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,—