In 1843, we find that the crater of Vesuvius contained three such small cones arranged in a line along its bottom as depicted in fig. 16. These drawings of the summit of Vesuvius give a fair notion of the changes which have been continually going on there during the whole of the historical period. Ever and anon a grand outburst, like that of 1822, has produced a vast and deep crater such as is represented in fig. 13, and then a long continuance of quiet and regular ejections has built up within the crater small cones like those shown in figs. 14, 15 and 16, till at last the great crater has been completely filled up, and the cone reconstructed. [Illustration: Fig. 17.--Outlines of Vesuvius, showing its Form at different periods of its history.] [Sidenote: CHANGES IN OUTLINE OF VESUVIUS.] In the series of outlines in fig. 17, we have endeavoured to illustrate the succession of changes which has taken place in Vesuvius during historical times. In the year 79 one side of the crater-wall of the vast mountain-mass was blown away. Subsequent ejections built up the present cone of Vesuvius within the great encircling crater-wall of Somma, and the form of this cone and the crater at its summit have been undergoing continual changes during the successive eruptions of eighteen centuries. What its _future_ history may be we can only conjecture from analogy. It may be that a long continuance of eruptions of moderate energy may gradually raise the central cone till its sides are confluent with those of the original mountain; or it may be that some violent paroxysm will entirely destroy the modern cone, reducing the mountain to the condition in which it was after the great outburst of 79. On the other hand, if the volcanic forces under Vesuvius are gradually becoming extinct (but of this we have certainly no evidence at present), the mountain may gradually sink into a state of quiescence, retaining its existing form.