DOES THE NUMBER OF AEROLITIC FALLS VARY WITH THE EARTH'S DISTANCE
FROM THE SUN?--RELATIVE NUMBERS OBSERVED IN THE FORENOON AND
AFTERNOON--EXTENT OF THE ATMOSPHERE AS INDICATED BY METEORS.
An analysis of any extensive table of meteorites and fire-balls proves
that a greater number of aerolitic falls have been observed during the
months of June and July, when the earth is near its aphelion, than in
December and January, when near its perihelion. It is found, however,
that the reverse is true in regard to bolides, or fire-balls. Now the
theory has been held by more than one physicist, that aerolites are
the outriders of the asteroid ring between Mars and Jupiter; their
orbits having become so eccentric that in perihelion they approach
very near that of the earth. If this theory be the true one, the earth
would probably encounter the greatest number of those meteor-asteroids
when near its aphelion. The hypothesis therefore, it has been claimed,
appears to be supported by well-known facts. The variation, however,
in the observed number of aerolites may be readily accounted for
independently of any theory as to their origin. The fall of meteoric
stones would evidently be more likely to escape observation by night
than by day, by reason of the relatively small number of observers. But
the days are shortest when the earth is in perihelion, and longest when
in aphelion; the ratio of their lengths being nearly equal to that of
the corresponding numbers of aerolitic falls.
On the other hand, it is obvious that fire-balls, unless of very
extraordinary magnitude, would not be visible during the day. The
_observed_ number will therefore be greatest when the nights are
longest; that is, when the earth is near its perihelion. This, it will
be found, is precisely in accordance with observation.
It has been found, moreover, that a greater number of meteoric stones
fall during the first half of the day, that is, from midnight to noon,
than in the latter half, from noon to midnight. This would seem to
indicate that a large proportion of the aerolites encountered by the
earth have direct motion.
_Height of the Atmosphere._--The weight of a given volume of mercury is
10,517 times that of an equal volume of air at the earth's surface; and
since the mean height of the mercurial column in the barometer is about
thirty inches, if the atmosphere were of uniform density its altitude
would be about 26,300 feet, or nearly five miles. The density rapidly
diminishes, however, as we ascend above the earth's surface. Calling
it unity at the sea level, the rate of variation is approximately
expressed as follows:
Altitude in Miles. Density.
0 1
7 1/4
14 1/16
21 1/64
28 1/256
35 1/1024
70 1/1000000
105 1/1000000000
140 1/1000000000000
etc. etc.
From this table it will be seen that at the height of 35 miles the air
is one thousand times rarer than at the surface of the earth; and that,
supposing the same rate of decrease to continue, at the height of 140
miles the rarity would be one trillion times greater. The atmosphere,
however, is not unlimited. When it becomes so rare that the force of
repulsion between its particles is counterbalanced by the earth's
attraction, no further expansion is possible. To determine the altitude
of its superior surface is a problem at once difficult and interesting.
Not many years since about 45 or 50 miles were generally regarded as a
probable limit. Considerable light, however, has been thrown upon the
question by recent observations in meteoric astronomy. Several hundred
detonating meteors have been observed, and their average height at the
instant of their first appearance has been found to exceed 90 miles.
The great meteor of February 3d, 1856, seen at Brussels, Geneva, Paris,
and elsewhere, was 150 miles high when first seen, and a few apparently
well-authenticated instances are known of a still greater elevation. We
conclude, therefore, from the evidence afforded by meteoric phenomena,
that the height of the atmosphere is certainly not _less_ than 200
miles.
It might be supposed, however, that the resistance of the air at
such altitudes would not develop a sufficient amount of heat to
give meteorites their brilliant appearance. This question has been
discussed by Joule, Thomson, Haidinger, and Reichenbach, and may now
be regarded as definitively settled. When the velocity of a meteorite
is known the quantity of heat produced by its motion through air of
a given density is readily determined. The temperature acquired is
the equivalent of the force with which the atmospheric molecules are
met by the moving body. This is about one degree (Fahrenheit) for a
velocity of 100 feet per second, and it varies directly as the square
of the velocity. A velocity, therefore, of 30 miles in a second
would produce a temperature of 2,500,000°. The weight of 5280 cubic
feet of air at the earth's surface is about 2,830,000 grains. This,
consequently, is the weight of a column 1 mile in length, and whose
base or cross section is one square foot. The weight of a column of the
same dimensions at a height of 140 miles would be about 1/350000th of a
grain. Hence the heat acquired by a meteoric mass whose cross section
is one square foot, in moving 1 mile would be one grain raised 7-1/7
degrees, or one-fifth of a grain 2500° in 70 miles. This temperature
would undoubtedly be sufficient to render meteoric bodies brilliantly
luminous.
But there have been indications of an atmosphere at an elevation of
more than 500 miles. A discussion of the best observations of the
great aurora seen throughout the United States on the 28th of August,
1859, gave 534 miles as the height of the upper limit above the
earth's surface. The aurora of September 2d, of the same year, had an
elevation but little inferior, viz., 495 miles. Now, according to the
observed rate of variation of density, at the height of 525 miles, the
atmosphere would be so rare that a sphere of it filling the orbit of
Neptune would contain less matter than 1/30th of a cubic inch of air at
the earth's surface. In other words, it would weigh less than 1/90th
of a grain. We are thus forced to the conclusion either that the law
of variation is not the same at great heights as near the surface; or,
that beyond the limits of the atmosphere of air, there is another of
electricity, or of some other fluid.