The dose investigation of what goes on within a volcanic vent may appear at first sight to be a task beset with so many difficulties and dangers that we may be tempted to abandon it as altogether hopeless. At the first recorded eruption of Vesuvius the elder Pliny lost his life in an attempt to approach the mountain and examine the action which was taking place there; and during the last great outburst of the same volcano a band of Neapolitan students, whose curiosity was greater than their prudence, shared the same fate.

But in both these cases the inquirers paid the penalty of having adopted a wrong method. If we wish to examine the mode of working of a complicated steam-engine, it will be of little avail for us to watch the machinery when the full blast of steam is turned on, and the rapid movements of levers, pinions, and slides baffle all attempts to follow them, and render hopeless every effort to trace their connection with one another. But if some friendly hand turn off the greater part of the steam-supply, then, as the rods move slowly backwards and forwards, as the wheels make their measured revolutions, and the valves axe seen gradually opening and shutting, we may have an opportunity of determining the relations of the several parts of the machine to one another, and of arriving at just conclusions concerning the plan on which it is constructed. Nor can we doubt that the parts of the machine bear the same relation to one another, and that their movements take place in precisely the same order, when the supply of steam is large as when it is small.

Now, as we shall show in the sequel, a volcano is a kind of great natural steam-engine, and our best method of investigating its action is to watch it when a part of the steam-supply is cut off. It is true that we cannot at will control the source of supply of steam to a volcano, as we can in a steam-engine, but as some volcanoes have usually only a small steam-supply, and nearly all volcanoes vary greatly in the intensity of their action at different periods, we can, by a careful selection of the object or the time of our study, gain all those advantages which would be obtained by regulating its action for ourselves.

Spallanzani appears to have been the first to perceive the important fact, that the nature of volcanic action remains the same, however its intensity may vary. Taking advantage of the circumstance that there exists in the Mediterranean Sea a volcano—Stromboli—which for at least 2,000 years has been in a constant and regular, but not in a violent or dangerous, state of activity, he visited the spot, and made the series of careful observations which laid the foundation of our knowledge of the 'physiology of volcanoes.' Since the time of Spallanzani, many other investigators have visited the crater of Stromboli, and they have been able to confirm and extend the observations of the great Italian naturalist, as to the character of the action which is constantly taking place within it. We cannot better illustrate the nature of volcanic action than by describing what has been witnessed by numerous observers within the crater of Stromboli, where it is possible to watch the series of operations going on by the hour together, and to do so without having our judgment warped either by an excited imagination or the sense of danger.

In the sketch, fig. 1, which was made on April 20, 1874, I have shown the appearance which this interesting volcano usually presents, when viewed from a distance. The island is of rudely circular outline, and conical form, and rises to the height of 3,090 feet above the level of the Mediterranean. From a point on the side of the mountain, masses of vapour are seen to issue, and these unite to form a cloud over the mountain, the outline of this vapour-cloud varying continually according to the hygrometric state of the atmosphere, and the direction and force of the wind. At the time when this sketch was made, the vapour-cloud was spread in a great horizontal stratum overshadowing the whole island, but it was clearly seen to be made up of a number of globular masses, each of which, as we shall hereafter see, is the product of a distinct outburst of the volcanic forces.

Viewed at night-time, Stromboli presents a far more striking and singular spectacle. The mountain, with its vapour canopy, is visible over an area having a radius of more than 100 miles. When watched from the deck of a vessel anywhere within this area, a glow of red light is seen to make its appearance from time to time above the summit of the mountain; this glow of light may be observed to increase gradually in intensity, and then as gradually to die away. After a short interval the same appearances are repeated, and this goes on till the increasing light of the dawn causes the phenomenon to be no longer visible. The resemblance presented by Stromboli to a 'flashing light' on a most gigantic scale is very striking, and the mountain has long been known as 'the lighthouse of the Mediterranean.'

It must be pointed out, however, that in two very important particulars the appearances presented by Stromboli differ markedly from those rhythmical gleams exhibited by the 'flashing-lights' of our coasts. In the first place, the intervals between successive flashes are very unequal, varying from less than one minute to twenty minutes, or even more; and in the second place, the duration and intensity of the red glow above the mountain are subject to like variation, being sometimes a momentary scarcely visible gleam, and at others a vivid burst of light which illuminates the sky to a considerable distance round.

