The head of a great comet is brilliant, sometimes starlike. But its tail often takes on the most impressive appearance. Donati's comet, in 1858, assumed the most varied shapes—sometimes its tail was broad, with one bright and curving edge, the other fainter and finer, the whole making up a stupendous semi-circular blade-like object. Later, the tail was shaped like a scimitar, and later again, it assumed a duplex form.
Though the bulk of comets is huge, they contain extraordinarily little substance. Their heads must contain some solid matter, but it is probably in the form of a loose aggregation of stones enveloped in vaporous material. There is some reason to suppose that comets are apt to shed some of these stones as they travel along their paths, for the orbits of the meteors that cause some of our greatest "star showers" are coincident with the paths of comets that have been observed.
But it is not only by shedding its loose stones that a comet diminishes its bulk; it loses also through its tail. As the comet gets close to the sun its head becomes heated, and throws off concentric envelopes, much of which consists of matter in an extremely fine state of division. Now it has been shown that the radiations of the sun have the power of repelling matter, whilst the sun itself attracts by its gravitational force. But there is a difference in the action of the two forces. The light-pressure varies with the surface of the particle upon which it is exercised; the gravitational attraction varies with the mass or volume. If we consider the behaviour of very small particles, it follows that the attraction due to gravitation (depending on the volume of the particle) will diminish more rapidly than the repulsion due to light-pressure (depending on the surface of the particle), as we decrease continually the size of the particle, since its volume diminishes more rapidly than its surface. A limit therefore will be reached below which the repulsion will become greater than the attraction. Thus for particles less than the 1/25000 part of an inch in diameter the repulsion of the sun is greater than its attraction. Particles in the outer envelope of the comet below this size will be driven away in a continuous stream, and will form that thin, luminous fog which we see as the comet's tail.
We cannot tell whether such objects as these were present to the mind of Joel when he spoke of "blood and fire and pillars of smoke"; possibly these metaphors are better explained by a sand- or thunder-storm, especially when we consider that the Hebrew expression for the "pillars of smoke" indicates a resemblance to a palm-tree, as in the spreading out of the head of a sand- or thunder-cloud in the sky. The suggestion has been made,—following the closing lines of Paradise Lost (for Milton is responsible for many of our interpretations of Scripture)
"High in front advanced,
The brandished sword of God before them blazed,
Fierce as a comet,"
—that a comet was indeed the "flaming sword which turned every way, to keep the way of the tree of life." There is less improbability in the suggestion made by several writers that, when the pestilence wasted Jerusalem, and David offered up the sacrifice of intercession in the threshing floor of Ornan the Jebusite, the king may have seen, in the scimitar-like tail of a comet such as Donati's, God's "minister,"—"a flame of fire,"—"the angel of the Lord stand between the earth and the heaven, having a drawn sword in his hand stretched out over Jerusalem."
The late R. A. Proctor describes the wanderings of a comet thus:—
"A comet is seen in the far distant depths of space as a faint and scarcely discernible speck. It draws nearer and nearer with continually increasing velocity, growing continually larger and brighter. Faster and faster it rushes on until it makes its nearest approach to our sun, and then, sweeping round him, it begins its long return voyage into infinite space. As it recedes it grows fainter and fainter, until at length it passes beyond the range of the most powerful telescopes made by man, and is seen no more. It has been seen for the first and last time by the generation of men to whom it has displayed its glories. It has been seen for the first and last time by the race of man itself."[108:1]
"These are . . . wandering stars, to whom is reserved the blackness of darkness for ever."