[1264] Hasselberg, loc. cit., p. 58.
[1265] Ueber die Natur der Cometen, p. 112.
[1266] Hasselberg, loc. cit., p. 38.
CHAPTER XI
RECENT COMETS (continued)
The mystery of comets' tails had been to some extent penetrated; so far, at least, that, by making certain assumptions strongly recommended by the facts of the case, their forms can be, with very approximate precision, calculated beforehand. We have, then, the assurance that these extraordinary appendages are composed of no ethereal or supersensual stuff, but of matter such as we know it, and subject to the ordinary laws of motion, though in a state of extreme tenuity.
Olbers, as already stated, originated in 1812 the view that the tails of comets are made up of particles subject to a force of electrical repulsion proceeding from the sun. It was developed and enforced by Bessel's discussion of the appearances presented by Halley's comet in 1835. He, moreover, provided a formula for computing the movement of a particle under the influence of a repulsive force of any given intensity, and thus laid firmly the foundation of a mathematical theory of cometary emanations. Professor W. A. Norton, of Yale College, considerably improved this by inquiries begun in 1844, and resumed on the apparition of Donati's comet; and Dr. C. F. Pape at Altona[1267] gave numerical values for the impulses outward from the sun, which must have actuated the materials respectively of the curved and straight tails adorning the same beautiful and surprising object.
The physical theory of repulsion, however, was, it might be said, still in the air. Nor did it even begin to assume consistency until Zöllner took it in hand in 1871.[1268] It is perfectly well ascertained that the energy of the push or pull produced by electricity depends (other things being the same) upon the surface of the body acted on; that of gravity upon its mass. The efficacy of solar electrical repulsion relatively to solar gravitational attraction grows, consequently, as the size of the particle diminishes. Make this small enough, and it will virtually cease to gravitate, and will unconditionally obey the impulse to recession.
This principle Zöllner was the first to realise in its application to comets. It gives the key to their constitution. Admitting that the sun and they are similarly electrified, their more substantially aggregated parts will still follow the solicitations of his gravity, while the finely divided particles escaping from them will, simply by reason of their minuteness, fall under the sway of his repellent electric power. They will, in other words, form "tails." Nor is any extravagant assumption called for as to the intensity of the electrical charge concerned in producing these effects. Zöllner, in fact, showed[1269] that it need not be higher than that attributed by the best authorities to the terrestrial surface.
Forty years have elapsed since M. Brédikhine, director successively of the Moscow and of the Pulkowa Observatories, turned his attention to these curious phenomena. His persistent inquiries on the subject, however, date from the appearance of Coggia's comet in 1874. On computing the value of the repulsive force exerted in the formation of its tail, and comparing it with values of the same force arrived at by him in 1862 for some other conspicuous comets, it struck him that the numbers representing them fell into three well-defined classes. "I suspect," he wrote in 1877, "that comets are divisible into groups, for each of which the repulsive force is perhaps the same."[1270] This idea was confirmed on fuller investigation. In 1882 the appendages of thirty-six well-observed comets had been reconstructed theoretically, without a single exception being met with to the rule of the three types. A further study of forty comets led, in 1885, only to a modification of the numerical results previously arrived at.