Let us now draw near and examine this wonderful phenomenon of a mountain which seemingly ever burns with fire, and yet is not consumed. The general form of the Island of Stromboli will be gathered from an inspection of the plan, fig. 2, which is copied from a map published by the Italian Government. When we land upon the island, we find that it is entirely built up of such materials as we know to be ejected from volcanoes; indeed, it resembles on a gigantic scale the surroundings of an iron furnace, with its heaps of cinders and masses of slag. The irregularity in the form of the island is at once seen to be due to the action of the wind, the rain, and the waves of the surrounding sea, which have removed the loose, cindery materials at some points, and left the hard, slaggy masses standing up prominently at others.

This great heap of cindery and slaggy materials rises, as we have said, to a height of more than 3,000 feet above the sea-level, but even this measurement does not give a just idea of its vast bulk. Soundings in the sea surrounding the island show that the bottom gradually shelves around the shores to the depth of nearly 600 fathoms, so that Stromboli is a great conical mass of cinders and slaggy materials, having a height of over 6,000 feet, and a base whose diameter exceeds four miles.

The general form and proportions of this mass will be better understood by an examination of the section, fig. 3, which is also constructed from the materials furnished by the map of the island issued by the Italian Government. The same section, and the map, fig. 2, will serve to make clear the position and relations of the point on the mountain at which the volcanic activity takes place. At a spot on the north-west slope of the mountain, about 1,000 feet below its summit, and 2,000 feet above the level of the sea, there exists a circular depression, the present active 'crater' of the volcano; and leading down from this to the sea there is a flat slope making an angle of about 35° with the horizon, and known as the 'Sciarra.' The Sciarra is bounded by steep cliffs, as shown in the sketch fig. 1, and the plan fig. 2.

If we climb up to this scene of volcanic activity, we shall be able to watch narrowly the operations which are going on there. On the morning of the 24th of April, 1874, I paid a visit to this interesting spot in order to get a near view of what was taking place. On reaching a point upon the side of the Sciarra, from which the crater was in full view before me, I witnessed, and made a sketch of, an outburst which then took place, and this sketch has been reproduced in fig. 4. Before the outburst, numerous light curling wreaths of vapour were seen ascending from fissures on the sides and bottom of the crater. Suddenly, and without the slightest warning, a sound was heard like that produced when a locomotive blows off its steam at a railway-station; a great volume of watery vapour was at the same time thrown violently into the atmosphere, and with it there were hurled upwards a number of dark fragments, which rose to the height of 400 or 500 feet above the crater, describing curves in their course, and then falling back upon the mountain. Most of these fragments tumbled into the crater with a loud, rattling noise, but some of them fell outside the crater, and a few rolled down the steep slope of the Sciarra into the sea. Some of these falling fragments were found to be still hot and glowing, and in a semi-molten condition, so that they readily received the impression of a coin thrust into them.

But on the upper side of the crater, at the point marked 6, on the section fig. 3, there exists a spot from which we can look down upon the bottom of the crater, and view the operations taking place there. This is the place where Spallanzani and other later investigators have carried on their observations, and, when the wind is blowing from the spectator towards the crater, he may sit for hours watching the wonderful scene displayed before him. The black slaggy bottom of the crater is seen to be traversed by many fissures or cracks, from most of which curling jets of vapour issue quietly, and gradually mingle with and disappear in the atmosphere. But besides these smaller cracks at the bottom of the crater, several larger openings are seen, which vary in number and position at different periods; sometimes only one of these apertures is visible, at others as many as six or seven, and the phenomena presented at these larger apertures are especially worthy of careful investigation.

These larger apertures, if we study the nature of the action taking place at them, may be divided into three classes. From those of the first class, steam is emitted with loud, snorting puffs, like those produced by a locomotive-engine, but far less regular and rhythmical in their succession. In the second class of apertures masses of molten material are seen welling out, and, if the position of the aperture be favourable, flowing outside the crater; from this liquid molten mass steam is seen to escape, sometimes in considerable quantities. The openings of the third class present still more interesting appearances. Within the walls of the aperture a viscid or semi-liquid substance is seen slowly heaving up and down. As we watch the seething mass the agitation within it is observed to increase gradually, and at last a gigantic bubble is formed which violently bursts, when a great rush of steam takes place, carrying fragments of the scum-like surface of the liquid high into the atmosphere.

