On coming out upon the brow of the mountain, Rollo saw at a short distance before him an immense column of dense white vapor pouring up into the air. His first impulse was to run forward up the sandy slope that still remained between the place where he stood and the margin of the crater; but he checked himself, and stopped where he was, to wait for the rest of the party. As soon as the portantina bearers reached the place where he stood, they set down the chair, and immediately the whole set crowded around Mr. George, and again demanded buono manos.

"Philippe payera," said Mr. George, pointing down the mountain to the Hermitage—"Philippe payera, lÀ bas," which means, "Philippe will pay when you go down."

Mr. George said this in a very quiet manner, and then proceeded to help Rosie out of her chair. The guide who had come up the mountain with them then led the way, and Mr. George, Rollo, Rosie and Josie followed, towards the crater.

And here I must stop a moment in my story to explain a little what a crater is, and how it is formed. A crater is a great circular pit or depression in the top of a volcano, formed by the sinking of the ground in that part. This sinking of the ground is caused apparently by the cooling and shrinking of the melted matter below, after a time when it has been unusually heated.

Most boys have observed an effect similar to this in casting lead. When you attempt to cast any thing of lead,—a cannon, for example, or anchor, or even a bullet,—you will observe that as the lead cools, the portion of it which comes at the top of the mould shrinks and falls in, forming a little pit or depression, which you have to fill up by pouring in a little more lead. The reason is, that lead, as well as most other melted substances, shrinks when it cools. In the case of the bullet, for instance, all the lead which forms the mass of the bullet within the mould shrinks. The effect of this would be to collapse the sides, were it not that the sides have already become solid by contact with the cold mould. But the lead at the top, having been poured in last, is still fluid; and so that settles down as the lead cools below, and forms the little pit or depression, which the boy presently fills up by pouring in a little more lead.

It is much the same with a volcano. For some reason or other,—no one as yet knows what it is,—the interior of a volcano changes its temperature very much at different times. Sometimes for a period of several months, or years, it seems to be all the time growing hotter and hotter. The substances below become more and more melted, and formed into lava. The water, which is all the time filtering in through the crevices and openings, in the rocks around the sides of the mountain, is forced down under this molten mass by the immense pressure given to it by the height of the mountain. There it is turned into steam. For a time it is kept down by the vast weight of the lava which is over it, but after a time the elastic force of it gets so great that a bubble of it bursts up, and comes out at the top of the mountain in a great, thundering puff, bringing up some portion of the melted lava with it, and throwing it high into the air.

The lava thus thrown up falls down again, and when there is no wind it falls down close around the opening. Some of it falls into the opening, where it is melted again. The rest falls on the sides, and in process of time it begins to build up a small hill, as it were, all around the opening, though the puffs and explosions of steam that are continually coming out keep a mouth open at the top.

Things go on in this way for some time, until at length, for some mysterious reason which nobody understands, the interior of the mountain begins to moderate its heat, and finally to grow cool—not entirely cool, but cooler than it has been. The puffs and explosions gradually cease. The lava within the bowels of the mountain shrinks as it cools. The sides of the mountain being firm and solid, do not collapse; but the top, being still more or less soft, falls in, not suddenly, but by a slow and gradual motion, corresponding with the progress of the cooling below. So slow, indeed, is this progress, that sometimes the ground continues sinking slowly in this way for several years before the crater is fully formed.

All this time, although the puffs and explosions have in a great measure ceased, the steam continues to blow out, more or less steadily, from a great many small openings, some of them in the bottom of the crater, and some, perhaps, in the sides. This steam is changed into visible vapor when it comes out where the air is cool, and the several streams, mingling together as they rise into the air, form a cloudy column, which is often called smoke. Strictly speaking, however, it is not smoke. It is almost entirely composed of steam.

