and the Nature of the Liquidity of Lavas. 139 
Roman territory, the Grecian Archipelago, Madeira, Teneriffe, 
the Azores, Bourbon, St. Helena, Barren Island, the Leeward 
Isles, &c.), will see numerous unquestionable examples of this 
law by which crater is formed within crater, and new cones upon 
the ruins of old ones. 
History of Vesuvius.—At the risk of repetition, I must be per- 
mitted to illustrate this law by the trite, but instructive, example 
of Vesuvius,—which only comes so often before us because from 
its proximity to Naples it has been open to more constant and 
accurate observations than any other known volcanic mountain. 
What, in brief, is the history of this voleano during the last cen- 
tury? Precisely one hundred years ago, in the year 1756, 
Vesuvius possessed no less than three cones and craters, one 
within the other, like a nest of boxes, besides the great encir- 
cling crater and cone of Somma (fig. 1). Sir W. Hamilton 
gives us a drawing of its appearance in this state. 
Fig. 1.—Outline-sketch of Vesuvius as it existed in 1756. 
(After Sir W. Hamilton.) 
a. Somma. 
By the beginning of the year 1767, the continuance of mode- 
rate eruptions had obliterated the inmost cone and increased the 
intermediate one, until it very nearly filled the principal crater 
(fig. 2, 4,8). An eruption in October of that year, 1767, com- 
pleted the process, and re-formed the single cone into one con- 
tinuous slope all round from the apex downwards (fig. 2, c). 
The dotted lines in fig. 2 (after Hamilton) represent the shape 
of the outer and inner cones before this eruption, and the space 
between them and the firm outline represents the amount by 
which the cone was in the intervening ten years augmented in 
bulk and height by the ejectamenta of that eruption. An in- 
terval of comparative tranquillity followed, until, in 1794, the 
paroxysmal eruption occurred, described by Breislak, which com- 
pletely gutted this cone, then solid, lowered its height, and left 
