Quantifying inference: The source challenges of the master’s logs

Exactly three months ago, I started transcribing the sections of Sultana’s master’s logs recording her ocean passages—156 days total, from August 1768 to early December 1772, for the purpose of extracting all the data and information on her sailing performance that I could possibly wring out of them. Ultimately, this project generated 38 separate summary documents and spreadsheets, the records of processing this information through multiple stages of analysis. I also plotted the wind directions and Courses Made Good for each of the 156 days on a 360-degree compass card pasted into 156 individual Word documents. Tomorrow, I am going to read a book. (“Required reading” for the project, but still, it’ll be nice to look at a different set of black characters on a white background for a few days.)

Working with this data brought up the basic challenges of trying to extract information from most historical sources. Those challenges start with this basic one: the source was not created for you, or with your agenda in mind. Whoever created it did so for their own reasons, and it’s highly unlikely it ever occurred to them that a historian three hundred years later would have the slightest interest in it. David Bruce’s log was written as standard procedure for the master of a vessel in the eighteenth-century British Atlantic, not for me. That set me up for the following basic challenge. My ultimate hope was to learn what sails Sultana was flying on what points of sail (angles to the wind), and in what wind strengths. This is the core of sailing performance, especially when the day’s progress is recorded, which in a log, it is. Unfortunately, I had to go about approaching this task in an oblique way, as the log only tells me some of what I want to know.

Generally, the log records three to four wind directions for the 24-hour day (noon to noon); North Atlantic weather is more fickle than steady. It also records between one and five wind strengths, along with other weather conditions—such as “moderate” or “strong gales and squally.” The third element of the equation is a Course Made Good (CMG) for the entire day’s run. Problem Number One is that the correlations between wind direction and wind strength are rough rather than exact. Much of the correlation must be inferred, by considering all the information presented. Problem Number Two is that only the day’s CMG, not the individual courses steered, is presented. So, if the wind came from three different directions that day, and it’s clear from the Remarks column that the crew made several sail changes indicating more than one point of sail, then we can assume that more than one course was steered. In order to know precisely the point of sail Sultana was on at any given time, we need to know what course she was steering when the wind was coming from a specific direction. The log rarely gives us that. So, plotting her points of sail meant plotting wind directions and CMG for the day, and inferring the points of sail from the angles of those wind directions to the CMG. Those inferences are, by necessity, approximations.

Problem Number Three is that Bruce is clearly inconsistent in recording sail changes. This is perfectly understandable. He was sailing master of a small vessel with 25 crew aboard, trying to stay alive and afloat in the unforgiving North Atlantic in all the wrong months of the year. He had much to do. He records that he reefs sails that he has not recorded ever setting. He lets one or more days pass without recording any sail sets or changes, yet then makes a remark that makes it difficult to assume that his sail plan was simply carried on from the last entry. Again, inferences must be made, and the unavoidable element of guesswork must be accepted.

So, in the end, my tables of percentages to the first decimal point would be most misleading if I did not qualify them with disclaimers like these; they imply a degree of precision of interpretation that is simply beyond our grasp at this point. We can say the same about so many such quantitative exercises in history—perhaps most of them. We have to take care not to ask more of our analyses than they can provide, and we must make clear to our readers what we have done, what assumptions we have made, and why.

Having said that, I am excited about what my analyses ultimately suggest. I am fortunate in that I should have the opportunity to discuss the results with the captain of the current Sultana; insights from his experience should clarify much of the uncertainty that remains, though even then, there are sure to be judgment calls of Bruce’s whose reasoning will elude us. We cannot, in the end, get inside another person’s mind. We can only know what they thought to tell us. Even then, that’s only what they wanted to record, or their own understanding of why they did what they did, and there’s likely even more to it than that. But that is a discussion for another post.

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