514 On the Presence of a Medium pervading all Space. 
before the natural term of their existence. This difficulty will, 
however, disappear when the effects of planetary attraction in 
condensing the medium are taken into consideration. How far 
observation of primary or secondary worlds give evidence of un- 
periodical changes in our system has not been yet determined 
with positive certainty. The constant acceleration of the moon’s 
orbital velocity, during the past 2000 years, has been traced by 
Laplace to a periodical change in the eccentricity of the earth’s 
orbits. But an error in his investigations being lately pointed 
out by Mr. Adams, there appears to be some definite ground for 
regarding the lunar orbit as subject to a very slow permanent 
diminution, which, after some allowance for the effects of tidal 
action, we may consider as depending, to some extent, on the 
resistance of a medium. The great oblateness which Arago and 
Sir William Herschel assign to Mars would indicate that the 
time of its rotation has been considerably lengthened, since the 
remote period at which it was moulded into its present form ; 
and this may be looked upon as circumstantial evidence of the 
effects of an ethereal resistance in changing the diurnal motion 
of planets. It is only to smaller worlds that we could look for 
such results, for in larger orbs the strength of their solid matter 
can have little influence in preventing alterations of form to cor- 
respond with the relations of gravity and centrifugal force. 
As doubts are entertained by some eminent astronomers as to 
the sensible ellipticity of Mars, it may be well to refer to certain 
appearances which show a slight deviation, at least, in his form 
from a figure of equilibrium. The marked indications of atmo- 
spheric phenomena around his poles, while they are either wholly 
absent or only faintly exhibited in the vicinity of his equator, is so 
much opposed to everything we might expect from the condition 
of our own globe and the belted appearance of Jupiter, that we 
cannot avoid concluding that the aérial ocean of Mars is much 
deeper in his polar than in his equatorial regions. Perhaps this 
may account for the very discordant results of observations in 
determining the extent of the atmosphere of this planet by oc- 
cultations of the fixed stars. 
It is to the revolutions of comets that astronomical curiosity 
has chiefly turned for evidence of the contents of interplanetary 
space ; but the advantages of low density in these bodies have 
been counterbalanced by the great elongation of their orbits, 
which exposes them to very great disturbances from the planets. 
During the past eighteen centuries Halley’s comet has occupied 
in its revolution a period varying from 74°88 to 79°34 years, ac- 
cording to the Table in Mr. Hind’s work on Comets (page 57): 
Of the twenty-four consecutive revolutions here recorded, the 
first eight average 77°59 years, the next eight 76°84, and the last 