If we visit the crater by night, the appearances presented are found to be still more striking and suggestive. The smaller cracks and larger openings glow with a ruddy light. The liquid matter is seen to be red- or even white-hot, while the scum or crust which forms upon it is of a dull red colour. Every time a bubble bursts and the crust is broken up by the escape of steam, a fresh, glowing surface of the incandescent material is exposed. If at these moments we look up at the vapour-cloud covering the mountain, we shall at once understand the cause of the singular appearances presented by Stromboli when viewed from a distance at night, for the great masses of vapour are seen to be lit up with a vivid, ruddy glow, like that produced when an engine-driver opens the door of the furnace and illuminates the stream of vapour issuing from the funnel of his locomotive.

Let us now endeavour to analyse the phenomena so admirably displayed before us in the crater of Stromboli. The three essential conditions on which the production of these phenomena seems to depend are the following: first, the existence of certain apertures or cracks communicating between the interior and the surface of the earth; secondly, the presence of matter in a highly heated condition beneath the surface; and thirdly, the existence of great quantities of water imprisoned in the subterranean regions—which water, escaping as steam, gives rise to all those active phenomena we have been describing.

We have said, at the outset, that there exists no analogy whatever between the action which takes place in volcanoes and the operation of burning or combustion. Occasionally, it is true, certain inflammable substances are formed by the action going on within the volcano, and these inflammable substances, taking fire, produce real flames. Such flames are, however, in almost all cases only feebly luminous, and do not give rise to any conspicuous appearances. What is usually taken for flame during volcanic eruptions is simply, as we have already pointed out, the glowing red-hot surface of a mass of molten rock, reflected from a vapour-cloud hanging over it. The red glow observed over a volcano in eruption is indeed precisely similar in its nature and origin to that which is seen above London during a night of heavy fog, and which is produced by the reflection of the gas-lights of the city from the innumerable particles of water-vapour diffused through the atmosphere. Fires, of course, occur when the molten and incandescent materials poured out from a volcano come in contact with inflammable substances, such as forests and houses, but in these cases the combustion is quite a secondary phenomenon.

There is another popular delusion concerning volcanic action, which it may be necessary to refer to and to combat. From the well-known fact that sulphur or brimstone is found abundantly in volcanic regions, the popular belief has arisen that this highly inflammable substance has something to do with the production of the eruptions of volcanoes. In school-books which were, until comparatively recent years, in constant use in this country, the statement may be found that by burying certain quantities of sulphur, iron-pyrites, and charcoal in a hole in the ground, we may form a miniature volcano, and produce all the essential phenomena of a volcanic eruption. No greater mistake could possibly be made. The chemical reactions which take place when sulphur and other substances are made to act upon each other differ entirely from the phenomena of volcanic action. The sulphur which is found in volcanic regions is the result and not the cause of volcanic action. Among the most common substances emitted from volcanic vents along with the steam are the two gases, sulphurous acid and sulphuretted hydrogen. When these two gases come into contact with one another, chemical action takes place, and the elements contained in them—oxygen, hydrogen, and sulphur—are free to group themselves together in an entirely new fashion; the consequence of this is that water and sulphuric acid (oil of vitriol) are formed, and a certain quantity of sulphur is set free. The water escapes into the atmosphere, the sulphuric acid combines with lime, iron, or other substances contained in the surrounding rocks, and the sulphur builds up crystals in any cavities which may happen to exist in these rocks.

If, however, careful and exact observations, like those carried on at Stromboli, compel us to reject the popular notions concerning the supposed resemblance between volcanic action and the combustion of sulphur or other substances, they nevertheless suggest analogies with certain other simple and well-known operations. And in pursuing these analogies, we are led to the recognition of some admirable illustrations both of the attendant phenomena and of the true cause of volcanic outbursts.

No one can look down on the mass of seething material in violent agitation within the fissures at the bottom of the crater of Stromboli, without being forcibly reminded of the appearances presented by liquids in a state of boiling or ebullition. The glowing material seems to be agitated by two kinds of movements, the one whirling or rotatory, the other vertical or up-and-down in its direction. The fluid mass in this way appears to be gradually impelled upwards, till it approaches the lips of the aperture, when vast bubbles are formed upon its surface, and to the sudden bursting of these the phenomena of the eruption are due.

