We must now proceed to compare the grand and terrible appearances presented during a great eruption with those more feeble displays which we have been describing, to show that in all their essential features these different kinds of outbursts are identical with one another, and must be referred to the action of similar causes.

The volcanic eruption which has been most carefully studied in recent times is that which we have already referred to as occurring at Vesuvius, in the month of April 1872. With the exception, perhaps, of that which took place in October 1822, this eruption was the grandest which has broken out at Vesuvius during the present century. Owing to the circumstance of its proximity to the great city of Naples, Vesuvius has always been the most carefully watched of all volcanoes, and in recent years the erection of an observatory, provided with instruments for recording the smallest subterranean tremors affecting the mountain, has facilitated the carrying on of those continuous and minute observations which are so necessary for exact scientific inquiry.

On the occasion of this outburst, the aid of instantaneous photography was first made available for obtaining a permanent record of the appearances displayed at volcanic eruptions. In fig. 5 we have one of these photographs, which was taken at 5 o'clock P.M. on April 26, 1872, transferred to a wood-block and engraved. In examining it we feel sure that we

VESUVIUS ERUPTION OF 1872.

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are not being misled by any exaggeration or error on the part of the artist. Vesuvius rises to the height of nearly 4,000 feet above the level of the sea, and an inspection of the photograph proves that the vapours and rock-fragments were thrown to the enormous height of 20,000 feet, or nearly four miles, into the atmosphere.

The main features of this terrifying outburst were as follows. For more than a twelvemonth before, the activity of the forces at work within the mountain appeared to be gradually increasing, and the great eruption commenced on April 24, attained its climax on the 26th, and began to die out on the following day. During the eruption the bottom of the crater was entirely broken up, and the sides of the mountain were rent by fissures in all directions. So numerous were these fissures and cracks that liquid matter appeared to be oozing from every part of its surface, and, as Professor Palmieri, who witnessed the outburst from the observatory, expressed it, 'Vesuvius sweated fire.' One of the fissures was of enormous size, extending from the summit to far beyond the base of the cone; the scar left by this gigantic rent being plainly visible at the present day.

From the great opening or crater at the summit, and from some of the fissures on the sides of the mountain, enormous volumes of steam rushed out with a prodigious roaring sound, the noise being so terrific that the inhabitants of Naples, five miles off, fled from

their houses and spent the night in the open streets. Although this roaring sound appeared at a distance to be continuous, yet those upon the mountain could perceive that it was produced by detonations or explosions rapidly following one another. Each of these explosions was accompanied by the formation of a great globe of white vapour, which, rising into the atmosphere, swelled the bulk of the vast cloud overhanging the mountain. An inspection of the photographs (see fig. 5) shows that the great vapour-cloud over Vesuvius was made up of the globular masses ejected at successive explosions. Each of these explosive upward rushes of steam carried along with it a considerable quantity of solid fragments, and these fell in great numbers all over the surface of the mountain, breaking the windows of the observatory, and making it dangerous to be out of doors.

We have said that lava, or molten rock, appeared to be issuing from the very numerous cracks formed all over the flanks of the mountain. But at three points this molten rock issued in such quantities as to form great, fiery floods, which rushed down the sides of the mountain, and flowed to a considerable distance beyond its base. The largest of these lava-floods overwhelmed and destroyed the two villages of Massa di Somma and San Sebastiano, besides many country houses in the neighbourhood.

A very marked and interesting feature exhibited by these three lava-floods was the quantity of watery