The logbooks contain detailed records of weather in the oceans, which were updated daily or even hourly by senior officers. There are measurements of wind speed and direction as well as temperature and pressure recordings. As time went on the measurements became more instrumented and accurate. The researchers are cross referencing the data with historical records for crop failures, droughts and storms and will compare it with data for the modern era in order to predict similar events in the future.
You can read more at the BBC's web site at http://news.bbc.co.uk/2/hi/science/nature/8291267.stm.