CHAPTER X.
THE ORIGIN OF COMETS AND METEORS.
The fact that comets and meteors, or at least a large proportion of such bodies, have entered the solar system from stellar space, is now admitted by all astronomers. The question, however, in regard to the origin and nature of these cosmical clouds still remains undecided. The theory that they consist of matter expelled with great velocity from the fixed stars appears to harmonize the greatest number of facts, and is accordingly entitled to respectful consideration. The evidence by which it is sustained may be briefly stated as follows:
1. The observations of Zollner, Respighi, and others, have indicated the operation of stupendous eruptive forces beneath the solar surface. The rose-colored prominences, which Janssen and Lockyer have shown to be masses of incandescent hydrogen, are regarded by Professor Respighi as phenomena of eruption. "They are the seat of movements of which no terrestrial phenomenon can afford any idea; masses of matter, the volume of which is many hundred times greater than that of the earth, completely changing their position and form in the space of a few minutes." The nature of this eruptive force is not understood. We may assume, however, that it was in active operation long before the sun had contracted to its present dimensions.
2. With an initial velocity of projection equal to 380 miles per second, the matter thrown off from the sun would be carried beyond the limits of the solar system, never to return. With velocities somewhat less, it would be transported to distances corresponding to those of the aphelia of the periodic comets.
3. On the 7th of September, 1871, Professor Young, of Dartmouth College,[29] witnessed an extraordinary explosion on the sun's surface. The observer, with his telescope, followed the expelled matter to an elevation of over 200,000 miles. The mean velocity between the altitudes of 100,000 and 200,000 miles was 166 miles per second. This rate of motion _in vacuo_ would indicate an initial velocity of about 260 miles per second. But the sun is surrounded by an extensive atmosphere, whose resistance must have greatly retarded the velocity of the outrush before reaching the height of 100,000 miles. The original velocity of these hydrogen clouds was therefore sufficient, in all probability, to have carried them, if unresisted, beyond the solar domain. Solid or dense matter propelled with equal force would doubtless have been driven off never to return.[30]
[29] Boston Journal of Chemistry, November, 1871.
[30] See Mr. Proctor's interesting discussion of this subject in the Monthly Notices of the R.A.S., vol. xxxii.
4. This eruptive force, whatever be its nature, is probably common to the sun and the so-called fixed stars. If so, the dispersed fragments of ejected matter ought to be found in the spaces intervening between sidereal systems. Accordingly, the phenomena of comets and meteors have demonstrated the existence of such matter, widely diffused, in the portions of space through which the solar system is moving.
5. According to Mr. Sorby the microscopic structure of the aerolites he has examined points evidently to the fact that they have been at one time in a state of fusion from intense heat,--a fact in striking harmony with this theory of their origin.
6. The velocity with which some meteoric bodies have entered the atmosphere has been greater than that which would have been acquired by simply falling toward the sun from any distance, however great. On the theory of their sidereal origin, this excess of velocity has been dependent on the primitive force of expulsion. The shower of aerolites which fell at Pultusk, Poland, on the 30th of January, 1868,[31] is not only a remarkable illustration of the fact here stated, but also of another which may be accounted for by the same theory, viz.: that meteoric bodies sometimes enter the solar system in groups or clusters.
[31] See Chapter VII.
7. A striking argument in favor of this theory may be derived from the researches of the late Professor Graham, considered in connection with those of Dr. Huggins and other eminent spectroscopists. Professor Graham found large quantities of hydrogen confined in the pores or cavities of certain meteoric masses. Now, the spectroscope has shown that the sun's rose-colored prominences consist of immense volumes of incandescent hydrogen; that the same element exists in great abundance in many of the fixed stars, and even in certain nebulæ; and that the star in the Northern Crown, whose sudden outburst in 1866 so astonished the scientific world, afforded decided indications of its presence.
THE END.
_BY THE AUTHOR OF THIS VOLUME._
METEORIC ASTRONOMY: A TREATISE ON SHOOTING STARS, FIRE BALLS, AND AEROLITES.
BY DANIEL KIRKWOOD, LL.D.
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Transcriber's Note: Obvious errors in spelling and punctuation have been corrected. Footnotes have been renumbered and moved from the page end to the end of the paragraph. Images have been moved from the middle of a paragraph to the closest paragraph break.