, when they do, men cannot paddle,
or row boats upon them. A current that should run at the rate of four or
five miles an hour, would be very hard to row against. But the air is
seldom in a state of less motion than that. It is very often moving at
the rate of fifteen or twenty miles an hour; sometimes sixty. So, you
see, there is a double reason why men cannot fly in the air, as well as
paddle on the water."

"If we were only light enough," said Rollo, "to float in the air, then
we could fly."

"We could paddle about in it, when it was calm," replied his father,
"but that would not be flying."

"Is there anything light enough to float in the air?" said Rollo.

"No," said his father, "I don't think of any visible substance that is."

"What do you mean by visible substance?" said Nathan.

"Why, anything that you can see," replied his father. "There are some
other kinds of air, which are lighter than common air, but there is
nothing else, so far as I know."

"Why, father, there are clouds. They float, and they are visible, I am
sure."

"Yes," said his father. "There is some mystery about the floating of
clouds. I don't fully understand it. Clouds are formed of small
globules or little balls of water; and water, I should think, whatever
the size of the little drops might be, would be heavier than air. And
yet they seem to float. If they are large, like rain drops, they fall
quickly to the ground. If they are small, like mist, they fall slowly.
That I should expect. If they are finer still, like vapor or fog, I
should think that they would fall still more slowly; but still I should
suppose that they would descend. But they do not appear to descend; they
seem to float, nearly at rest; though perhaps all the clouds we see, may
be slowly descending all the time, while we do not perceive it."

"The smoke goes up from the top of the chimney," said Rollo.

"Yes," said his father, "there is no difficulty about that. The vapor
from a fire is carried up by the warm air, no doubt. Air swells when it
is heated, and so becomes lighter, and rises; and the hot air from the
top of the chimney carries the vapor up with it, no doubt. After it
rises a little way, and becomes cool, it ceases to ascend, but floats
away horizontally. Perhaps it begins to descend when it gets cool,
though very slowly; and perhaps all clouds are really descending all
the time, though too slowly for us to perceive the motion."

"Only," said Rollo, "af