“You know,” volunteered Alan, “that you can’t pick out a compass direction and stick to it for a long voyage. You’ve got to allow for the lessening tendency in the compass needle to point directly toward the magnetic pole.”
“I’d have got to that in a second,” protested Bob. “Well, where the distances are not great and you have land marks or lighthouses or buoys to bring you up if you go wrong, sailors generally set a course by compass and allow nothing for the variation of the needle. They may shy off a few miles but a headland or a known lighthouse will bring ’em right again. But, on the high seas, it’s a different story. When they begin a voyage a course is laid to an imaginary point—say three hundred miles out. You can pretty nearly tell when you’ve covered this on a steamer by the speed of the engines. On a sailing vessel it’s often necessary to find the point with an observation for longitude and latitude. When that point is reached, the course is altered and a new point taken. The direction of this is marked down by compass. That’s called the ‘true course.’ Then, knowing your latitude and longitude, you’ve got to calculate what the magnetic variation will be in that place and from that you get what’s called a ‘magnetic course.’ That’s the one you sail. And you do this over and over till you get there.”
Poor Buck shook his head, not much better off than before. Ned picked up the engineer’s table of courses for the ocean flight.
“Look here,” he began, pointing to the chart. “Let me try. See Fogo Island up here—where we leave Newfoundland?” Buck wrinkled his forehead and looked. “Well, the bearing of that is known—it’s 54° and 5´ west longitude and 49° 43´ north latitude. Our course from that looks like a curve but the curve is really made up of short straight lines. We make our first straight line end at a convenient place on our curve. The first one stops at 50° west longitude and 54° 40´ north latitude. For a navigator it is now a question in mathematics to find the true direction between these two bearings. In this instance you’d reach the end of the first straight line by sailing N. 67° E., from Fogo Island. That is, you would if it were not for the variation in the compass due to the magnetic pole. In this latitude and longitude the variation would be 33-1/2° W. Then it’s a question of more mathematics and your true course of N. 67° E., becomes a magnetic course of S. 79-1/2° E., or, by compass, E. by S.”
“I ain’t goin’ to be called on to do that figurin’, am I?” asked the alarmed Buck.
“It’s all done,” laughed Ned, “thanks to our engineer.”
“Well, or try to understand it?” persisted Buck. “For I might as well let you in on a secret: I don’t understand a bit of it. If you can start goin’ N. 67° E., and then go the same way by runnin’ S. 79° E., you’ve got my goat.”
“Oh, you’re not so much wiser,” exclaimed Alan when Ned and Bob laughed at Buck’s perplexity and frankness. “Neither of you could make one of these calculations.”
“Right,” retorted Ned, “but you watch me use ’em.”
Bob had been studying the Ipswich-Fogo courses and data.