Chapter VII.
The Summit.
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.