dest as it is, it is that unit about which we have by far the
fullest and most reliable knowledge. The earth not only furnishes the
physical baseline of celestial observation, but supplies all the
appliances by which inquiry penetrates the depths of the heavens. Not
alone earth-science, as such, but several of the intensive sciences
brought into being through the intellectual evolutions that have
attended the later history of the earth, have been prerequisites to
the development of the broad science of the outer heavens. The science
of the lower heavens is a factor of earth-science in the definition we
are just about to give. At the same time, the whole earth, including
the lower heavens, is enveloped by the more comprehensive domain of
celestial science.

If we seek the most logical limit that may be assigned the realm of
earth-science, as distinguished from that of celestial science, of
which it is the home unit, it may be found at that borderline _within
which_ any passive body obeys the call of the earth, as against the
call of the outer worlds, and _without which_ such a passive body
obeys the call of the outer worlds, the call of the sun in particular.
This limit is the _dynamic dividing line_ between the kingdom of the
earth and the kingdom of the outer heavens. This boundary, according
to Moulton, incloses a spheroid whose minimum radius is about 620,000
miles, and whose maximum radius is about 930,000 miles. We may, then,
conveniently say that the earth's sphere of control stretches out a
million kilometers from its center and that this defines its true
realm. At the same time, this defines the logical limit of the earth's
ultra-atmosphere and appears to mark a zone of exchange between the
ultra-atmosphere of the earth and the ultra-atmosphere of the sun. It
thus appears to imply the place and the mode of an exchange of vital
elements upon which probably hangs the wonderful maintenance of the
earth's atmosphere for many millions of years and the equally
wonderful regulation of the essential qualities of the atmosphere so
that these have always remained within the narrow range subservient to
terrestrial life. It is needless to add that this regulation also
conditions the present intellectual status of the thinking factor
among the inhabitants of the earth out of which--may I be pardoned for
saying?--has grown the present educational discussion.

If this last shall seem to squint toward special pleading, let it