and glasses in their hands. These were tourists that had been
travelling in Switzerland, and were coming home by way of the Rhine; and
as they were now just entering the part of the river where the grand and
imposing scenery was to be seen,--though Mr. George and Rollo were just
leaving it,--they were full of wonder and admiration at the various
objects which appeared around them on every side. Rollo had but a very
brief opportunity to look at these strangers, for the steamer which
conveyed them passed by very swiftly, and in a moment they were gone.

"How swift!" said Rollo.

"Yes," said Mr. George, "they go down the stream much faster than they
go up; for in going down they have the current to help them, but we have
it to hinder us in going up."

"And does it help just as much as it hinders?" asked Rollo.

"Yes," said Mr. George, "for any given time. If the current flows two
miles an hour, it will carry forward a boat that is going _with_ it just
two miles faster than it would go in still water. And if the boat is
going _against_ it, it will go just two miles an hour slower.

"Thus, you see," continued Mr. George, "if a steamer had an engine
capable of driving her twelve miles an hour through the water, in
navigating a stream that flows _two_ miles an hour, she would go
_fourteen_ miles an hour in going down, and _ten_ miles an hour in going
up."

"Then," said Rollo, "it seems that the _help_ of a current is just as
much as the _hinderance_ of it, and that a river running fast is just as
good for navigation as if the water were still. Because, you see," he
added, "that though they lose some headway in going up, they gain it
just the same in coming down."

"That reasoning seems plausible," replied Mr. George, "but it is not
sound."

"What do you mean by _plausible_?" asked Rollo.

"Why, it _appears_ to be good, when it really is not so. Reasoning very
often appears to be good, while there is all the time some latent flaw
in it which makes the conclusion wrong. Very often something is left out
of the account which ought to be taken in and calculated for, and that
is the case here. The truth is, that the current helps the steamer in
going down just as much as it retards her in coming up _for any given
time_; as for instance, for an hour, or for six hours. But we are to
consider that in accomplishing any given _distance_, the steamer is
longer in coming up than she is in going down, and so is exposed to the
re