[CHAPTER VIII.]
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.