Now if we take a tall narrow vessel and fill it with porridge or some similar substance of imperfect fluidity, we shall be able, by placing it over a fire, to imitate very closely indeed the appearances presented in the crater of Stromboli. As the temperature of the mass rises, steam is generated within it, and in the efforts of this steam to escape, the substance is set in violent movement. These movements of the mass are partly rotatory and partly vertical in their direction; as fresh steam is generated in the mass its surface is gradually raised, while an escape of the steam is immediately followed by a fall of the surface. Thus an up-and-down movement of the liquid is maintained, but as the generation of steam goes on faster than it can escape through the viscid mass, there is a constant tendency in the latter to rise towards the mouth of the vessel. At last, as we know, if heat continues to be applied to the vessel, the fluid contents will be forced up to its edge and a catastrophe will occur; the steam being suddenly and violently liberated from the bubbles formed on the surface of the mass, and a considerable quantity of the material forcibly expelled from the vessel. The suddenness and violence of this catastrophe is easily accounted for, if we bear in mind that the escaping steam acts after the manner of a compressed spring which is suddenly released. Steam is first formed at the bottom of the vessel which is in contact with the fire; but here it is under the pressure of the whole mass of the liquid, and moreover, the viscidity of the substance tends to retard the union of the steam bubbles and their rise to the surface of the mass. But when the pressure is relieved by the bursting of bubbles at the surface, the whole of the generated steam tends to escape suddenly.

Now within the crater of Stromboli we have precisely the necessary conditions for the display of the same series of operations. In the apertures at the bottom there exists a quantity of imperfectly fluid materials at a higher temperature, containing water entangled in its mass. As this water passes into the state of steam it tends to escape, and in so doing puts the whole mass into violent movement. When the steam rises to the surface, bubbles are formed, and the formation of these bubbles is promoted by the circumstance that the liquid mass, where exposed to the atmosphere, becomes chilled, and thereby rendered less perfectly fluid. By the bursting of these bubbles the pressure is partially relieved, and a violent escape of the pent-up steam takes place through the whole mass. Equilibrium being thus restored, there follows a longer or shorter interval of quiescence, during which steam is being generated and collected within the mass, and the series of operations which we have described then recommences.

There is one other consideration which must be borne in mind in connection with this subject. It is well known that if water be subjected to sufficiently great pressure it may be raised to a very high temperature and still retain its liquid condition. When this pressure is removed, however, the whole mass passes at once into the condition of steam or water-gas; and the gas thus formed at high temperatures has a proportionably high tension. In a Papin's digester water confined in a strong vessel is raised to temperatures far above its ordinary boiling-point, and from any opening in such a vessel the steam escapes with prodigious violence. Now, at considerable depths beneath the earth's surface, and under the pressure of many hundreds or thousands of feet of solid rock, water still retaining its liquid condition may become intensely heated. When the pressure is relieved by the formation of a crack or fissure in the superincumbent mass of rock, the escape of the superheated steam will be of very violent character, and may be attended with the most striking and destructive results. In the existence of high temperatures beneath the earth's surface, and the presence in the same regions of imprisoned water capable of passing into the highly elastic gas which we call steam, we have a cause fully competent to produce all the phenomena which we have described as occurring at Stromboli.

It may at first sight appear that the grand and terrible displays of violence witnessed during a great volcanic eruption differ fundamentally in their character and their origin from those feeble outbursts which we are able to examine closely and analyse rigorously at Stromboli. But that such is not the case a few simple considerations will soon convince us.

Although Stromboli usually displays the subdued and moderate activity which we have been describing, yet the intensity of the action going on within it is subject to considerable variation. Occasionally the violence of the outbursts is greatly increased—the roaring of the steam-jets may be heard for many miles around, considerable streams of incandescent liquefied rock flow down the Sciarra into the sea, and the explosions in the crater are far more frequent and energetic, cinders and fragments of rock being scattered all over the island and the surrounding seas.

On the other hand, volcanoes like Vesuvius, which are sometimes the scene of eruptions on the very grandest scale, at others subside into a temporary state of moderate activity quite similar in character to that which is the normal condition of Stromboli. Thus, shortly before the great eruption of Vesuvius in April 1872, a small cone was formed near the edge of the crater, and during some months observers could watch, in ease and safety, a series of small explosions taking place, quite similar in their character and attendant phenomena to those which we have described as occurring at Stromboli. French geologists are in the habit of defining the condition of activity in a volcano by speaking of the more quiet and, regular state as the 'Strombolian stage,' and the more violent and paroxysmal as the 'Vesuvian stage'; but the two conditions are, as we have seen, presented by the same volcano at different periods, and pass into one another by the most insensible gradations.

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 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 vapour which they gave off during their flow. All along their course, enormous volumes of steam were evolved from them, as will be seen by an inspection of the photograph. Indeed, such was the abundance and tension of the steam thus escaping from the surfaces of the lava-currents that it forced the congealing rock up into great bubbles and blisters, and gave rise to the formation of innumerable miniature volcanoes, varying in size from a beehive to a cottage, some of which remained in a state of independent activity for a considerable time.

