"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 retarding effect of the current longer than she has the benefit of its cooperation. "For example," continued Mr. George, "suppose the distance from one place to another, on a river flowing two miles an hour, is such that it takes a steamer three hours to go down and four hours to come up. In going down she would be aided how much?" "Two miles an hour," said Rollo. "And that makes how much for the whole time going down?" asked Mr. George. "Six miles," said Rollo. "Now, it takes her _four_ hours to go up," said Mr. George. "How much would she be kept back then by the current?" "Why, two miles an hour for _four_ hours," said Rollo, "which would make eight miles." "Thus in the double voyage," said Mr. George, "the boat would be helped _six_ miles and hindered _eight_, so that the current would on the whole be a serious disadvantage. For a steamer, therefore, which is to be navigated equally both ways, the current is an evil. "But for that sort of navigation which goes only one way, it is a great advantage. For instance, the rafts have to come down, but they never have to go back again; and so they have the whole advantage of the current in bringing them down, without any disadvantage to balance it.