After continuing in this state for some time, the interior of the mountain begins to grow hot again. Then the steam and hot lava begin to puff out at some one or other of the vents in the bottom of the crater. If the heating goes on, the lava comes out hotter and hotter from the opening, and by melting away the sides of it and blowing it out, it gradually enlarges it. The lava that is blown out, too, falls down all around the hole, and gradually builds up a mound around it, like a little dome, while the successive blasts keep the outlet open all the time at the top. This small cone, rising up gradually thus, in the bottom of the crater formed by the sinking in of the mountain before, and the chimney opening up through the centre of it, gives vent to all the steam from below, while a great many of the other orifices are stopped up by the lava which comes up out of the great opening falling into them. After a time, the lava that is thrown out spreads over the whole floor of the crater in a mass of black, corrugated slag, with the small cone rising from the centre of it, and the opening at the top glowing like the mouth of a fiery furnace, and bursting out every now and then, with explosions of steam, and red-hot stones, and melted lava.

This was precisely the condition of Vesuvius at the time that Rollo visited it. The top of the mountain had fallen in, in two places, some time before, on account of the cooling below, and two great craters had been formed. Now, the furnace had been for some time heating up again, and in each crater a black cone, with a fiery mouth open at the apex of it, was gradually growing up, and covering the whole floor of the crater with the black and molten matter which it was ejecting.

It was to the edge of one of these craters that the party now advanced, and the engraving will give you some idea of the view which it presented.

[Pg 137-138]

There were several persons, both ladies and gentlemen, standing on the margin of the crater when our party arrived. Mr. George led Rosie to the place, and looked down with her into the abyss. The sides of it were formed of precipitous cliffs of rocks and sand, all beautifully colored, in every shade of red and yellow, by the deposits of sulphur which had accumulated upon them from the fumes of the volcano. The floor of the crater was black as jet, being covered by the molten lava, which had gradually spread over

it. The surface of this lava lay in wave-like corrugations, like the hide of a rhinoceros, showing that it was or had been semi-fluid. In the centre rose a great, black, rounded cone, like the cupola of an immense blast furnace. This cone was about fifty feet high, and there was an opening at the top eight or ten feet in diameter, which glowed with a furious heat, and emitted quietly, but continually, a red-hot breath of sulphurous vapor.

After remaining thus quiet for a few moments, suddenly it would give a gasp, and immediately afterwards there would burst forth a thundering explosion, which seemed to come up from a great depth below, and threw into the air a shower of stones and scraps of molten lava, which, after ascending to a great height, came down again, and fell, with a dripping sound, upon and around the cone. Similar explosions occurred at intervals of a few minutes, all the time that the party remained.

Rosie was at first very much afraid of these explosions, and she wished to go back. Mr. George himself was also afraid at first to stand very near the edge of the crater; but it was not on account of the explosions, but for fear that the cliff might cave in. Indeed, the cliffs all around were cracked off, and in some places leaning over, apparently ready to fall; and even at the spot where the spectators stood looking into the crater, there was a fissure running along parallel to the cliff, some feet behind them. At first Mr. George was afraid to step over this crack.

"How do they know," said he to himself, "but that the whole mass will fall and carry them all down into the gulf below?"

He found, however, after waiting a little while, that it did not fall, and there were besides other masses a little farther along, as seen in the engraving, which had become separated entirely from the cliff behind them, leaving a chasm open two or three feet wide; and yet they did not fall. So Mr. George gradually acquired more confidence, and at length went cautiously forward, and looked over the brink.

Rosie, however, hung back. She was alarmed to see Rollo and Josie go so near.

"Come back, Josie," said she; "come back. You must not go so near."

So Mr. George called the boys back, and they obeyed.

The walls of this crater were on every side almost perpendicular. As the central part had gradually sunk, the sides had caved off and fallen in, and then afterwards the lava that had been thrown up had spread over the floor, and covered it with a bed of a half-fluid looking substance, that was as black as pitch, and which, though it was really now pretty hard, looked as if a stone thrown down upon it would sink immediately into it, out of sight.

The crater seemed to be four or five hundred feet across, and the walls of it were eighty or a hundred feet high.

After Mr. George and the children had been standing upon the brink of this abyss some time, watching the explosions, the guide who had come up with them from the Hermitage beckoned to Mr. George, and saying something at the same time in Italian, made signs as if he wished the party to go with him to some other place.

"Come, boys," said Mr. George; "he wants us to go with him."

"Where does he want us to go?" asked Rollo.

"I don't know," said Mr. George. "I cannot understand what he says; but let us go and see."

