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 onc