Zealand

42. Of Navigation

Wednesday, 27th February 1666

While I cleaned from Captain's breakfast on Wednesday, he beckoned me from the chart table. "Leave that be for the nonce, Boy, and hie thee here. We have another half-hour before the tide is proper, so let me continue answering your many questions."

Spread on the table when I arrived was a chart, and pointing to it, he said, "This depicts the ocean we will cross, and our initial destination is the island of Barbados, here."

"What are all the lines, sir? I saw those on the other charts, as well, but we strayed to other topics before I could ask."

"The ones radiating from the circles show the points of the compass.[1] Without land in sight, we must steer by a magnetic needle that points toward the north. These parallel horizontal lines show the latitude."

"Latitude, sir?"

"The distance from the Equator. They are measured in degrees, beginning with zero at the Equator and arriving at ninety at the pole. And with each degree being sixty sea miles,[2] that spans five thousand and four hundred miles."

"So, one-quarter of the way around the world – twenty-one thousand and six hundred would be the whole way around."

"Exactly, Boy. On this chart, the lines of latitude are spaced at every five degrees, their numbers shown here in the side margins. And it is from these that we measure distances." He lifted a brass stick from the table. "This is a divider, hinged here to allow the legs to open. With it, we span between two places on the chart and find the distance from one to the other by using the latitude scale like this."

I nodded at the simplicity of it, then I puzzled. "For longer distances, have you a larger divider?"

"Nay, Boy. For distances greater than the span of the legs, we take a measure from the marginal scale like this. Here, I take ten degrees – how many miles is that?"

"Six hundred, sir."

"Yes, then we walk the divider across the chart like this, counting the steps. From here to the island of Barbados is six steps, so three thousand and six hundred miles."

"But that goes across dry land where Zealand cannot go."

"Indeed, Boy. And this is the role of a navigator – to find a route clear of land, both that which is seen and that which lurks close beneath the surface. Once we are clear of the Channel and beyond the tip of France, our best route is southward to the islands of Cape Verde, here off the African coast. Then we will turn westward to Barbados, here. A total of four thousand and eight hundred miles."

"One-third the distance farther that way." I ran a finger across the chart. "Why do we not go directly across here?"

"Neither the winds nor the currents favour that. Around the entire Western Ocean runs a great gyre, which follows –"

"Gyre, sir?"

"A circular motion, Boy. Both the winds and the currents follow a pattern. They run southerly down the coasts of Europe and Africa, then across westward above the Equator to the Caribbean before turning northward along the coasts past our colonies in Virginia, New York and Newfoundland, then returning to England to complete the circle. In the centre of the gyre is a vast area of fickle and confused winds, trapping – sometimes for months – ships with foolish or unaware navigators."

"Oh! How will I ever remember all of this?"

"It is not to remember, Boy; rather, it is to understand. With a comprehension of the fundamentals, the details will come to mind."

I nodded while I pondered all of this, then pointing to the chart, I asked, "Why are the latitude lines spaced farther apart toward the north?"

"It pleases me that you noticed. This is caused by the projection used to draw the chart, and before you ask, a projection is the manner in which the globe is represented on a flat piece of paper. There is no way to do this without distorting some aspects, and here with Mercator's projection, it is with the size of the land and the seas. The farther from the equator, the larger they become. In reality, these vertical lines, called longitude, meet at the poles, as with segments of an orange."

I pointed again at the chart. "But these lines are parallel, not coming together."[3]

"Aye, to convert the spherical shape into a flat one while retaining the proportions, the latitude is expanded at the same rate as the lines of longitude are straightened. The bearings from point to point are all true and straight lines, but the distances must be found using the scale in the margin beside the area measured." Captain shrugged. "But we are getting deeper into this than you need at the moment. Have you another question?"

I put my finger on Barbados. "How long will it take us to reach here?"

"It depends upon the weather, but if we can maintain a mean[4] of ten knots, it will take us four hundred and eighty hours, which is twenty days – three weeks. But eight knots is more likely to be our progress, so twenty-five days at sea plus stops in Madiera and Cape Verde for water, ale, wine and fresh meat."

"Knots?" I shook my head. "How do you mean, knots?"

"Captain chuckled. "Knots in this meaning is speed. The speed of the wind, the speed of the current, and in this instance, the speed of the ship."

"How fast does Zealand go?"

"The proper nautical expression is what speed does Zealand make? A ship's speed is related to her waterline length – the longer she is, the faster she can make."

"Oh! That is why you bade me measure that – one hundred and twenty-three feet."

"Exactly, Boy. There is an arithmetical process[5] for converting that to maximum theoretical speed – four-thirds the square root of the waterline length in feet is the maximum speed in knots."

"What is square root, sir?"

"A number, when multiplied by itself, arrives at what is called a square. The square root is the number which was multiplied. For instance, the square of five is twenty-five; thus, the square root of twenty-five is five."

"Aha! Mother had called it a radix.[6] So, eleven times eleven is a hundred and twenty-one, and four-thirds of eleven is fourteen and two-thirds, meaning Zealand can make almost fifteen knots."

"Aye, in theory, Boy. But that requires ideal winds and sea state, perfectly set sails and a clean bottom. Also, the fastest speed is often not in the direction we wish to sail. The art of navigation is in finding the best course for the conditions, a course which will take us most safely and most swiftly where we wish to go."

He pointed at the staircase. "And now, I wish to head up top to ensure all is ready for our departure. When you have finished your tasks, come up, and I will show you where you may stand to be out of the way."

Notes:
[1] The term compass rose wasn't coined until the 1890s.
[2] The term nautical mile didn't enter the language until 1830.
[3] Converge didn't enter the language until the 1690s.
[4] The word average in this sense is from 1755.
[5] The word formula in this mathematical sense didn't appear until 1796.
[6] Radix is the Latin word for root, and the square root symbol √ is stylized from its first letter.