So the whole party followed the guide, Mr. George leading Rosie by the hand. The guide conducted them along a narrow path through the sand, which led away from the crater behind a hill which formed one of the sides of it at a place where it was so steep below the path down the mountain side, that Rosie was almost afraid to go. Mr. George, however, held her firmly by the hand, and he charged Rollo and Josie to follow very carefully. After going on in this way for some distance, they came to another crater very similar to the first, only the sides of it, instead of being formed, like the first, of perpendicular cliffs, consisted of steep, sloping banks of volcanic sand and gravel. There was, however, the same pitchy bed of lava spread out all over the bottom of it below, and in the centre a black cone thirty feet high, with a fiery furnace mouth at the top, glowing with heat, and throwing out continually the same thundering puffs of steam, and projecting the same masses of melted lava and hot stones into the air.

"Ah, here is another crater!" said Mr. George.

"Yes," said Rollo; "only it is smaller than the first. I like the first the best."

While they were standing on the narrow ridge which formed the brink of the crater, looking down, their guide by their side, another guide came by, conducting two young men; and they, instead of stopping on the brink, as Mr. George and his party had done, began at once to go down. There was a sort of track in the sand down the slope, and in this track the young men, half walking, half sliding, descended.

"Why, uncle George!" exclaimed Rollo, "they are going down into the crater. Let Josie and me go too."

Mr. George saw by a glance that the descent into the crater must be safe, for the young men were led by one of the regular mountain guides; and besides, there was a track in the sand, showing that other parties had gone down before. So he said that Rollo and Josie might go.

"You may go down with this party," said Mr. George, "and then you can come up and take care of Rosie while I go down with our guide."

So Rollo and Josie followed the two young men down. Mr. George watched them from above. They went down very easily, for the sand was soft, and the track turned this way and that, so as to avoid the steepest places. The black lava covered the whole floor of the crater, and Mr. George and Rosie supposed that those who had gone down would be able only to go to the edge of it; but, to their great surprise, they found that the guide, as soon as he reached it, stepped upon it, and walked boldly out, followed by the young men and by Rollo and Josie, like a party of boys walking out upon the ice on a pond.

"Why, uncle George!" exclaimed Rosie, "they are walking over the lava. Why don't they sink in?"

"I cannot imagine," said Mr. George. "I supposed it was soft."

In fact, it was soft; that is, it was soft enough to flow if it had been on a slope, but yet it was hard enough to walk upon. A current of lava, when it is coming down the mountain side, can often be walked upon while it is still in motion. Its fluidity at the best is very imperfect, and its motion is very slow. The lava which Rollo was upon in the floor of the crater, though pretty nearly cool and hard on the surface, was hot below. Rollo could see the redness of the heat in the holes and crevices. Probably, if a heavy stone were laid upon the bed of lava, it would gradually have sunk into it. And yet persons could walk over it without any difficulty.

Rollo and Josie followed the young men over the lava until they came so near the cone in the centre that if they were to advance farther they would be in danger of having the lava which was thrown up from it fall upon their heads. Here they found some boys, who belonged to the mountain, engaged in getting out small pieces of the lava, where it was hot and soft, and pressing coins into it, to sell to the people above. Rollo and Josie bought some of these specimens of the boys, and put them hot in their pockets.

While the boys were thus near the cone in the centre of the crater, they were sometimes lost to view from Mr. George and Rosie, on account of the puffs of vapor which the wind blew over them. Rosie was very much afraid whenever this happened. She thought that Josie and Rollo were lost; but Mr. George assured her that there was no danger.

"I should think there would be a great deal of danger," said she.

"So should I," said Mr. George, "of my own judgment. But I do not go by my own judgment in such cases."

"Whose judgment do you go by?" asked Rosie.

"By the guides'," replied Mr. George. "The guides know all about the mountain. They are up here every day. They have been watching it for years, and they can tell where it is safe to go, and where it is dangerous, better than any stranger. So I give up my judgment entirely, and go altogether by theirs. You will see Rollo and Josie coming back out of the smoke pretty soon, as safe as they went in."

