{p. 77}

The very fact that these waves of motion run through the tail of the
comet, and that it is capable of expanding and contracting on an
immense scale, is conclusive proof that it is composed of small,
adjustable particles. The writer from whom I have already quoted,
speaking of the extraordinary comet of 1843, says:

"As the comet moves past the great luminary, it sweeps round its tail
as a sword may be conceived to be held out at arm's-length, and then
waved round the head, from one side to the opposite. But a sword with
a blade one hundred and fifty millions of miles long must be a
somewhat awkward weapon to brandish round after this fashion. Its
point would have to sweep through a curve stretching out more than
six hundred millions of miles; and, even with an allowance of two
hours for the accomplishment of the movement, the flash of the weapon
would be of such terrific velocity that it is not an easy task to
conceive how any blade of _connected material substance_ could bear
the strain of the stroke. Even with a blade that possessed the
coherence and tenacity of iron or steel, the case would be one that
it would be difficult for molecular cohesion to deal with. But that
difficulty is almost infinitely increased when it is a substance of
much lower cohesive tenacity than either iron or steel that has to be
subjected to the strain.

"There would be, at least, some mitigation of this difficulty if it
were lawful to assume that the substance which is subjected to this
strain was not amenable to the laws of ponderable existence; if there
were room for the notion that comets and their tails, which have to
be brandished in such a stupendous fashion, were sky-spectres,