Running down the longitude?

30 Mar 2012

I have just been reviewing a new book, The World of John Secker (1716–95), Quaker Mariner, for the Journal for Maritime Research. I came across some interesting passages in it that describe how seafarers navigated in the early eighteenth century but which also left me slightly puzzled. One of them describes sailing in the 1730s:

After we had left Tercera & it was concluded to run for Madera we stood to the eastward till by the reckoning of the captain & mate we were in the longitude of Madera, then steering a south course to get into the latitude they expected to have fell in with the middle of the island. I keeping a journal my own self was pretty certain we had not run far enough to the eastward & told the captain & mate so, but they were too positive in their own accounts to give credit to my estimation till they found by sad experience it was too late for after we got into the latitude of the island, & could see nothing of it: they to their error concluded we were to the eastward of the island and notwithstanding all I could say to the contrary would stand to the westward in hopes of making the island.

Secker goes on to describe how they ran low on food and water during the prolonged search for their destination, but what struck me was that the process he describes is slightly different from what I was expecting. What we normally assume is that because navigators were less confident in their determinations of longitude, they would do something called 'running down the latitude'. This involved sailing well to either the east or west of the destination until its latitude was reached, then sailing west or east (depending on which side they were) while maintaining the same latitude (which could be measured fairly well from the Sun or Pole Star) until the destination came into sight. I'm slightly puzzled that Secker describes something that sounds more prone to error, in that they were trying to sail to the destination's longitude, then sail directly towards it on a southward course. I'd love to hear of other similar examples.