This prediction proved to be true. In a few minutes, on account of some change in the gusts of wind, the masses of vapor in the crater broke into openings, and rolled off towards the other side, and in the openings Rosie could see the boys coming back over the black surface of the lava, their footsteps making a curious sound upon it, as if they were walking over clinkers. Very soon they reached the side, and then came toiling up the path which ascended the slope of sand.

Rollo and Josie were both full of enthusiasm in describing what they had seen at the bottom of the crater, and near the cone, and they strongly recommended to Rosie to go down too.

"I'll go with you, Rosie," said Josie, "and show you the way."

But Rosie declined the adventure, and Mr. George told her that she did right to do so.

"Why, what is there to be afraid of?" asked Josie. "There is no danger—not the least in the world."

"True," said Mr. George; "but going into such places does not give so much pleasure to young ladies as it does to such courageous young gentlemen as you. But I wish to go down myself, and I will leave Rosie under your care here while I am gone."

Pretty near where the party stood while engaged in this conversation, several persons were gathered about what seemed to be a fire. A sort of smoke came up from the ground in the centre of the group, and by the side of it were one or two baskets containing eggs, bread, bottles of wine, and other refreshments. Mr. George led the way to this place, and then he found that what seemed to be a fire was really a jet of hot steam and sulphurous gases that was issuing from a cleft among the rocks. The place was very near the crest of the crater, and the people that stood around it were watching to see men cook in the jets of steam. There was a little level place inside the crevice, just beneath the ground, where they could put eggs and other such things, and after leaving them there a short time, they were found to be nicely cooked. As fast as they were done, the men took them out and sold them to the bystanders.

Mr. George left Rosie and the two boys here while he went down into the crater. The guide went with him to show him the way. In about ten minutes Mr. George returned, and found the three children standing round the cuisine, as the men called the place where they cooked. Rollo had been buying some of the eggs, and he and Josie and Rosie were eating them.

"Mr. George," said Josie, "are these boiled eggs, or baked eggs, or roasted eggs, or what?"

"They seem to be steamed eggs," said Mr. George.

"I suppose," said Rollo, "that by digging about here in the sand, we might find a place where it would be just warm enough to hatch eggs."

"No doubt," said Mr. George.

Just then Rollo observed that the two young men whom he and Josie had followed down into the crater were standing at a little distance, and attentively regarding some sort of instrument which they had in their hands.

"I mean to go and see what they are doing," said Rollo.

So saying, he looked into Mr. George's face, and waited to see if Mr. George had any objection to his going.

"Very well," said Mr. George.

So Rollo went off to the place where the young men were standing, and soon afterwards Mr. George and the others of the party could see that the strangers were showing him the instrument, and apparently explaining it to him. Pretty soon Rollo returned and reported that the two young men were students, and that the instrument which they had was a metallic barometer, and that they were measuring the height of the mountain with it.

This metallic barometer is quite a curious instrument. You will often read, in books, of measuring the height of a mountain, or other lofty place, by the barometer; and to most people this is quite a mystery. The explanation of it is, however, very simple. It is this: The earth is surrounded on all sides by the atmosphere, which, though very light, has a certain weight, and it presses with considerable force upon the ground, and upon every thing that is exposed to it. If, however, you go up from the ground, as, for instance, when you ascend a mountain, the higher you go, the less the pressure is. This is naturally to be expected, for the higher you go in such a case, the less air there is above you to press. Now, a barometer is an instrument to measure the pressure of the air, just as a thermometer measures the heat or coldness of it. A metallic barometer is a new kind, in which the air presses on a curiously contrived ring or band of brass, and according as it presses more or less, it moves an index like the hand of a watch, which is placed on the face of it. It was such an instrument as this that the two students had, on Vesuvius.

The way in which you use such an instrument to measure the height of a mountain is this: You look at the instrument when you are at the bottom of the mountain, before you begin your ascent, and see how it stands. There is a little index like the hour hand of a watch, which is movable. This you set at the point where the other index stands when you are at the foot of the mountain. Then you begin your ascent. You shut up your barometer if you please, and put it in your knapsack, or in the chaise box, or any where else you please. Wherever you put it, the pressure of the air will find it out, and penetrate to it, and as you gradually rise from the surface of the earth, the index, which is connected with the curious brass ring, moves slowly backward as the pressure diminishes. This motion continues as long as you continue ascending. If you come to a level place, it remains stationary as long as the level continues. If you descend, it goes forward a little, and then begins to go back again as soon as you once more begin to ascend. Then, when you get to the top of the mountain, you look at it, and you see at once how much the pressure of the air has diminished. From this, by an easy calculation, you tell at once how high you have come.

