[7] Judd, Report, p. 21.
[8] Report, Part ii.
[9] In this eruption, 36,380 human beings perished, of whom 37 were Europeans; 163 villages (kampoengs) were entirely, and 132 partially, destroyed.—Verbeek, loc. cit., p. 79.
[10] Verbeek, loc. cit., p. 144-5. The dust put a girdle round the earth in thirteen days.
[11] Verbeek, loc. cit., p. 30.
CHAPTER II.
EARTHQUAKES.
Connection of Earthquakes with Volcanic Action.—The connection between earthquake shocks and volcanic eruptions is now so generally recognised that it is unnecessary to insist upon it here. All volcanic districts over the globe are specially liable to vibrations of the crust; but at the same time it is to be recollected that these movements visit countries occasionally from which volcanoes, either recent or extinct, are absent; in which cases we may consider earthquake shocks to be abortive attempts to originate volcanic action.
(a.) Origin.—From the numerous observations which have been made regarding the nature of these phenomena by Hopkins, Lyell, and others, it seems clearly established that earthquakes have their origin in some sudden impact of gas, steam, or molten matter impelled by gas or steam under high pressure, beneath the solid crust.[1] How such impact originates we need not stop to inquire, as the cause is closely connected with that which produces volcanic eruptions. The effect, however, of such impact is to originate a wave of translation through the crust, travelling outwards from the point, or focus, on the surface immediately over the point of impact.[2] These waves of translation can in some cases be laid down on a map, and are called "isoseismal curves," each curve representing approximately an equal degree of seismal intensity; as shown on the chart of a part of North America affected by the great Charleston earthquake. ([Fig. 37].) Mr. Hopkins has shown that the earthquake-wave, when it passes through rocks differing in density and elasticity, changes in some degree not only its velocity, but its direction; being both refracted and reflected in a manner analogous to that of light when it passes from one medium to another of different density.[3] When a shock traverses the crust through a thickness of several miles it will meet with various kinds of rock as well as with fissures and plications of the strata, owing to which its course will be more or less modified.