on the planet.

Fourthly, we are to find something that would produce cyclonic
convulsions upon a scale for which the ordinary operations of nature
furnish us no parallel.

Fifthly, we are to find some external force so mighty that it would
crack the crust of the globe like an eggshell, lining its surface
with great rents and seams, through which the molten interior boiled
up to the light.

Would a comet meet all these prerequisites?

I think it would.

Let us proceed in regular order.

{p. 65}

CHAPTER II.

WHAT IS A COMET?

IN the first place, are comets composed of solid, liquid, or gaseous
substances? Are they something, or the next thing to nothing?

It has been supposed by some that they are made of the most
attenuated gases, so imponderable that if the earth were to pass
through one of them we would be unconscious of the contact. Others
have imagined them to be mere smoke-wreaths, faint mists, so rarefied