Mr. George knew all about the barometer, and the means of measuring heights with it, though he had never seen an instrument of this particular kind. He was accordingly very much interested in Rollo's account of it, and he said he had a great mind to go and see it himself.

"I wish you would," said Rollo. "I told them that I thought you would like to see it, and they said that they should be very happy to show it to you."

Mr. George accordingly went to see the instrument, and the students gave him so cordial a reception, that he formed at once quite an intimate acquaintance with them. Indeed they were quite pleased to find a person on the mountain who sympathized with them in their scientific inquiries and pursuits, and was capable of understanding and appreciating them. They told Mr. George that they were going to remain on the mountain until after dark, in order to see it in its night aspects, and they invited him to remain with them.

"Then to-morrow," said they, "we are going across the mountain down through the back ravines, to study the geological structure of the old lava beds, and so come out at Pompeii."

Mr. George said there could be nothing that he should enjoy more, were it not that he had ladies under his charge, and that he felt bound to accompany them back to Naples.

Rollo, when he heard this invitation, immediately felt a strong desire that Mr. George should go, and that he might go too. He instantly perceived, however, that this was out of the question; but he thought that by cordially falling in with the plan of allowing Mr. George to go, he might, perhaps, be the means of accomplishing it. Many boys, in such a case, when they find that a plan of enjoyment that is proposed is one which they cannot themselves share, do all they can to hinder and oppose it altogether. But Rollo had now travelled about the world so much, and had acquired so much experience, that he was above such folly as this.

"Uncle George," said he, "you can go just as well as not. I can take care of Rosie down the mountain to the Hermitage, and then we shall have nothing to do but to get into the carriage and ride home."

Mr. George saw at once how generous it was in Rollo to make this offer, and he said he would so far accept it as to let Rollo take charge of the party going home from the Hermitage in the carriage; but he felt bound, he said, not to leave Rosie until he had returned her safe to her mother's hands. So he said to the students,—

"I will go down the cone with Rosie and the two boys, and accompany them as far as the Hermitage. There I shall find Mrs. Gray and the carriage. If Mrs. Gray seems cordially willing to go home with the children alone, I will come back here and join you; but if I find she does not seem entirely willing,—if she looks sober about it,—then I will go back to Naples; though in that case I shall come to Pompeii to-morrow, and shall hope to meet you there."

"I hope the lady will be willing to release you," said one of the students.

[Pg 153-154]

"I have but little doubt that she will," said Mr. George.

Accordingly, after rambling about on the margin of the crater a little time longer, and gathering all the specimens which they required, Mr. George and the children commenced their descent. One of the students went down with them, in order to accompany Mr. George back. The descent was very easy, for the path led down a slope, where, instead of being rocky as it was where they came up, there was little else but loose sand, so that at every step they took they slid down a great way, and thus went, very fast and very easily, from the top to the bottom.

When they reached the foot of the slope, they found the mules and donkeys there. Rollo and Josie insisted that Mr. George and the student should ride, because they had got to ascend the cone again.

"Besides," said Rollo, "if you ride you can get there quicker, and arrange the business with Mrs. Gray."

Mr. George was right in anticipating that Mrs. Gray would give her cordial consent to have him leave the party.

"I shall miss your company," said she, "but I feel perfectly safe in going home in the carriage with Philippe and the boys. Besides, I shall want to hear an account of your adventures on the mountain in the night, and in crossing over by the ravines to-morrow. And then if you are willing," she added, "we will all come and meet you at Pompeii to-morrow."

"I should like that very much indeed," said Mr. George. "Philippe will arrange every thing for you."

This being all settled, Mrs. Gray and the children entered the carriage and set out for Naples, while Mr. George and the student turned their faces towards the mountain again.