pear-shaped, or with the hinder part (corresponding to the tail of a comet)
irregular, studded with projecting points, and even concave. Their surfaces
are rough, and fissured with branching cracks; their internal structure is
either irregularly scoriaceous and compact, or it presents a symmetrical
and very curious appearance. An irregular segment of a bomb of this latter
kind, of which I found several, is accurately represented in Figure 3. Its
size was about that of a man's head. The whole interior is coarsely
cellular; the cells averaging in diameter about the tenth of an inch; but
nearer the outside they gradually decrease in size. This part is succeeded
by a well-defined shell of compact lava, having a nearly uniform thickness
of about the third of an inch; and the shell is overlaid by a somewhat
thicker coating of finely cellular lava (the cells varying from the
fiftieth to the hundredth of an inch in diameter), which forms the external
surface: the line separating the shell of compact lava from the outer
scoriaceous crust is distinctly defined. This structure is very simply
explained, if we suppose a mass of viscid, scoriaceous matter, to be
projected with a rapid, rotatory motion through the air; for whilst the
external crust, from cooling, became solidified (in the state we now see
it), the centrifugal force, by relieving the pressure in the interior parts
of the bomb, would allow the heated vapours to expand their cells; but
these being driven by the same force against the already-hardened crust,
would become, the nearer they were to this part, smaller and smaller or
less expanded, until they became packed into a solid, concentric shell. As
we know that chips from a grindstone (Nichol "Architecture of the
Heavens.") can be flirted off, when made to revolve with sufficient
velocity, we need not doubt that the centrifugal force would have power to
modify the structure of a softened bomb, in the manner here supposed.
Geologists have remarked, that the external form of a bomb at once bespeaks
the history of its aerial course, and few now see that the internal
structure can speak, with almost equal plainness, of its rotatory movement.