Before and during the great outbreak of April 1872, Vesuvius itself and the whole country round were visited with earthquake-shocks, or tremblings of the ground. The sensitive instruments in the Vesuvian Observatory showed the mountain daring the eruption to be in a constant state of tremor. These earthquakes are not, as is commonly supposed, actual upheavings of the earth's surface, but are vibrations propagated through the solid materials of which the earth is built up. We cannot stamp our feet upon the ground without giving rise to such vibrations, though our senses may not be sufficiently acute to perceive them. The explosive escape of steam from a crack is a cause sufficiently powerful to produce a shock which is propagated and may be felt for a considerable distance round. Even on Stromboli an observer at the edge of the crater may notice that each explosive outburst of steam is accompanied by a perceptible tremor of the ground, and in the case of Vesuvius the violent shocks produced by the escape of far larger volumes of steam give rise to proportionately stronger vibrations. The nature and origin of those far more terrible and destructive shocks which sometimes accompany, and more frequently precede, great volcanic eruptions, we shall consider in the sequel. [Sidenote: CAUSE OF LIGHTNING DURING ERUPTIONS.] Another striking phenomenon which was exhibited in the great eruption of Vesuvius in 1872 was the vivid display of lightning accompanied by thunder. The uprushing current of steam and rock-fragments forms a vertical column, but as the steam condenses it spreads out into a great horizontal cloud which is seen to be made up of the great globes of vapour emitted at successive explosions. When there is little or no wind the vertical column with a horizontal cloud above it bears a striking resemblance to the stone-pine trees which form so conspicuous a feature in every Neapolitan landscape. Around this column of vapour the most vivid lightning constantly plays and adds not a little to the grand and awful character of the spectacle of a volcanic eruption, especially when it is viewed by night. In the eruption of 1872 a strong wind blowing from the north-west destroyed the usual regular appearance of this 'pine-tree appendage' to the mountain, which is so well known to, and dreaded by the inhabitants of Naples; the cloud, as will be seen from the photograph (fig. 5, _facing_ p. 24), was blown on one side, and most of the falling fragments took the same direction.