | seven | 3 | Direct | |
|Biela | 1826 | years. |