So far, what we have described as taking place at Vesuvius, in April 1872, has been only the repetition on a £Eur grander scale of the three kinds of action which we have shown to be constantly taking place at Stromboli; namely, the formation of cracks or fissures in the earth's surface, the escape of steam with explosive violence from these openings, often propelling rock-fragments into the atmosphere, and the outwelling, under the influence of this compressed steam, of masses of molten materials.

There were some other appearances presented at the great outburst at Vesuvius, which do not seem at first sight to find any analogies in the manifestations of the more feeble action continually going on at Stromboli.

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.

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.

It is well known that when high-pressure steam IS allowed to escape through an orifice, electricity is abundantly generated by the friction, and Sir William Armstrong's hydro-electric machine is constructed on this principle. Every volcano in violent eruption is a very efficient hydro-electric machine, and the uprushing column is in a condition of intense electrical excitation. This result is probably aided by the friction of the solid particles as they are propelled upwards and fall back into the crater. The restoration of the condition of electrical stability between this column and the surrounding atmosphere is attended with the production of frequent lightning-flashes and thunder-claps, the found of the latter being usually, however, drowned in the still louder roar of the uprushing steam-column.

The discharge of Buch large quantities of steam into the atmosphere soon causes the latter to be saturated with watery vapour, and there follows an excessive rainfall; long-continued rain and floods were an accompaniment of the great Vesuvian outbreak of 1872, as they have been of almost all great volcanic eruptions. The Italians, indeed, dread the floods which follow an eruption more than the fiery streams of lava which accompany it—for they have found the mud-streams (lave di fango), formed by rain-water sweeping along the loose volcanic materials, to be more widely destructive in their effects than the currents of molten rock (lave di fuoco).

Besides the phenomena which we have now described as accompanying a great volcanic outburst, many others have undoubtedly been recorded by apparently trustworthy authorities. But, in dealing with the descriptions of these grand and terrible events, we must always be on our guard against accepting as literal facts, the statements made by witnesses, often writing at some distance from the scene of action, and almost always under the influence of violent excitement and terror. The desire to administer to the universal love of the marvellous, and the tendency to exaggeration, will usually account for many of the wonderful statements contained in such records; and, even where the witness is accurately relating events which he thinks passed before his eyes, we must remember that it is probable he may have had neither the opportunity nor the capacity for exact observation.

The more carefully we sift the accounts which have been preserved of great volcanic outbursts, the more are we struck by the fact that the appearances described can be resolved into a few simple operations, the true character of which has been distorted or disguised by the want of accurate observation on the part of the witnesses.

We are thus led to the conclusion that the grand and terrible appearances displayed at Vesuvius and other volcanoes in a state of violent eruption do not differ in any essential respect from the phenomena which we have witnessed accompanying the miniature outbursts of Stromboli. And we are convinced, by the same considerations, that the forces which give rise to the feeble displays in the latter case would produce, if acting with greater intensity and violence, all the magnificent spectacles presented in the former.

In Vesuvius and Stromboli alike, the active cause of all the phenomena exhibited is found to be the escape of steam from the midst of masses of incandescent liquefied rock. The violence, and therefore the grandeur and destructive effects of an eruption, depend upon the abundance and tension of this escaping steam.

There is one respect in which volcanic phenomena are especially calculated to excite the fear and wonder of beholders—namely, in the sudden and apparently spontaneous character of their manifestations. Eclipses were regarded as equally portentous with volcanic eruptions till astronomers learned not only to explain the causes which gave rise to them, but even to predict to the second the times of their occurrence. If we were able in like manner to warn the inhabitants of volcanic regions of the approach of a grand eruption, the fear and superstition with which these events are now regarded would doubtless be in great part dispelled. The power of prediction is alike the crucial test and the crowning triumph of a scientific theory.

But, although natural philosophers are able to assign the causes to which the grand operations of volcanoes are due, and also to explain all the varied appearances which accompany them, they have not as yet so far mastered the laws which govern volcanic action as to be able to predict the periods of their manifestation.

That these operations, like all others going on upon the globe, are governed by great natural laws we cannot for a moment doubt. And that, in all probability, more careful and exact observation and reasoning will at some future time lead us to the recognition of these laws, every student of nature is sanguine. But at the present time, it must be confessed, we are very far indeed from being able to afford that crowning proof of the truth of our theories of volcanic action which is implied in the power of predicting the period and degree of intensity of their manifestations.

