ll 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.