"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 coöperation.
"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.