There are, however, some observations which lead us to hope that the time may not be far distant when we shall have so £Eur obtained a knowledge of the conditions on which volcanic action depends as to be able to form some judgment as to its manifestations in the future at any particular locality. But we must recollect that these conditions axe very numerous and complicated, and that some of them may lie almost entirely outside our sphere of observation; hence hasty attempts in this direction, such as have recently been made, are to be deprecated by every true lover of science.

Concerning the eruptions that have taken place at those volcanic centres which have been known from a remote antiquity, we have records from which we can determine the intervals separating these outbursts and their relative violence. A critical examination of these records leads to the following conclusions:—

(1.) A long period of quiescence is generally followed by an eruption which is either of long duration or of great violence.

(2.) A long-continued, or very violent eruption is usually followed by a prolonged period of repose.

(3.) Feeble and short eruptions usually succeed one another at brief intervals.

(4.) As a general rule, the violence of a great eruption is inversely proportional to its duration.

It will be seen that these general conclusions are in perfect harmony with the theory that volcanic outbursts are due to the accumulation of steam at volcanic centres, and that the tension of this imprisoned gas eventually overcomes the repressing forces which tend to prevent its manifestation. Before astronomers had learnt to determine all the conditions on which the production of eclipses depends, they had found that these phenomena succeed one another at regular intervals. The discovery of such astronomical cycles was a great advance in our knowledge of the heavenly bodies, and in the same way the determination of these general relations between the intensity and duration of volcanic outbursts and the intervals of time which separate them may be regarded as the first step towards the discovery of the laws which govern volcanic activity.

In the actual determination of the conditions upon which the occurrence of volcanic eruptions depends, it must be confessed, however, that very little has as yet been done. This is in part due to the fact that some at least of these conditions lie beyond the limits of direct observation. But it must also be admitted, on the other hand, that little has been as yet accomplished towards the careful and systematic observation of those phenomena which may, and probably do, exert an influence in bringing about volcanic outbursts.

In the Lipari Islands there has prevailed a belief, from the very earliest period of history, that the feeble eruptions of Stromboli are in some way dependent upon the condition of the atmosphere. These islands were known to the ancients as the Æolian Isles, from the fact that they were once ruled over by a king of the name of Æolus. It seems not improbable that Æolus was gifted with natural powers of observation and reasoning far in advance of those of his contemporaries. A careful study of the vapour-cloud which covers Stromboli would certainly afford him information concerning the hygrometric condition of the atmosphere; the form and position assumed by this vapour-cloud would be a no less perfect index of the direction and force of the wind; and, if the popular belief be well founded, the frequence and violence of the explosions taking place from the crater would indicate the barometric pressure. From these data an acute observer would be able to issue 'storm-warnings' and weather-prognostics of considerable value. In the vulgar mind, the idea of the prediction of natural events is closely bound up with that of their production; and the shrewd weather-prophet of Lipari was after his death raised to the rank of a god, and invested with the sovereignty of the winds.

Whether the popular idea that the outbursts of Stromboli are regulated by atmospheric conditions has any foundation is still open to grave doubt. It seems to be certain, however, that during autumn and winter the more violent paroxysms of the volcano occur, and that in summer the action which takes place is far more regular and equable. It would be of the greatest benefit to science if an observatory were erected beside the crater of Stromboli, where a careful record might be kept of all atmospheric changes, and of the synchronous manifestations of the volcanic forces.

A little consideration will show that it is a by no means unreasonable supposition that variations in atmospheric pressure may exercise a very important influence in bringing about volcanic outbursts. Changes in the barometer to the extent of two inches within a very short period are not uncommon occurrences. A very simple calculation will show that the fall of the mercury in the barometer to the extent of two inches indicates the removal of a weight of two millions of tons from each square mile of the earth's surface where this change takes place. Now, if we suppose, as we have good ground for doing, that under volcanic areas vast quantities of superheated water are only prevented from flashing into steam by the superincumbent pressure, a relief of this pressure to the extent of two millions of tons on every square mile could scarcely fail to produce very marked effects. The way in which explosions in fiery coal-mines generally follow closely upon sudden falls in the atmospheric pressure is now well known; and coal-mine explosions and volcanic outbursts have this in common, that both result from the sudden and violent liberation of subterranean gases. There are not a few apparently well-authenticated accounts of volcanic and earthquake phenomena following closely on peculiar atmospheric conditions, and the whole question of the relation of the volcanic forces to atmospheric pressure, as Spallanzani himself so long ago pointed out, is deserving of a most careful and rigorous investigation.