The great comet of 1858, Donati's comet, which many now living will
well remember, and which was of such size that when its head was near
our horizon the extremity of the tail reached nearly to the zenith,
illustrated this continual movement of the material of the tail; that
appendage shrank and enlarged millions of miles in length.

Mr. Lockyer believed that he saw in Coggia's comet the evidences of a
_whirling_ motion--

"In which the regions of greatest brightness were caused by the
different coils _cutting_, or appearing to cut, each other, and so in
these parts leading to compression or condensation, and _frequent
collision of the luminous particles_."

Olbers saw in a comet's tail--

"A sudden flash and pulsation of light which vibrated for several
seconds through it, and the tail appeared during the continuance of
the pulsations of light to be lengthened by several degrees and then
again contracted."[1]

[1. "American Cyclopædia," article "Clay."

2. "Edinburgh Review," October, 1874, p. 208,

3, "Cosmos," vol. i, p. 143.]

{p. 76}

Now, in this perpetual motion, this conflict, these great thrills of