"That will do," said the Doctor; "you have given one quadrant, or a quarter of the circle; I'm sure you can do the rest easily, for it goes on in the same way."
"You see," Frank continued, "that you know by the compass exactly in what direction you are going; then, if you know how many miles you go in a day or an hour, you can calculate your place at sea.
"That mode of calculation is called 'dead-reckoning,' and is quite simple, but it isn't very safe."
"Why so?" Fred asked.
"Because it is impossible to steer a ship with absolute accuracy when she is rolling and pitching about, and, besides, the winds make her drift a little to one side. Then there are currents that take her off her course, and sometimes they are very strong."
"Yes, I know," Fred replied; "there's the Gulf Stream, in the Atlantic Ocean, everybody has heard of; it is a great river in the sea, and flows north at the rate of three or four miles an hour."
"There's another river like it in the Pacific Ocean," Frank explained; "it is called the Japan Current, because it flows close to the coast of Japan. It goes through Behring Strait into the Arctic Ocean, and then it comes south by the coast of Greenland, and down by Newfoundland. That's what brings the icebergs south in the Atlantic, and puts them in the way of the steamers between New York and Liverpool.
"On account of the uncertainty of dead-reckoning, the captain doesn't rely on it except when the fog is so thick that he can't get an observation."
"What is that?"